VALVE TRAIN et CYLINDER HEADS

Cylinder heads are cast out of iron or aluminum. Cylinder heads bolt onto the top of the engine block and are used to seal the top of the cylinders. They contain the passages that intake and exhaust gases flow through as well as, passages for coolant and oil flow. The intake and exhaust valves are also located in the cylinder heads. On overhead camshaft equipped engines, the cylinder head will also contain the camshaft.

The valves located in the cylinder heads allow the air/fuel mixture to flow into the combustion chamber and provide a means to remove the combustion by-products from the cylinder. Intake and exhaust valves look similar, but the intake valve diameter is usually larger than the exhaust valve. The large circular end of the valve that faces the piston is called the valve head. The beveled edge of the valve that is on the rear side of the valve head is called the valve face. The valve face is held tightly closed against a circular hole in the cylinder head called the valve seat. The valve seat has an edge that is ground at a different angle than the valve face. This is done in order to allow an interference fit between the valve and the valve seat. This interference fit provides a better seal and helps to center the valve in the valve seat when it is closing. The long shaft that extends from the valve head is called the valve stem. The valve stem extends up through a machined guide into the cylinder head. The portion of the valve stem that protrudes from the top of the cylinder head, is retained by a coiled spring and retainer clips. This coiled spring holds the valve closed.

The valves are opened by a rotating camshaft. The camshaft is rotated by a belt or chain connected to the crankshaft. The camshaft contains eccentric lobes that provide the opening motion for the valve when the shaft is rotated. The camshaft lobes are designed to open and close the valves quickly without damaging them. The round portion of the camshaft lobe is called the base circle. The first part of the eccentric is called the ramp or the flank. The tip of the camshaft lobe is called the nose. The opening ramp is cut in an angle to quickly unseat the valve without causing damage. The nose is cut in a measured distance to hold the valve open for a precise time. The closing ramp is cut at an angle to allow the valve to close quickly without damaging the valve.

The amount of time a valve remains open is called the duration. Duration is measured in degrees. The distance that a valve travels when opening is called the lift. The amount of time that the intake and exhaust valves are open at the same time is called overlap. Changes in engine performance, economy, and emissions can be made by using camshafts with different variations of these measurments.

The camshaft does not directly contact the valve stem. Instead, the camshaft contacts a tappet or lifter, sometimes also referred to as follower. The lifter is used to transmit the action of the spinning camshaft lobe to their respective valves. As the camshaft lobe pushes up the lifter, the lifter in turn moves a long rod called the pushrod. The pushrod connects the lifter to an arm that pivots on a shaft or stud. This arm is called the rocker arm. The rocker arm presses directly on the valve stem on one end and contacts the pushrod on the other. The rocker arm is designed to multiply the lifting action of the camshaft and provide a higher lift to the valve. Overhead camshaft engines work in a similar principle, but use less components. By placing the camshaft on top of the cylinder, the need for complex lifter-pushrod designs are eliminated. While overhead camshaft designs vary from manufacturer to manufacturer, the elimination of the cumbersome lifter pushrod designs, allow an engine to produce power more efficiently.

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TROUBLESHOOTING DRIVABILITY PROBLEMS

To control vehicle emissions and improve performance and fuel economy, automobile manufacturers began installing "on-board" computer systems on vehicles in the late 1970's. This provides a monitoring system that continuously checks the readings from various sensors and turns on the "Check Engine" or "Service Engine soon" light when a sensor is not providing the correct information to the computer. Whenever the "Check Engine" or "Service Engine Soon" light appears, there is a service code, often called the trouble code, stored in the computer's memory circuit that can be extracted and used to help troubleshoot the problem. Today's computerized vehicle systems can actually help you diagnose and troubleshoot themselves if you have the right information and a few basic hand tools.

If you are experiencing a drivability problem, your first step is to extract any "trouble codes" from the vehicle's computer. The following are the most commonly experienced drivability problems and a short description of what components may be affected. In each case, these suggestions are assuming the engine is mechanically sound (camshaft, pistons, timing chain or belt, valves, etc). The engine must also be in good tune (spark plugs, wires, distributor cap and rotor, air and fuel filters been recently replaced, etc). The ignition timing and base idle speed must also be adjusted properly.

Complaint #1 - I HAVE AN INTERMITTENT "CHECK ENGINE" LIGHT FLASHING
An intermittent "Check Engine" light is an indication that a service code has been stored in the vehicle's computer system. First, extract the service codes prior to making auto repairs. A common sensor that creates an intermittent "Check Engine" light is the Oxygen Sensor.

Complaint #2 - MY CAR STARTS HARD
Hard starting can be caused by a number of different items. First, inspect all vacuum lines on the engine and replace them if any cracks or brittleness is found. If your vehicle is carbureted, the choke system must be functioning properly before any sensor testing is performed. If it is operating correctly, the Coolant Temperature Sensor could be affecting how well the vehicle starts. On fuel injected vehicles, the sensors that can commonly be attributed to this problem include the Coolant Temperature sensor, Manifold Absolute Pressure (MAP) Sensor, or the Mass Air Flow Sensor.

Complaint #3 - MY FUEL INJECTED CAR IDLES ROUGH AND STALLS AT STOP SIGNS
Stalling at stop signs while the vehicle is in gear on carbureted vehicles could be caused by a failing Idle Speed Control Motor. On fuel injected vehicles, you should look for failure in the Manifold Absolute Pressure Sensor, Mass Air Flow sensor, or restricted fuel injectors. If you suspect restricted fuel injectors, we suggest using an in-tank fuel injector cleaner prior to automotive troubleshooting procedures.

Complaint #4 - MY CAR IDLES ROUGH
Rough idle is most commonly caused by a tune-up problem or vacuum leak due to deteriorated vacuum lines. Once these parts have been replaced or ruled out, the sensors that could be at fault include: Coolant Temperature Sensor, Manifold Absolute Pressure Sensor, Air Charge Temperature Sensor, or Idle Speed Control Motor. Check your auto repair manual for the testing of each of the sensors.

Complaint #5 - THE ENGINE SEEMS TO IDLE ERRATICALLY, WAY UP AND THEN WAY DOWN AGAIN
The items that may cause an erratic idle include the Throttle Position Sensor and the Idle Speed Control Motor.

Complaint #6 - THE ENGINE HESITATES ON ACCELERATION, BUT ONLY WHILE THE ENGINE IS WARMING UP
If you own a carbureted vehicle, first make sure the choke system is functioning properly. A hesitation on acceleration during the engine warm-up period may indicate a failure in one of the following areas: Coolant Temperature Sensor or Air Charge Temperature Sensor.

Complaint #7 - THE ENGINE HESITATES ON ACCELERATION AFTER IT IS FULLY WARMED UP
A hesitation or stumble during acceleration is most likely caused by the Throttle Position Sensor (whether the engine is carbureted or fuel injected).

Complaint #8 - I AM EXPERIENCING POOR FUEL ECONOMY, AND OCCASIONALLY SEE BLACK SMOKE OUT THE TAILPIPE
This situation is typically caused by an overly rich air/fuel mixture. This is commonly caused by a defective Oxygen Sensor, Coolant Temperature Sensor, Manifold Absolute Pressure Sensor, or a bad Mass Air Flow Sensor.

Complaint #9 - I HAVE POOR FUEL ECONOMY AND PERFORMANCE, BUT THERE AREN'T ANY SERVICE CODES IN THE COMPUTER
Sluggish performance is often attributed to plugged or restricted injector nozzles, in which case you can use an in-tank fuel injector cleaner to help restore performance.

Complaint #10 - THE ENGINE SEEMS TO "PING" OR "KNOCK" AFTER IT IS WARM
The first item to check is the base ignition timing. If that is correct, the electronic component to test, if your vehicle has one, is the Knock Sensor. An inoperable knock sensor will not send a signal to the computer to retard timing therefore pinging may result.

Complaint #11 - MY CAR SURGES AT HIGHWAY SPEEDS
Surging at highway speeds is commonly caused by a lean air/fuel mixture. This may be due to a failing Coolant Temperature Sensor, a defective Manifold Absolute Pressure Sensor, or a Mass Air Flow Sensor problem.

Complaint #12 - MY ENGINE HAS A CARBURETOR AND IT DIESELS ("RUNS ON") AFTER I TURN OFF THE KEY
Engine "run on" is commonly attributed to a high idle speed setting caused by a defective Idle Speed Control Motor, an incorrect adjustment, or excessively advanced ignition timing.

Complaint #13 - I AM EXPERIENCING A "ROTTEN EGG" SMELL OUT OF THE TAILPIPE
Rotten egg smell out of the tailpipe is caused by an overly rich air/fuel mixture or by running leaded fuels in a vehicle equipped for unleaded. Overly rich fuel mixtures can be caused by a leaking fuel injector, a bad Oxygen Sensor, or defective Coolant Temperature Sensor.

Complaint #14 - MY CAR "BUCKS" AND OCCASIONALLY BACKFIRES
Other than a mechanical or ignition type of problem, bucking or backfiring can be caused by a defective Manifold Absolute Pressure Sensor, a failing Mass Air Flow Sensor, or a bad Throttle Position Sensor.

As you can see, many of the sensors of a computerized vehicle system can create the same types of drivability problmes. But with a few inexpensive hand tools and the right troubleshooting information you can diagnose and repair the electronic computerized systems on today's vehicles. Remember the above are only to be used as a guideline for testing. Many times, other components or sensors can cause the same symptoms. Consult the manufacturer's auto repair manual covering your specific vehicle before proceeding with auto repairs.

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MY TIRES ARE WEARING ABNORMALLY

SYMPTOM SUMMARY
The tires show signs of abnormal wear.

USUAL CAUSE
The usual cause of abnormal tire wear is infrequent tire rotations, worn suspension components, or misaligned front end.

DIAGNOSIS
The type of tire wear usually indicates the type of malfunction. If the tires are worn only in the middle of the tread area it is an indication of over inflation. If the tires are worn on the outer section of the tread area on both the inside and outside of the tire, it is an indication of under inflation. If the tires are worn on the outside of the tread area on the outside of the tire, it is an indication of improper front end alignment or infrequent tire rotation. For proper tire wear, the tires should be rotated every six thousand miles or every other oil change. If the tires are not rotated frequently, wear on the outside of the tires can result. If the tires have been rotated properly and this type of wear is still present, the alignment of the front end should be checked. Usually the "toe" adjustment will have to be re-adjusted to correct this type of wear. Tires that have peaks and dips accross the tread, it usually indicates worn shocks or struts. Worn shocks or struts will cause excessive tire bounce when going over bumps or dips in the road, which will cause this type of wear.

CORRECTIVE ACTION
If the tire wear is caught soon enough they may not require replacement. Frequent tire rotations and proper inflation can go a long way in extending tire life. If the tires have been rotated and inflated properly, the alignment should be inspected by qualified personel using the proper alignment equipment.

PRECAUTIONS, TIPS, and NOTES
Tires contain steel belts that if exposed on a worn or damaged tire, can easily cut your hand. Use extreme caution when inspecting the tires not to run your hand over the tire which may have exposed belts.

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ENGINE LOSING COOLANT

SYMPTOM SUMMARY The engine loses coolant. This may be in the form of a leak or an internal malfunction that causes the coolant to be consumed in the combustion process. If the engine is exhibiting an overheating condition and there is no leak present, refer to the "Engine Overheating" symptom on this website.

USUAL CAUSE Coolant loss is usually the result of a coolant leak. A blown head gasket or cracked cylinder head can also cause the coolant to be consumed. In this case, no leak will be present but the coolant level will continually go low.

DIAGNOSIS Check and fill the coolant level as necessary while the engine is cold. Start the engine and allow the engine to reach operating temperature. Shut the engine off. The cooling system should be under pressure at this point and any leaks should be easily detectable. Inspect the hoses and the fittings for leaks or loose clamps. Inspect the water pump and thermostat housing for leaks. Inspect the radiator tanks and cooling tubes for signs of leakage.

PRECAUTIONS, TIPS, and NOTES The cooling system is under pressure when the engine is hot. Use caution not to open the system when it is hot. Hot coolant can rapidly discharge from the opening causing severe burns. A cooling system pressure tester can be rented from some auto parts stores. This system allows you to pressurize the cooling system so that it can be inspected for leaks. This can be an aid when trying to locate small or intermittent leaks.

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MY VEHICLE IS LEAKING SOME FLUID

BACKGROUND INFO
A leak is present, however, its origin is unknown.

DIAGNOSIS
Identifying the type of leak is critical in determining its origin. Automotive fluids are often colored and by identifying the color of the leak you can determine from which component it is leaking from. Taking a small sample from each reservoir or dipstick can help you locate the type of fluid leak. For instance, automatic transmission fluid is red, engine oil is brown or black, differential fluid or gear lube has a foul odor to it. The location of the fluid leak is an indication of its origin, i.e. center of vehicle, front of vehicle, etc. Be aware that a leaking hose on top of the engine will leak fluid down over the engine. The engine parts may route the leak away from its source. It is sometimes easier to clean and degrease the engine prior to inspection. This will prevent you from identifying components that are actually leaking from components that are only in the path of the leak. Your diagnosis should start with an inspection of each fluid level. Refer to "Vehicle Maintenance Tips" and your owners manual for more information. Low fluid level is an indication of a leak. Once the type of fluid has been identified you should refer to that components section for further diagnosis.

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EXCESSIVE ODOR EMITTED FROM TAIL PIPE (Rotten Egg Smell)

SYMPTOM SUMMARYThere is excessive odor emitted from the tail pipe. The odor may smell like "rotten eggs". If there is exhaust smoke present, refer to the "Excessive Smoke From Tail Pipe" symptom on this website. If the vehicle has failed an emissions test, refer to the "Emissions Test Failure" symptom also on this website.

USUAL CAUSE This symptom can be caused by an excessively rich air/fuel mixture. If the vehicle only exhibits this condition after severely loading the engine, as with mountainous driving, the condition may be normal. If the "Check Engine" or "Service Engine Soon Light" is on, refer to that symptom for diagnosis.

DIAGNOSIS An exhaust odor that smells like rotten eggs is the result of a rich air/fuel mixture. This condition may be the normal result of placing a heavy load on the engine, as with pulling a trailer or heavy grade. The catalytic converter is responsible for reducing the Carbon Monoxide and Hydrocarbon levels in the exhaust. When the air/fuel mixture is too rich, the catalyst cannot convert all of the additional fuel in the exhaust. The result is a foul odor that is often asscociated with rotten eggs. If the vehicle only emits this odor under the above conditions, the symptom is probably normal. However, if the vehicle also has a running problem, engine warning light on, or emits the odor under all operating conditions, further diagnosis should be performed.

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ENGINE STALLS WHEN COMING TO A STOP OR ON DECELERATION

SYMPTOM SUMMARY
The engine stalls when coming to a stop or on deceleration. The engine runs normally otherwise. If the engine runs poorly you should address that symptom first.

USUAL CAUSE
A problem in the idle speed control system or transmission is the usual cause of this symptom. Almost all modern vehicles utilize computer controlled idle speed control. This system automatically compensates for loads placed on the engine, such as the air conditioning compressor or heavy electrical loads. A failure in the automatic transmissions torque converter clutch that prevents it from disengaging on deceleration, can also cause this symptom.

DIAGNOSIS
The first step in the diagnostic process is a visual inspection of the throttle body. Inspect the throttle body for binding or sticking linkage. If the vehicle is port fuel injected, a visual inspection of the throttle body for excessive carbon build up will be necessary. Remove the rubber duct that attaches to the throttle body. Inspect for black carbon build up around the throttle body. If the throttle body requires cleaning, a spray cleaner is available at any auto parts store. Spray the cleaner on a clean cloth towel. With the engine off, open the throttle and wipe the carbon from the throttle body plate and bore. If the throttle body is clean and functioning properly, the vehicle may have a defective idle speed control motor or valve. Refer to a manufacturer specific repair manual for specific test procedures.

PRECAUTIONS, TIPS, and NOTES
Use caution when working around hot or rotating engine parts. If the symptom is only present while driving then coming to a stop, the transmission torque converter should be inspected for proper operation. This might best be done by a qualified transmission shop. This should only be done after the idle speed control system is found to be functioning.

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ENGINE DOES NOT CRANK

SYMPTOM SUMMARY
The engine fails to crank over.

USUAL CAUSE
This symptom is usually a result of a defect in the electrical system. It is usually a result of a drained battery, defective starter, or a defect in the ignition switch circuitry.

DIAGNOSIS
The first step in the diagnosis is an inspection of the fuses. Refer to the fuse box placard and owner's manual for fuse ratings and designations. Some vehicles use fusible links that are at or near the batteries positive battery cable. These fusible links are designed to open when an electrical overload is present. They will not reset and will require replacement when blown. An inspection of the fusible links should be performed. Clean the battery terminals using a wire brush. Ensure that the inside of the battery terminal and posts are clean and free from corrosion. You should ensure the vehicle has a fully charged battery. Attempt to jump start the vehicle. If the vehicle cranks, a functional check of the battery and charging system should be performed. Refer to the "Battery Goes Dead" or "Battery or Charge Light On" articles for further diagnosis. If the engine will not jump start, a defect in the starter or ignition circuit is the most probable cause. Using a voltmeter, measure the voltage on the small wire of the starter solenoid while an assistant cranks the vehicle. If voltage is present, the most likely cause is a defective starter. If there is no voltage present, a defect in the ignition switch or circuitry is indicated. Refer to a vehicle specific wiring diagram and service manual for specifics on diagnosing the ignition switch circuitry.

PRECAUTIONS, TIPS, and NOTES
Use caution when working around hot or rotating engine parts. When using test equipment, refer to the manufacturers instructions for proper usage.

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ENGINE PINGS OR KNOCKS ON ACCELERATION

SYMPTOM SUMMARY
The engine "pings" or knocks on acceleration. The symptom is more pronounced when the engine is at operating temperature.

USUAL CAUSE
Engine ping is the result of abnormally high combustion temperatures. The usual cause of engine ping is over advanced ignition timing, an inoperative EGR system or improper fuel usage. A defect in the cooling system can also result in engine ping.

DIAGNOSIS
First, do not confuse engine ping with a mechanical failure that produces an internal knock in the engine. An engine knock will be a deep sound that eminates from the engines lower section and will be present under almost all operating conditions. Engine ping is usually present during moderate to heavy engine acceleration after the engine has reached or is near operating temperature. The ignition timing should be inspected and adjusted according to manufacturers specifications. Some vehicles use computer controlled ignition timing that is not adjustable. Consult your owners manual for the recommended fuel rating. A vehicle that is designed to operate on high octane fuel should not be operated using lower octane fuel. Switching to a higher octane fuel may be the only corrective action necessary. If the ignition timing and fuel octane rating are correct, a functional check of the EGR valve will be necessary. Inspect the vehicles cooling system operation. If the vehicle is running hotter than normal or overheating, than that symptom should be diagnosed first. Refer to the "Engine Overheats" article for further diagnosis.

PRECAUTIONS, TIPS, and NOTES
Use caution when working around hot or rotating engine parts.

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ENGINE STALLS INTERMITTENTLY

DIAGNOSE - ENGINE STALLS INTERMITTENTLY

by Kyle McFadden

SYMPTOM SUMMARY
The engine stalls intermittently. The symptom may be more present after the engine has reached operating temperature or during hot ambient temperatures. The vehicle may restart after a short cool down. When the vehicle is running, it runs normally. If the vehicle misses or idles poorly refer to the "Engine Misses or Idles Poorly" article.

USUAL CAUSE
This symptom can be caused by numerous things. If the engine is operating then stalls as if the key was turned off, the cause could be a defective ignition control module or poor connection in the vehicles electrical system.

DIAGNOSIS
Diagnosis of this symptom can be difficult due to the number of possible causes. You must determine what is occuring when the engine stalls. The engine could be losing fuel pressure, ignition spark or the computer itself may be defective (ie. the EEC). Once the type of failure has been isolated such as no spark, no fuel or injection pulse, you will need to focus your diagnosis on that particular sub-system. Refer to the applicable section to obtain a working knowledge of each system involved. By understanding each components role you will be able to more accurately troubleshoot the entire system.

PRECAUTIONS, TIPS, and NOTES
Some vehicles are prone to defective ignition control modules. The module may fail when it reaches operating temperature and then becomes functional again after it has cooled off. Use caution when working around hot or rotating engine parts. The ignition system produces extremely high voltage. Use caution when working around the igntion coils and spark plug wires.

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ENGINE IDLES TOO HIGH

SYMPTOM SUMMARY
The engine idles too high.

USUAL CAUSE
The usual cause of abnormally high idle speeds are vacuum leaks or an inoperative idle speed control motor. A vacuum leak on a fuel injected engine will cause high idle. The oxygen sensor will detect the extra oxygen that is entering the engine through the vacuum leak and will signal the computer the exhaust is lean. The computer in turn will then provide additional fuel which will result in an abnormally high idle.

DIAGNOSIS
The first step in the diagnosis is visual inspection. Inspect the throttle cable and linkage for binding. Inspect the engine for vacuum leaks. With the engine at idle, visually inspect each vacuum hose on the vehicle. Listen for a hissing sound which is a key indicator of a vacuum leak. Repair the vacuum hoses as necessary. Inspect the PCV valve and hose. Using pliers, pinch off the hose that supplies the PCV valve. The engine speed should drop slightly. If the engine returns to a normal idle speed, the PCV valve should be replaced. Consult a vehicle specific service manual for location and specific testing procedures. If the vehicle is free from vacuum leaks and the PCV valve is functioning normally, the idle speed control motor or valve may be defective.

PRECAUTIONS, TIPS, and NOTES
You should consult a vehicle specific service manual for proper diagnosis of the system used on your vehicle. Use caution when working around hot or rotating engine parts.

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DIAGNOSE - ENGINE MISSES OR IDLES POORLY

SYMPTOM SUMMARY
The engine misses or idles poorly. The miss usually follows engine speed and may be more pronounced when the engine is under load.

USUAL CAUSE
There are many items that can cause the engine to miss or run roughly. Defective ignition wires, restricted injectors, or engine mechanical malfunctions are all possible.

DIAGNOSIS
Start your diagnosis with an underhood inspection. Look for broken or cracked vacuum hoses. Inspect the spark plug wires for chafing or signs of arching to the engine block. If the vehicle is due for routine service, this should be done prior to spending too much time on a diagnosis. A complete tune-up including spark plugs, spark plug wires, distributor cap and rotor (if so equipped), fuel filter, and air filter will go a long way in correcting the most common problems. If a misfire is still present after a complete tune-up is performed, diagnosis of engine mechanical problems may be necessary. A vacuum test using a vacuum gauge should be performed. A vacuum gauge can be obtained at any auto parts store. An engine should draw at least 15" of vacuum at idle when connected to a vacuum hose that is connected to the intake manifold. The vacuum should be smooth and not fluctuating. A severely fluctuating vacuum gauge is an indication that there is a valve train malfunction, such as a defective valve or slipped timing belt. Retarded ignition timing can cause abnormally low engine vacuum. A compression check should be performed on the engine to determine if lower than normal compression is present in any cylinder. A compression tester is available at most auto parts stores. Refer to a manufacturers specific service manual for compression specifications. As a general rule, the compression should be above 120 PSI and the lowest cylinder should be at least 80% of the highest cylinder. If the above tests do not indicate a failure, a diagnosis of the fuel and ignition system must be performed.

PRECAUTIONS, TIPS, and NOTES
Often, diagnosing an engine misfire requires the use of special test equipment that is capable of performing ignition and fuel systems test. If the above testing procedures do not reveal the problem, it may be more cost effective to have a repair facility with the above equipment perform a diagnostic test. This usually requires about one hour of labor time. You should ask the repair facility for a diagnostic print-out detailing what was found during the test. Use caution when working around hot or rotating engine parts. When performing a compression test, the ignition system should be disabled to prevent the engine from starting. This is usually done by connecting the spark plug wires or coil wire to a good engine ground away from the area you are working on. Consult a manufacturers specific repair manual for further information.

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DIAGNOSE ENGINE HESITATES WHEN ACCELERATING

SYMPTOM SUMMARY
The engine hesitates when accelerating. The vehicle may stumble or stall when the accelerator is pressed or may exhibit an immediate lack of power.

USUAL CAUSE
This symptom may be caused by several items. A defective Throttle Position Sensor (TPS), Mass Air Flow Sensor (MAF), low fuel pressure, or something as simple as worn spark plugs or defective spark plug wires can be the cause.

DIAGNOSIS
If the "Service Engine Soon" or "Check Engine" light is on, extract the trouble code using a scanner prior to diagnosing this symptom. If the vehicle is due for maintenance, a complete tune-up should be performed prior to an extensive diagnosis. A complete tune-up including spark plugs, spark plug wires, distributor cap and rotor (if so equipped), fuel filter and air filter will go a long way in correcting the most common problems. If the problem still persists, further diagnosis on the vehicles fuel management system is necessary. The throttle position sensor, and mass air flow sensor (if equipped) should be checked. For an understanding of how these sensors operate, refer to the "Fuel Injection and Computer System" article on this website. On most vehicles, the TPS is a three wire sensor. The wires include a 5 volt reference, a ground, and the output voltage. The output voltage of the throttle position sensor should be checked using a digital volt meter. The voltage output should increase smoothly as the throttle is increased. Consult a vehicle specific wiring diagram for a schematic of the computer system which will include the TPS details. The Mass Air Flow sensor is difficult to test without the proper test equipment. There are many styles of sensors utilizing many methods of output signals. You should consult a vehicle specific service manual for testing procedures.

PRECAUTIONS, TIPS, and NOTES
The computer system and its sensors are sensative devices and can be damaged using improper testing procedures. Never use an older needle type volt meter to measure signals on vehicles equipped with computers. Only digital meters should be used. Use caution when working around hot or rotating engine components.

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DIAGNOSE ENGINE CRANKS SLOWER THAN NORMAL

DIAGNOSE ENGINE CRANKS SLOWER THAN NORMAL

by Kyle McFadden

SYMPTOM SUMMARY
The engine cranks over slower than normal. The engine may fail to start as a result of the slow crank speed.

USUAL CAUSE
Low battery charge can result in slow cranking. This can be caused by a defect in the charging system or something as simple as leaving the lights on. A defective starter can also be the cause.

DIAGNOSIS
Start your diagnosis with determining whether the battery is in a proper state of charge. If the vehicle is able to be jump started and cranks over normally, the batteries state of charge could be low. If the "battery" or "charge" light comes on after the vehicle is started, a failure in the charging system is likely. You should refer to the "Battery or Charge Light On" symptom article on this website. If the battery and the charging system are functional and the symptom is more pronounced when the engine is hot, the most likely cause is a defective starter.

PRECAUTIONS, TIPS, and NOTES
Most part stores or repair facilities can charge and test your battery for a nominal charge. Battery cell testers are available at most parts stores andt can test each cell of the battery. This will only work on batteries that have removable cell covers. Use caution when working around batteries. Batteries contain acid that is very corrosive and can burn your skin. Always wear safety glasses and use caution when working around hot or rotating engine parts.

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DIAGNOSE ENGINE CRANKS FAST BUT DOESN'T START

SYMPTOM SUMMARY
The engine cranks over at normal speed but fails to start.

USUAL CAUSE
A no-start condition can be caused by numerous items. If the vehicle is eqiupped with a timing belt and the engine has over 60,000 miles on it and it has never been serviced, it could be broken. This is especially true if the vehicle stalled while driving and will not restart. A defect in any of the vehicles electronics can cause the same symptom. A defective ignition control module or distributor pick-up, ranks as one of the highest probabilities.

DIAGNOSIS
A diagnosis should start with an inspection of the vehicles fuses. Refer to the fuse block placard and owners manual for fuse ratings and designations. An engine that is cranking over unevenly or abnormally fast, is a key indicator of an engine mechanical failure. A compression check should be performed to detect a mechanical failure. If the engine is in proper mechanical order, the ignition system should be inspected for spark while the engine is cranked. A spark tester is available at most auto parts stores. If there is no spark, the ignition control module, distributor pick-up or crankshaft sensor should be tested. A visual inspection of the distributor cap and rotor (if so equipped) should be performed. If spark and proper engine compression are present, a diagnosis of the fuel system will be necessary. Fuel pressure and injector pulse should be tested. You will need to determine if the electric fuel pump is operating, the injectors are firing and there is adequate fuel pressure. You should consult a vehicle specific repair manual for diagnosis of the specific fuel system components on your vehicle.

PRECAUTIONS, TIPS, and NOTES
Use caution when working around hot or rotating engine components. When test equipment is used, always refer to the manufacturers instructions for proper usage. Always disable the ignition system prior to performing a compression test. This is usually done by connecting the spark plug wires or coil wire to an engine ground. Refer to a vehicle specific repair manual for instructions on how to disable the ignition system.

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THERMOSTATIC AIR CLEANER

Troubleshooting a car's emission system often involves inspection of the thermostatic air cleaner (TAC system). The TAC is used to provide warm air to the air cleaner during cold start up. This helps to improve cold engine performance and reduce automotive emissions by providing better fuel vaporization. The TAC system is primarily used on carbureted and throttle body injected vehicles.

The typical TAC system uses a vacuum motor to actuate an air flow door, located in the air cleaner inlet. The vacuum motor is operated by a signal from a thermostatic switch located in the air cleaner assembly. Warm air is supplied to the air cleaner via a hose that connects from a shroud mounted on the exhaust manifold, to a port on the underside of the air cleaner inlet. During cold engine operation, the actuator closes the air flow door and warm air is drawn into the engine from around the exhaust manifold (take special note of this during a diagnosis of the TAC system). As the engine warms up, the vacuum signal is removed from the actuator and the door slowly opens to allow cooler air to enter through the air cleaner inlet, while at the same time, blocking warm air from the exhaust manifold.

Some TAC systems do not use a vacuum actuator to control the air inlet door. Instead, a sealed actuator containing wax is used. When the engine is cold, the wax solidifies and contracts, causing the door to close. As the engine warms up, the wax liquefies and expands, placing pressure on a piston that is used to actuate the door. The TAC system functions well to aid in cold start up. However, if the air cleaner door were to stay in the closed position after the engine reaches operating temperature, lack of power and hard starting could result. Visual inspection of the TAC system can usually determine if this is the cause of these symptoms. If you are doing the auto repairs yourself or if you have hired your local auto repair mechanic to do it for you, the first step should be to visually inspect the TAC system.

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The "CHECK ENGINE" LIGHT CAME ON

SYMPTOM SUMMARY
The "Service Engine Soon" or "Check Engine" light comes on while the engine is running.

USUAL CAUSE
The vehicles electronic control unit constantly monitors and adjusts various engine parameters. If the control unit detects a failure in one of the connected circuits, it will illuminate the malfunction indicator lamp and wll store a diagnostic trouble code in its memory that corresponds to the failed circuit. If the malfunction indicator light is on, the computer has detected a malfunction in the engine management system.

DIAGNOSIS
The diagnosis of the symptom requires interfacing with the on-board computer and retrieving the stored diagnostic trouble code. Manufacturers troubleshooting procedures are then used for the specific trouble code that was retrieved. A special hand held computer, usually referred to as a scan tool, is required to interface with the computer. However, some manufacturers use the malfunction indicator lamp to flash a series of blinks to indicate the specific code. On these vehicles, diagnostics are usually entered by grounding or connecting a pair of diagnostic connectors. Once connected, the computer transmits the stored code(s) to the lamp. The lamp flashes are counted to determine the corresponding code. You will need to refer to the manufacturer specific repair manual to determine the type of system utilized on your vehicle.

PRECAUTIONS, TIPS, and NOTES Working on the computer system requires a good working knowledge of electrical system theory and the proper testing equipment. The computer system can be damaged by using the incorrect test procedures. There are relatively inexpensive test equipment available to retrieve the diagnostic codes from the most common vehicles. Consult your auto parts store for availability.

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POSITIVE CRANKCASE VENTILATION (PCV)

When troubleshooting cars which fail an emissions inspection, one of the most common culprits is the PCV valve. When an engine is running, a certain amount of combustion gas escapes past the rings into the crankcase; this is normal. Combustion gases consists mostly of carbon dioxide and water vapor, but do contain small amounts of unburned fuel and other contaminants. Water and fuel vapors in the crankcase can react to rapidly break down and shorten the life of engine oil. In addition, these vapors contribute to the formation of sludge in the crankcase. Incidentally, this sludge should be cleaned out periodically. Any auto repair garage can perform this for you. In the past, a tube was installed that vented crankcase vapors to the atmosphere while the vehicle was moving. This tube was called the road draft tube. Later it was discovered that a large portion of the emissions of a vehicle were produced by crankcase vapors vented into the atmosphere. This resulted in the development of the PCV system and the disappearance of road draft tubes on engines.

The PCV system removes vapors from the crankcase and delivers them to the intake manifold, to be burned in the combustion chamber. The system consists of a fresh air intake for the crankcase, hoses, and PCV valve. The PCV valve provides flow control for the introduction of crankcase vapors into the intake manifold and acts as a check valve in case of an engine backfire. It contains a spring loaded check valve that is operated by manifold vacuum. When the engine is at idle, vacuum overcomes the spring tension of the check valve and causes it to move inward and restrict vapor flow into the intake manifold. When the vehicle is cruising and vacuum is lower, the spring tension moves the check valve away from the seat, allowing higher flow of crankcase vapors into the intake manifold. Control of crankcase vapor flow is used to prevent excessive amounts of vapors into the intake manifold at idle speed. This could cause rough idle or incorrect idle speeds.

The PCV system operates by using intake manifold vacuum through the PCV valve to create negative pressure in the crankcase. Filtered air is drawn into the crankcase through a hose connected to the air cleaner or air intake duct. The flow of air is circulated through the crankcase and into the intake manifold, via the PCV valve. It then carries the crankcase vapors to the combustion chamber to be burned. The fresh air intake of the PCV system also serves to function as a pressure relief point for crankcase vapors, in the event of an internal engine malfunction or PCV valve blockage. If this occurs, excess vapors are vented through the air intake into the air cleaner. One symptom of crankcase vapors flowing into the fresh air intake is oil residue in the air filter housing, as any auto repair shop will tell you.

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How to Check Your Fluids

General Description of Problem:
You want to check all of your vehicle's fluids

Summary:
Fluids are integral to your vehicle and as an intelligent owner you realize the value in keeping your vehicle well maintained. With just a few moments time, you can check all the essential fluids throughout your vehicle.

Step-by-step:
Position you vehicle on a level surface after having driven it for a minimum of 15 minutes. This will ensure all your fluids are at their normal operating level, temperature, and viscosity. Once you have positioned the vehicle, grab a rag or paper towel to use to wipe the dipstick and clean up any small spills you make when filling. You may also want to have a funnel handy.

When you check the fluids, note their color and texture. Oil should be a golden brown in appearance and whould be free from particulates. The consistency should be similar to that of cooking oil. The fluid in your transmission will be a light pink to red in hue and also should be free of debris. The coolant in your radiator should be a bright green and be thin and almost water like.

Let's start by checking the engine oil. Locate the engine oil dipstick and remove it completely. After removing the dipstick, use your rag or paper towel to wipe it clean and look at the end of the dipstick for the markings indicating full. Most cars will have some manner of notches, holes, or other lines on the dipstick to indicate a "Full" level and a level where oil needs to be added. Once you have found the markings, replace the dipstick from the tube which you removed it. be careful to insert it completely, and then remove it to check the level of oil. The location of oil on the dipstick will indicate whether your oil is full, or in need of a fill. If you need to add oil, only add enough to bring it to full and do not overfill. The location to add oil is usually located on top of the engine and indicated with a marked screw cap.

Your next check will be the radiator fluid. We always want to check this a bit after the car has had some time to cool down as removing the radiator cap on a "Hot" car can be very dangerous. Be very careful to insure that the cap is cool to the touch and that the radiator is not hot before opening and checking the fluid. NEVER OPEN THE RADIATOR CAP IF IT IS HOT, YOU COULD BE SEVERLY BURNED BY THE SCALDING PRESSURIZED COOLANT!

The above precaution noted, lets take a look at the coolant. If your vehicle has a coolant overflow resevoir you a quick glance at the level of coolant in this location should indicate your status. This resevoir is usually a clear or nearly transparent plastic container located to the side of the radiator. It will typically be marked with appropriate levels on the side and you can fill your coolant to the recommended level. If your vehicle does not have an overflow resevoir, carefully remove the radiator cap and check the level of coolant in the radiator. If the level of coolant is low in either location, use a 50/50 mixture of coolant and water to fill as needed.

Next, we'll check a number of other fluids that are important to your vehicle... On many newer cars you can check brake fluid simply by looking at the side of the master Cylinder. This has been made transparent or in many vehicles and with a quick glance you can determine the level of fluid. On older vehicles, you may need to open the lid of the Master Cylinder by removing the small swing top bar that typically holds it in place. As brakes wear the level of fluid will gradually decline. You do not need to refill the fluid unless it drops below the minimum mark indicated.

On vehicles equipped with power steering, the level of fluid can be checked by simply removing the screw top lid to the fluid resevoir. In most vehicles, a dipstick will be attached to the cap and have a measurement for both hot and cold readings.

The windshield wiper fluid resevoir is another translucent resevoir located under the hood. Though not imperative to the mechanical operation of your vehicle, having clean windows is imperative to the safe driving of your vehicle so you should take a quick look when you are already under the hood. If the resevoir is low fill it with additional windshield washer fluid. Finally, if you have an automatic transmission, you should check the transmission fluid.

On most vehicles the transmission fluid is checked with a dipstick similar to that used to check engine oil and it is checked while the vehicle is running. Turn on the car and let your car idle for a moment while you locate the dipstick. Once you have located it, we will do the same as we did when checking engine oil:

- Remove the dipstick and wipe it clean
- Check the far end of the dipstick for the notches or holes that indicate "Full" and "Low"
- Replace the dipstick fully
- Remove and recheck to assess levelUnlike engine oil, if you need to add transmission fluid, most times you will use a narrow funnel and pour the fluid directly into the tube from which you removed the dipstick. Again, add only enough to fill the fluid and never overfill.

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DIAGNOSING ENGINE OVERHEATING

SYMPTOM SUMMARY
The engine overheats or boils over. The temperature gauge reads higher than normal or the instrument panel "hot" light is illuminated.

USUAL CAUSE
The most common cause of an engine overheating condition is a defective thermostat. However, there are numerous items that can cause an overheating condition. Coolant loss will also cause an overheating condition. If you are experiencing coolant loss in the form of a leak, refer to the "Coolant Loss" symptom on this site. If the vehicle overheats and has excessive white smoke from the tail pipe, refer to the "Excessive Smoke From Tail Pipe" symptom (also on this site).

DIAGNOSIS
You should start your diagnosis with a visual inspection of the cooling system. Ensure the coolant level is at the proper level. Do not remove the radiator cap when the engine is hot! Inspect for warped belts and leaking hoses. If the engine overheats a few minutes after start up, the most probable cause is a defective thermostat. If the engine overheats after several miles of driving or only overheats at low speeds, you should inspect the cooling fan operation and radiator. On vehicles that use an electric cooling system, ensure the cooling fan is coming on at when the engine reaches operating temperature, usually between 195F - 220F. Let the engine idle with all of the accessories in the off position (such as A/C). Monitor the engines temperature by observing the temperature gauge. The cooling fan should automatically come on prior to the gauge reading hot. If it does not, promptly shut the engine off. If the vehicle is equipped with air conditioning, turn it on and observe the cooling fan. If the fan comes on, the most likely cause is a cooling fan thermostatic switch. If the fan does not come on, further diagnosis of the fans electrical system will be necessary.

On vehicles with a conventional radiator fan, inspect the fan for damage. Ensure the fan clutch is operating properly. Have an assistant turn the engine off while observing the fan. The fan should only spin for about 2 seconds after the engine is shut off. If the fan continues to spin for several seconds after the engine is shut off and the engine only overheats at low vehicle and engine speeds, fan clutch replacement may be necessary. If the cooling fan operates properly, inspect the radiators external cooling fins for excessive debris, such as bugs and dirt. When the engine has cooled down, remove the radiator cap and inspect the inside of the radiator for excessive scaling. The tubes that run accross from one tank to the other should be clear of build up. If the radiator is not equipped with a conventional cap, it may be necessary to remove one of the radiator hoses to perform an internal inspection. You should do this when the engine is cold. If necessary, the radiator can be removed and inspected by a radiator shop that will provide advice. A radiator that is plugged may require a "Rod Out" to restore proper operation. If the radiator, cooling fan, coolant level and thermostat are in proper working order further diagnosis will be necessary. It is possible that the impeller on the water pump is damaged or the vehicle has a blown head gasket or cracked cylinder head. White smoke from the tail pipe is a key indicator of such a failure.

PRECAUTIONS, TIPS, and NOTES
Use extreme caution when working on the cooling system. The cooling system is under pressure when hot and can cause extreme burns if opened when hot. Use caution when working around hot or rotating engine parts. On vehicles with electric cooling fans the cooling fan starts automatically. Keep your body away from the cooling fan at all times. Radiator flush agents may be an aid in removing scaling from the radiator. Use these products as per the manufacturers recommendations

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