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.
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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