AIR INJECTION SYSTEM
by Kyle McFadden

The air injection system reduces hydrocarbon and carbon monoxide emissions by creating a combustion process in the exhaust system. As the exhaust gases leave the combustion chamber, an air stream is injected into the exhaust port. The temperature of the exhaust gas combined with the addition of oxygen from the AIR system, causes any unburned fuel in the exhaust gas to ignite and burn more completely. The air injection system is also used on some vehicles to provide additional oxygen to the catalytic converter. Some designs of catalytic converters require a supply of fresh air for correct operation. These systems will usually supply air to the exhaust manifolds when the engine is cold and switch the air downstream to the catalytic converter when the engine is at operating temperature. As part of a regular emission check, you should have a car repair mechanic inspect the air injection system.

The air injection system consists of an air pump, diverter or air management valve, check valves and plumbing. The air pump is designed to supply a constant supply of air for the air injection system. The air pump can either be belt driven or on some late model vehicles, operated by an electric motor. The air pump typically contains no serviceable parts and is repaired by replacement only.

The diverter valve is typically equipped on carbureted vehicles to control backfire on deceleration. During deceleration on carbureted engines, an excessively high amount of fuel is drawn into the exhaust. The addition of air into this fuel rich atmosphere can cause an explosion that could result in exhaust system damage. The diverter valve switches air flow from the exhaust to the atmosphere when it senses deceleration. The diverter valve uses a manifold vacuum signal to determine deceleration. Most fuel injection systems do not require a diverter valve, since they are able to shut down fuel flow on deceleration.

The air management valve or air switching valve, can be used in addition to or in place of a diverter valve. The air management valve is typically used on computerized emission control systems to switch air flow from the exhaust manifold to the catalytic converter or the air cleaner. During cold start up, air is supplied to the exhaust manifold, to reduce emissions and provide quicker warming of the oxygen sensor. When specific operating conditions are reached, air is switched downstream to the catalytic converter or on some models, directed to the air cleaner. Air injection is not supplied upstream of the oxygen sensor during closed loop operation to prevent false oxygen sensor readings. The check valves are attached to the air injection plumbing and are used to prevent exhaust gases from damaging air injection components. They are designed to allow air to flow into the exhaust system and block exhaust from flowing back through the air supply system.

Some vehicles use a pulse type of air injection system. This type of air system does not require an air pump. The pulses of air pressure located in the exhaust stream draw fresh air into the exhaust through a one way check valve assembly. The check valve prevents the hot exhaust gases from backing up into the valve assembly. Refer to an auto repair manual for specifics covering the diagnosing and repair of the AIR system for your vehicle’s make and model.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)