2008 Audi S4 Air Injection System Control Valve Replacement Cost

Generally speaking, control valves allow or disallow air or fluid from moving between connected cavities. In this case, it will allow or disallow air from moving between the air injection pump and the exhaust manifold. The purpose it to prevent excess clean air from moving into the exhaust during lean conditions, and allowing air to move into the exhaust during rich conditions.

When the air injection system control solenoid is actuated by vacuum or computer control, the solenoid will act on the air injection system control valve, causing it to open or close. When it opens, it creates a path for clean air from the air injection pump to move through the air injection system line between the air injection pump and exhaust manifold. Once in the exhaust manifold, the air will allow improved operation of the catalytic converter by encouraging complete burning of unused fuel before exhaust gases are passed into the atmosphere. When the valve is closed, the lack of air will prevent lean running conditions to be sensed by the oxygen sensors to maintain engine efficiency.

If the control solenoid fails to open or close the control valve for the air injection system, the oxygen sensors will detect a lean condition when the valve is open and a rich condition when the valve is closed. This is because the oxygen sensors are expecting less or more clean air to be injected when the engine control module commands the solenoid open. At the very least, the check engine light will illuminate, and codes will be stored for the air injection system, catalytic converter, oxygen sensors, mass air flow sensor, or manifold absolute pressure sensor. The vehicle may also hesitate during acceleration, misfire, or even stall under some conditions. Rough idling and acceleration are also typical if the valve is fully open at all times.

Since this error can result in the ECM operating in a closed loop with the oxygen sensors, the air to fuel ratio for the engine may become excessively rich or excessively lean. At the very best, this will result in additional fuel consumption, but lean conditions are hazardous to engine components due to excessive heat. This should be addressed right away to prevent adverse effects and collateral damage to the catalytic converter, exhaust manifold, and engine cylinder head valves.

The air injection system is typically trouble free, however, when issues do arise from these systems it seems to occur between the 80,000 and 120,000 mile mark. This is no guarantee, and most vehicles will never have an issue with this system due to its lack of complexity and design to operate near high heat.

If the air injection control valve is no longer able to prevent or allow the flow of air to the exhaust system, it will become evident through physical inspection. In the most difficult case, the valve may need to be tested using low pressure air or vacuum.

Replacement of the valve will require separating the valve from the metallic line moving from the air pump to the exhaust manifold and installing the new valve. Note that some valves will be an individual unit which may be integrated with the control solenoid for that valve. These can be on or in the metallic line, or mounted adjacent to the rest of the system.

Since the air injection system uses rubber press-fit hoses and other components subject to deterioration, any time the system is serviced it is advised to perform a complete inspection of the secondary air system. This includes inspecting for exhaust leaks where the air injection system connects to the exhaust manifold or exhaust pipe.

Portions of this system become extremely hot to the touch as they are directly connected the the exhaust system. Any time this system is service, ensure that the engine has had adequate time to cool. Laser thermometers are advised to locate high temperature areas prior to servicing the system.

Control valves are on the hot side of the air injection system, and will likely be mounted between metal lines or fittings. This means replacing the valve will likely require working in tight, hot spaces, and this may also limit the ability to diagnose the control valve. If diagnostics for the control valve is unclear, or replacement seems to be overwhelming, trust a professional to make sure this repair is done correctly.