2002 Audi S8 Air Injection System Air Bypass Valve Replacement Cost

Air injection systems are emissions components that send clean air to the exhaust system which allows the catalytic converter to completely burn fuel before it is released through the exhaust pipe. Modern vehicles may still use vacuum operated air pumps, but that is much less common that computer controlled actuation. In these systems, the engine control module (ECM) will determine when the pump should not send clear air to the exhaust system and open the air bypass valve to allow clean air into the engine instead.

The air bypass valve is installed on the secondary air pump, or inside a line from the secondary air pump to the intake manifold. When the valve is opened, the air pressure generated by the air pump will be sent into the engine instead of the exhaust. Once the ECM closes the valve, the air pressure from the secondary air injection pump will be sent to the exhaust and catalyze the fuel at the catalytic converter.

If the air injection system air bypass valve is no longer able to open or close, or stays open at all times, the fuel to air ratio for the vehicle will suffer as will engine performance. If the valve sticks closed, the pump will only be capable of pumping air into the exhaust. This will cause the oxygen sensors to sense a lean condition and the check engine light will appear. Also, the ECM will detect insufficient flow of air to the engine which may result in storage of OBD-II trouble codes related to the mass airflow sensor or manifold absolute pressure sensor. On the other hand, a valve that is stuck open will result in the oxygen sensors reporting rich running conditions, and the mass air flow or manifold absolute pressure sensor reporting errors. The vehicle will likely run roughly, can hesitate on acceleration, and misfires may be present.

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.

This component is diagnosed differently depending on the form of actuation. Vacuum actuated valves will be tested by pulling vacuum on the solenoid and watching the valve for opening. For computer controlled systems, a digital scanner or outside power source can be used to open the valve on demand. If the valve fails to open or close in either case, it will require replacement. Of course, that is after confirming associated electrical circuits are functioning properly.

Replacing the bypass valve will differ from vehicle to vehicle, but generally speaking the valve should be simple to remove from the air pump or hose, and there should only be one electrical connector to remove. Many times, the use of tools may not be necessary. Installation is also straight forward, and the only concern is assuring the valve and hose is sealed from air leaks.

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.

Anyone can replace the air bypass valve, and the failure may be obvious due to leaks in an air line or a disconnected hose. However, if the fault is not obvious, diagnosis should be thoroughly understood and performed correctly in order to ensure the valve is not replaced erroneously.