BMW 318/325/M3/525/535/M5 1989-1993 Repair Information

Air Injection System



The S38 engine used in the M5 is equipped with an air injection system to help control emissions. The system injects air into the exhaust manifold during engine warm-up. By providing oxygen to the unburned fuel in the exhaust, the hydrocarbons burn and the carbon monoxide combines with the oxygen to form carbon dioxide and water. Air injection occurs only during the period of catalytic converter warm-up. Once the catalytic converters are at operating temperature, the air injection stops and the catalysts take over the emission reduction.

If the DME receives the start signal, the engine speed is below 4600 rpm and the engine coolant temperature is below 86°F.(30°C.), pin 23 will be grounded for up to 125 seconds. When the pin is grounded, the circuit to the air pump clutch relay is completed. The relay will activate the air pump clutch and an electrically controlled vacuum valve. The vacuum valve will allow vacuum to pass to a vacuum controlled air valve and the valve will open. Air from the pump will pass through the valve, through a check valve and into the injection manifold.

The vacuum controlled valve, when closed, will force the air to be released into the atmosphere via the pressure control valve. The valve also prevents air from being drawn into the manifold when it is not needed. The check valve prevents exhaust gases from reversing into the system.

The air pump clutch relay is located in the electronics box in the engine compartment along with the DME unit. The air pump is located on the engine and is driven by a belt. The electrically controlled vacuum valve is next to the idle control valve, below the intake runners for cylinders 1 and 2. The vacuum controlled air valve is on the front right side of the engine near the check valve. the pressure control valve is located in the same line bolted to the block.


Air Injection Manifold
  1. Disconnect the negative battery cable. Remove the exhaust system. Drain the coolant below the level of the head. Disconnect the right side engine mount and ground strap.
  3. Disconnect the air injection pipe at the check valve and the heat shield. Remove the fan cowl and coolant expansion tank. Remove the air pump air filter housing.
  5. Lift the right side of the engine as far as possible without damaging any connections.
  7. Remove the heat shield holder and the air injection pipe. It may be necessary to use an universal joint on an extension. There is a mounting tab at cylinder number 1.

To install:
  1. Clean the manifold mounting surfaces. Replace the air injection pipe gaskets and the air injection pipe. Torque to 25-33 ft. lbs. (35-45 Nm).
  3. Install the heat shield. Lower the engine. Install the air pump filter, coolant expansion tank and the fan cowl. Connect the air injection pipe and tighten to 25-33 ft. lbs. (35-45 Nm). Install the heat shield holder.
  5. Connect the ground strap and the engine mount. Torque to 30 ft. lbs. (42 Nm). Install the exhaust system and fill the cooling system. Connect the negative battery terminal.