Secondary Air System Monitoring

SECONDARY AIR INJECTION MONITORING

In order to reduce HC and CO emissions while the engine is warming up, BMW implemented the use of a Secondary Air Injection System in. Immediately following a cold engine start (-10 - 40°C) fresh air/oxygen is injected directly into the exhaust manifold. By injecting oxygen into the exhaust manifold:

-The warm up time of the catalyst is reduced.

-Oxidation of the hydrocarbons is accelerated.

The activation period of the air pump can vary depending on engine type and operating conditions.




Conditions for Secondary Air Pump Activation:

NOTE: Below -10°C the air injection pump is activated only as a preventive measure to blow out any accumulated water vapor that could freeze in the system.

System Components:

-Electric Air Injection Motor/Pump

-Electric Motor/Pump Relay

-Non- Return Valve

-Vacuum Valve

-Stainless Steel Air Injection Pipes

-Vacuum Reservoir

-Vacuum Reservoir Check Valve

-Activated by Engine Control Module

-In-Line resistor for speed control (M73 engine only)

The Secondary Air Injection System is monitored via the use of the pre-catalyst oxygen sensor(s). Once the air pump is active and is air injected into the system the signal at the oxygen sensor will reflect a lean condition. If the oxygen sensor signal does not change within a predefined time a fault will be set and identify the faulty bank(s). If after completing the next cold start and a fault is again present the 'Check Engine" light will be illuminated.

Example: Secondary Air Injection Monitoring (M52-Siemens System).

During a cold start condition air is immediately injected into the exhaust manifold and since the oxygen sensors are in open loop at this time the voltage at the pre catalyst sensor will reflect a lean condition (4.8 - 3.9 V) and will remain at this level while the air pump is in operation. Once the pump is deactivated the voltage will change to a rich condition (O.7V) until the system goes into closed loop operation.

M52 & M44 System Operation:




The pump draws air through its own air filter and delivers it to both exhaust manifolds through a non-return (shutoff valve). The non-return valve is used to:

1.Control air injection into the exhaust manifold - A vacuum controlled valve will open the passageway for air to be injected once a vacuum is applied.

2.Prevent possible backfires from traveling up the pipes and damaging the air pump when no vacuum is applied.

The control module activates the vacuum vent valve whenever the air pump is energized. Once the vacuum vent valve is energized a vacuum is applied to the non-return valve which allows air to be injected into the exhaust manifold. A vacuum is retained in the lines, by the use of a check valve, in order to allow the non-return valve to be immediately activated on cold engine start up. When the vacuum/vent valve is not energized, the vacuum to the non-return valve is removed and is vented to atmosphere.




Duration of time after engine starts (In Seconds).

M73 System Operation

The secondary air injection system is controlled by both Engine Control Modules to provide two pump speeds. After engine start-up DME II simultaneously switches the stage I relay control circuit to ground and activates the secondary air vacuum vent valve allowing engine vacuum to open the non-return valves.

The air pump is energized through the closed contacts of the stage I relay. The circuit has a 0.8 OHMS in-line resistor which causes the pump motor to run at a slow speed on initial start. The supplied air pressure is directed through the non-return valves to the air injection inlet pipe directly into the exhaust manifolds.




After approximately 5 seconds, DME I switches the Stage II relay control circuit to ground and closes the contacts of the relay. The Stage II relay supply circuit bypasses the resistor causing the pump to run at fast speed for up to 85 seconds. The faster speed of the motor injects additional air volume into the exhaust manifolds.

Main System View:

Possible Causes:

-Faulty Secondary Air Pump Relay

-Faulty Secondary Air Pump

-Faulty Check Valve

-Inadequate Vacuum Supply

-Port in exhaust manifold blocked (Casting flaw)