Secondary Air Injection System

Vacuum Reservoir And Check Valve:

The M73 is equipped with a secondary air injection system. This system is required to meet 1995 federal emission requirements.

The air injection system helps the catalytic converter to function more efficiently at its most inefficient period of operation. Catalytic converters need heat and oxygen to operate effectively. When first starting and idling a cold engine, heat and oxygen are at their lowest levels in the exhaust stream.

The Air injection system injects fresh air into the exhaust manifolds. The air mixes with the exhaust gas as it leaves the engine. The additional air helps the catalytic converter to function earlier and further reduce the exhaust emissions during the critical higher emission warm up period.

The mechanical components of this system on the M73 engine are:

Air Pump:



The air pump is located in the front right inner fender well. It is a 12 volt DC, two speed, electric motor pump that is controlled by the DME control modules through two relays. Operation is based on the programmed requirements.

The pump draws filtered air from the air filter housing of cylinder bank 7-12 and supplies it to two non-return valves.

Non-Return (One Way) Valves:



Located on the front of the engine are the two Non-Return Valves. The Non-Return valves serve the following functions.

- When activated by a controlled vacuum source the valves open and allow the supplied air from the electric motor pump to inject air into the exhaust manifolds.
- In the event of an engine backfire they will check the back pressure from damaging the pump, hoses or secondary air solenoid vacuum valve.

Stainless Steel Injection Pipes:
Outlet of each Non-Return Valve is a stainless steel injection pipe. Each pipe forks into two outlets for connection to each of the four exhaust manifolds (two per bank). The pipes are connected to the exhaust system to inject the air directly into the exhaust gas stream.

Secondary Air Vacuum/Vent Valve:
The Vacuum/Vent Valve is located in the main harness conduit tray on top of the engine. It is energized by DME II simultaneously with the Air Injection Pump.

When the air injection pump is switched off the Vacuum/Vent valve disconnects the vacuum supply from the Non-Return Valves leaving them in a closed position.

Vacuum Reservoir and Check Valve:
Also located in the main harness conduit tray are the vacuum reservoir and check valve. The vacuum reservoir stores a vacuum supply for the Non-Return Valves from the throttle housing.

The Check Valve ensures the vacuum reservoir has an ample vacuum supply for operation.