Secondary Air Injection (page 2 of 2)

ENGINE MANAGEMENT SYSTEM – V8

DESCRIPTION AND OPERATION 18-2-57

There are several reasons for the increase of HC emissions at low cold start temperatures, including the tendency for 
fuel to be deposited on the cylinder walls, which is then displaced during the piston cycle and expunged during the 
exhaust stroke. As the engine warms up through operation, the cylinder walls no longer retain a film of fuel and most 
of the hydrocarbons will be burnt off during the combustion process.

The SAI pump is used to provide a supply of air into the exhaust ports in the cylinder head, onto the back of the 
exhaust valves, during the cold start period. The hot unburnt fuel particles leaving the combustion chamber mix with 
the air injected into the exhaust ports and immediately combust. This subsequent combustion of the unburnt and 
partially burnt CO and HC particles help to reduce the emission of these pollutants from the exhaust system. The 
additional heat generated in the exhaust manifold also provides rapid heating of the exhaust system catalytic 
converters. The additional oxygen which is delivered to the catalytic converters also generate an exothermic reaction 
which causes the catalytic converters to 'light off' quickly.

The catalytic converters only start to provide effective treatment of emission pollutants when they reach an operating 
temperature of approximately 250

°

C (482

°

F) and need to be between temperatures of 400

°

C (752

°

F) and 800

°

C 

(1472

°

F) for optimum efficiency. Consequently, the heat produced by the secondary air injection “after burning”, 

reduces the time delay before the catalysts reach an efficient operating temperature.

The engine control module (ECM) checks the engine coolant temperature when the engine is started, and if it is above 
-9º C but below 75

°

C, the SAI pump is started. Secondary air injection will remain operational for a period controlled 

by the ECM. The SAI pump operation can be cut short due to excessive engine speed or load.

Air from the SAI pump is supplied to the cylinder head, via a metal pipe which splits the air flow evenly to each bank.

At the same time the secondary air pump is started, the ECM operates a SAI vacuum solenoid valve, which opens to 
allow vacuum from the reservoir to be applied to the non return valve on the metal delivery tube on the engine. 
Secondary air is injected into the inner most exhaust ports on each bank.

When the ECM breaks the ground circuit to de-energise the SAI vacuum solenoid valve, the vacuum supply to the 
SAI non return valve is cut off and the valve is closed to prevent further air being injected into the exhaust manifold. 
At the same time as the SAI vacuum solenoid valve is closed, the ECM opens the ground circuit to the SAI pump 
relay, to stop the SAI pump.

A vacuum reservoir is included in the vacuum line between the intake manifold and the SAI vacuum solenoid valve. 
This prevents changes in vacuum pressure from the intake manifold being passed on to cause fluctuations of the 
secondary air injection solenoid valve. The vacuum reservoir contains a one way valve and ensures a constant 
vacuum is available for the SAI vacuum solenoid valve operation. This is particularly important when the vehicle is at 
high altitude.