Vehicle Testing Procedures
6-7
Air Pump Test
An engine-driven air pump is sometimes used to lower CO and HC
by pumping fresh air into the exhaust system at pressures up to
approximately 5 psi (0.35 bar). This supplies additional O
2
to the
hot gases to allow continued combustion, resulting in lower levels
of HC and CO at the tail pipe. However, this combustion occurs
outside the combustion chamber.
A – Closed Loop Fuel Control
B – Electronic Control Module (ECM)
C – Reducing Catalyst
D – Oxidizing Catalyst
E –
O
2
Sensor
F – Check Valves
G – Air Pump
H – Port Solenoid
I – Converter Solenoid
J – Electrical Signals from ECM
K – Bypass Air to Atmosphere
These systems usually consist of a belt-driven air pump, air
bypass or diverter valve, check valves, connecting plumbing, and
sometimes an air pump intake filter.
Figure 6-1
shows a typical air
pump system, with other components, making up a feedback
control system.
When the air pump system is working properly, HC, CO and CO
2
readings are lowered and O
2
readings are raised. For this reason,
additional air from the air pump must be taken into consideration
when monitoring exhaust gas concentrations.
Figure 6-1
: Typical Air Pump System
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