6-10
Vehicle Testing Procedures
Accelerator Pump Test
When the throttle is opened rapidly, engine speed should increase
without hesitation. If there is a hesitation, the problem could be a
malfunctioning accelerator pump.
Test
1. Note the CO reading at idle.
2. Note the change in the CO readings from the idle readings
for each throttle change:
— 1/4 throttle snap,
— 1/2 throttle snap, and
— Full throttle snap.
3. Allow the engine to return to idle speed between throttle
changes and idle at least 10 seconds to allow the CO
reading to stabilize.
Interpretation
•
Unless otherwise specified by the vehicle manufacturer, each
throttle snap should result in an increased CO reading.
— The resulting increase at full throttle should be about twice
the 1/2 throttle increase, and the total increases in CO
reading for all three throttle changes will usually be
between 1.5% vol and 3.5% vol.
•
If the CO reading decreases before increasing, or does not
increase, the accelerator pump is not functioning properly.
— Check the linkage adjustment before attempting to repair
the pump mechanism.
Power Valve Test
The power valve supplements the fuel supply at higher speeds,
since richer mixtures are needed for maximum engine power and
performance. The power valve can either be a diaphragm or a
vacuum operated piston connected to metering rods. Regardless
of the type, verification of power valve operation can be performed
by creating a momentary “near zero” intake manifold vacuum.
Under this condition, the power valve lifts from its seat and
supplements the fuel flow. It is important to verify this operation. It
is also important to verify that the power valve is re-seated.
Test
1. Disconnect the accelerator pump linkage.
2. Stabilize the engine at approximately 2000 RPM using the
fast idle cam.
3. Note the CO reading.
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