GM Prizm/Nova 1985-1993 Repair Guide

Auxiliary Acceleration Pump (AAP)



See Figures 1 and 2

When accelerating with a cold engine, the main acceleration pump's capacity is insufficient to provide enough fuel for good acceleration. The auxiliary acceleration pump system compensates for this by forcing more fuel into the acceleration nozzle to obtain better cold engine performance.

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Fig. Fig. 1: Auxiliary acceleration pump system components

When engine coolant temperature is below 122°F (50°C), the thermostatic vacuum switching valve connects port K to port N which connects manifold vacuum to the AAP diaphragm. When engine rpm is relatively steady, the diaphragm moves against its spring causing its fuel chamber to fill. Whenever the engine is accelerated, the vacuum signal to the AAP diaphragm diminishes quickly. The diaphragm is pushed by its spring, forcing its fuel into the main acceleration circuit and out its nozzle.

After coolant temperature exceeds 154°F (68°C), the TVSV blocks Port K and Port N stopping the operation of the auxiliary acceleration pump system. The additional fuel requirement for warm engine acceleration is adequately handled by the main acceleration pump (carburetor) circuit.


Auxiliary Acceleration Pump System
  1. Check that the coolant temperature is below 122°F (50°C). Remove the cover from the air cleaner and start the engine.
  3. Pinch the AAP hose, and shut off the engine.

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Fig. Fig. 2: Auxiliary acceleration pump system chart

  1. Release the hose. Check that gasoline spurts out from the accelerator nozzle in the carburetor. Don't perform this test too often - you may flood the motor.
  3. Restart the engine and warm it to normal operating temperature. Repeat steps 2 and 3 above. Check that gasoline DOES NOT spurt out from the accelerator nozzle.
  5. Reinstall the air cleaner cover.

Auxiliary Acceleration Pump Diaphragm
  1. Start the engine.
  3. Disconnect the hose from the AAP diaphragm.
  5. Apply vacuum directly to the AAP diaphragm (at idle) with a hand held vacuum pump.
  7. Check that the engine rpm changes as the vacuum is released from the system.
  9. Reconnect the AAP hose.