See Figure 1
An auxiliary enrichment system, which Toyota calls an Auxiliary Accelerator Pump (AAP) System, is used on all models, starting in 1975.
When the engine is cold, an auxiliary enrichment circuit in the carburetor is operated to squirt extra fuel into the acceleration circuit in order to prevent the mixture from becoming too lean.
A thermostatic vacuum valve (warm-up valve), which is threaded into the intake manifold, controls the operation of the enrichment circuit. Below a specified temperature, the valve is opened and manifold vacuum is allowed to act on a diaphragm in the carburetor. The vacuum pulls the diaphragm down, allowing fuel to flow into a special chamber above it.
Under sudden acceleration manifold vacuum drops momentarily, allowing the diaphragm to be pushed up by spring tension. This in turn forces the fuel from the chamber through a passage and out the accelerator pump jet.
When the coolant temperature goes above specification, the thermostatic vacuum valve closes, preventing the vacuum from reaching the diaphragm which makes the enrichment system inoperative.
See Figure 2
- Check for clogged, pinched, disconnected, or misrouted vacuum lines.
- With the engine cold (below 75°F), remove the top of the air cleaner, and allow the engine to idle.
- Disconnect the vacuum line from the carburetor AAP unit. Gasoline should squirt out the accelerator pump jet.
- If gas doesn't squirt out of the jet, check for vacuum at the AAP vacuum line with the engine idling. If there is no vacuum and the hose are in good shape, the thermostatic vacuum valve is defective and must be replaced.
- If the gas doesn't squirt out and vacuum is present at the vacuum line in Step 4, the AAP unit is defective and must be replaced.
- Repeat Step 3 with the engine at normal operating temperature. If gasoline squirts out of the pump jet, the thermostatic vacuum valve is defective and must be replaced.
- Reconnect all of the vacuum lines and install the top of the air cleaner.
See Figure 3
- The engine must be completely cold for this test.
- Remove the air cleaner cover and start the engine.
- Pinch the AAP hose and then have a helper turn off the engine while you continue pinching the hose.
- With the engine off, release the hose. Gasoline should squirt out of the accelerator nozzle inside the carburetor.
- Run the engine until it reaches normal operating temperature. Repeat Steps 3-4 and check that gasoline does not squirt out of the nozzle.
Inspect the AAP diaphragm.
See Figures 4 and 5
- Start the engine and disconnect the hose from the diaphragm.
- Apply and release the vacuum directly to the diaphragm at idle.
- Check that the engine rpm changes when you release vacuum.
- Reconnect the AAP hose. If a problem is found, replace the diaphragm.