fuel pump question
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The operation of the fuel pump is not exactly straightforward, so you have to look at the whole picture to find out if everything is working correctly.
The ECM turns on the fuel pump (through the fuel pump relay) for 2 seconds when you turn on the ignition. The purpose of this is to pressurize the fuel rails in preparation for starting. The ECM shuts off the fuel pump after 2 seconds so it won't run continuously if you turn on the ignition but don't start the engine. If you hear the fuel pump then it means that the ECM and relay are applying a voltage to the pump.
When the ECM sees reference pulses from the distributor (while the engine is cranking or running) it turns on the fuel pump (through the relay). The ECM will keep the pump turned on as long as it sees these reference pulses.
If the relay has failed or the ECM does not see reference pulses, the oil pressure switch (which is in parallel with the fuel pump relay) will turn on the fuel pump when is sees at least 4 PSI of oil pressure. This pressure is very low and you can get that much just by cranking the engine. This does not mean that your engine will start, because the ECM needs to see the distributor reference pulses and will not turn on the injectors if the pulses are not there. The ECM uses the distributor reference pulses to synchronize the firing of the injectors with the rotation of the engine.
The only way to know that the fuel pump is working correctly is to connect a fuel pressure gauge to the fuel rail and monitor the fuel pressure. The pressure needs to be checked at key on, after the engine has started and while driving (especially at WOT). It should be checked under all three conditions to verify that it really is OK.
If the fuel filter is plugged or the pump is not working correctly, it's possible to get low fuel pressure at the fuel rails. In my case the sock on the fuel pump was ground to shreds by rubbing on the bottom of the fuel tank. The pump sucked up bits of the sock, which messed up the pump. The fuel pressure was very low and the car would start but would not go faster than about 5 MPH. The engine would die if I pressed on the gas pedal more than a tiny bit. It happened very suddenly. I went around a corner and stepped on the gas -- and there wasn't any.
The ECM turns on the fuel pump (through the fuel pump relay) for 2 seconds when you turn on the ignition. The purpose of this is to pressurize the fuel rails in preparation for starting. The ECM shuts off the fuel pump after 2 seconds so it won't run continuously if you turn on the ignition but don't start the engine. If you hear the fuel pump then it means that the ECM and relay are applying a voltage to the pump.
When the ECM sees reference pulses from the distributor (while the engine is cranking or running) it turns on the fuel pump (through the relay). The ECM will keep the pump turned on as long as it sees these reference pulses.
If the relay has failed or the ECM does not see reference pulses, the oil pressure switch (which is in parallel with the fuel pump relay) will turn on the fuel pump when is sees at least 4 PSI of oil pressure. This pressure is very low and you can get that much just by cranking the engine. This does not mean that your engine will start, because the ECM needs to see the distributor reference pulses and will not turn on the injectors if the pulses are not there. The ECM uses the distributor reference pulses to synchronize the firing of the injectors with the rotation of the engine.
The only way to know that the fuel pump is working correctly is to connect a fuel pressure gauge to the fuel rail and monitor the fuel pressure. The pressure needs to be checked at key on, after the engine has started and while driving (especially at WOT). It should be checked under all three conditions to verify that it really is OK.
If the fuel filter is plugged or the pump is not working correctly, it's possible to get low fuel pressure at the fuel rails. In my case the sock on the fuel pump was ground to shreds by rubbing on the bottom of the fuel tank. The pump sucked up bits of the sock, which messed up the pump. The fuel pressure was very low and the car would start but would not go faster than about 5 MPH. The engine would die if I pressed on the gas pedal more than a tiny bit. It happened very suddenly. I went around a corner and stepped on the gas -- and there wasn't any.
