2000 Grand Prix fuel guage problem


The fuel guage on this car reads full when you start the car, then it cycles back and forth slowly between full and empty no matter how much fuel is in the tank. I brought it to one mechanic and he told me it was doing some kind of self-test, but that’s all he knew. Anyone have any experience with this problem?


The PCM (engine computer) controls two relays which control the power to the fuel level sender and the fuel pump. The PCM receives the output signal from the fuel level sender. The PCM sends the fuel level signal to the dash fuel level gage. The mechanic could check the power at the fuel level sender input (AND the fuel pump input). If the power is not a stable 12 volts, it’s a problem for, both the fuel level sender AND the fuel pump. He would then check the signal (voltage) arriving at the PCM from the fuel level sender. It should be a voltage lower that the input reading AND stable. Then, he would check the signal voltage from the PCM to the dash fuel gage. It should be stable. Wherever the voltage is not stable, will be where the fault lies. Now MY question, "WHY in the HECH did the designers route this circuit through the PCM?!