Stumbles, stalls and wont start....

[greekguy7]

I am working on a 1988 Pontiac Grand Prix 2.8 litre that hesitates and stumbles real bad upon acceleration between 20-30mph, stalls out when hot and is hard to restart. Fuel pressure looks good and the car was recently tuned. It looks like it may be a faulty MAF sensor since it comes up with a code 34, also and it feel like its running a bit lean when you blip the throttle.

 

When it wont start though, if I disconnect the harness connector to the MAF sensor the car still wont start. If the sensor was in fact bad, wouldn't unplugging the MAF sensor cause it to start because the ECM will use default values instead of the "bad" ones coming from the defective MAF sensor?? Any assitance would be greatly appreciated.

[greekguy7]

While testing the fuel pump pressure when the car wont start, pressure is at 42psi while cranking. While the car is running, pressure is at 38psi.

 

When I turn the key to ON, but not cranking though, pressure is at 38psi for a few seconds while the fuel pump energizes, but then pressure drops down to zero after that. Is it supposed to drop down to zero while the key is set to ON?? If not, any ideas?

[Slade901]

It could be your fuel pressure regulator is leaking slowly. Check the vacuum hose coming out of the fuel pressure regulator to see if there is any sign of fuel in it. Try cranking the vehicle and if starts check the vacuum hose, turn the ignition ON and see if there is any fuel on the vacuum hose coming from the fuel pressure regulator.

[Canada]

When fuel pressure drops that quickly after it shuts off, its the pump. Be sure to always replace the fuel filter with a new fuel pump.....some pump warranties are void if you don't.