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Npt working

Tiny
CRAIG92481
  • MEMBER
  • 1998 PONTIAC GRAND PRIX
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 96,000 MILES
Pressure tests at 55-60, during prime. Start engine and pressure drops to 15psi engine dies, then pump reprimes back to 55-60. What would stop the pump from pumping while engine is running, does it sound like a oil pressure switch
Thursday, July 16th, 2009 AT 12:05 AM

7 Replies

Tiny
MMPRINCE4000
  • MECHANIC
  • 8,548 POSTS
Check fuel pump relay and also check the fuel pressure regulator. Disconnect FPR vacuum line, if gas is present, the FPR diaphram is ruptured.

Do you have a security light on?
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Saturday, March 20th, 2021 AT 7:32 PM
Tiny
CRAIG92481
  • MEMBER
  • 7 POSTS
I swapped the 2 relays and there was not any change. There is not a security light on. I jumped the fuel pump and the car runs fine, but there is still no power to the plug for the pump. I only have the power for the 3 second primes.
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Saturday, March 20th, 2021 AT 7:32 PM
Tiny
MMPRINCE4000
  • MECHANIC
  • 8,548 POSTS
If you jumped the pump to run off battery voltage and it runs fine, then the pump is OK.

The problem lies in the pump wiring harness, or oil pressure sending unit which serves as a backup to the FP relay.

The oil pump sending unit is designed as a backup to FP relay, but not designed to run pump all the time.

I would replace the OP sender, if problem persists, then you will have to get into the harness and check for open/shorted wires.
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Saturday, March 20th, 2021 AT 7:32 PM
Tiny
CRAIG92481
  • MEMBER
  • 7 POSTS
After checking the pump and testing several sensors found a ressistor in the right side in the wheel well above the washer fluid bottle was bad. GM has a updated part and location for this ressistor. The way I think this system works is full power, batt voltage to the pump on the prime and when the boost reaches a set level in the computer. All other time the sytem run on a lower voltage when the deman for fuel is lowwer.
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Saturday, March 20th, 2021 AT 7:32 PM
Tiny
MMPRINCE4000
  • MECHANIC
  • 8,548 POSTS
Generally on supercharged engines, as vacuum goes to boost, the fuel pressure increases as well as injector pulse width.

Air/fuel ratio goes to 12.5:1 or alittle lower to prevent a lean condition.

A lean condition on a super/turbo/nitrous is an immediate burnt piston as combustion temps skyrocket.
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Saturday, March 20th, 2021 AT 7:32 PM
Tiny
CRAIG92481
  • MEMBER
  • 7 POSTS
Then what would be the reason to ad a resistor to the fuel pump feed during normal operation.
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Saturday, March 20th, 2021 AT 7:32 PM
Tiny
MMPRINCE4000
  • MECHANIC
  • 8,548 POSTS
Less resistance would increase voltage, more would decrease.

Like LED bulbs, a resistor is added inline on a non LED light circuit to reduce voltage in order not to blow a fuse, since LED's require much less voltage than incandesent.
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Saturday, March 20th, 2021 AT 7:32 PM

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