1999 Pontiac Grand Prix High Fuel Pressure

Tiny
BANDITGTP
  • MEMBER
  • 1999 PONTIAC GRAND PRIX
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 135,000 MILES
I've been working on an issue with my car for some time already. I have dropped from getting 28mpg to 22 mpg. I am also getting codes for misfire and running rich. After a major tune up as well as a few other repairs (with no positive results), I have just found out my fuel pressure is 43psi when the key is in the 'ON' position but then jumps up to 83psi after I start the car. When I take the vacuum line off from the Fuel Pressure Regulator while the car is running, there is no change in the fuel pressure or the performance of the car.
Does it seem like the problem is the Fuel Pressure Regulator or is there a problem elsewhere?
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Tuesday, February 24th, 2009 AT 8:15 PM

26 Replies

Tiny
RASMATAZ
  • MEMBER
Check the fuel return line if its clogged-up if not replace the regulator then report back asap.
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Tuesday, February 24th, 2009 AT 8:25 PM
Tiny
BANDITGTP
  • MEMBER
How would I check to see if the return line is clogged?
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Wednesday, February 25th, 2009 AT 6:04 AM
Tiny
RASMATAZ
  • MEMBER
Disconnect the fuel return line on the fuel rail and apply air pressure to it, if not clogged-up replace the fuel pressure regulator
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Wednesday, February 25th, 2009 AT 1:54 PM
Tiny
BANDITGTP
  • MEMBER
Do you know if there is a trick to disconnect the return line? I was trying to disconnect the line where the steel line end and changes back into rubber/plastic line. The rubber/plastic line slips over the steel line and it has 4 clips on the inside of the rubber/plastic line that clip onto the steel line. I can't seem to get all 4 clips pushed back at the same time in order to disconnect the line.
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Wednesday, February 25th, 2009 AT 7:28 PM
Tiny
RASMATAZ
  • MEMBER
Do you have vacuum on the vacuum hose to the fuel pressure regulator?
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Wednesday, February 25th, 2009 AT 8:32 PM
Tiny
BANDITGTP
  • MEMBER
Yes there is vacuum. When the car is running and I take the vacuum line off from the regulator, there is vacuum on the vacuum line. When I remove the line there is also no change in how the car runs. I still get the same amount of pressure on the fuel rail.
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Thursday, February 26th, 2009 AT 6:09 AM
Tiny
RASMATAZ
  • MEMBER
You probably need a special disconnect tool.

Note: When you remove the vac hose on the regulator the pressure should rise to about 8psi-sounds like the regulator
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Thursday, February 26th, 2009 AT 2:03 PM
Tiny
BANDITGTP
  • MEMBER
I changed out my FPR earlier today and to my surprise, and disappointment, it still didn't solve my problem. I really had high hopes that this would do the trick.

With the key turned to ON, I get closer to 48psi now compared to 43psi before I changed it. After I start the car I am getting 84psi compared to the 78psi to 84psi before I changed it. I also pulled off the vacuum line with the engine running and there was no change. I was able to position the pressure gauge so I can see it from inside the car. While the car was in gear but holding down the brake, I am able to rev the engine enough so that it bogs down. The fuel pressure is still above 80psi while the engine is bogging down.

I'm not exactly sure of the mechanics behind the supercharger but it also seems that the bogging happens whenever the supercharger is giving boost according to the dashboard gauge. Could part of the problem be that the engine is getting extra fuel because of the boost but not enough air to match and is therefore dumping in way too much fuel with out enough air to back it up? Like I said, I'm not sure of the mechanics behind the S/C but I'm grasping for straws. I still don't think this would explain the reason for such high fuel pressure.

Any suggestions would be greatly appreciated.
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Saturday, February 28th, 2009 AT 7:42 PM
Tiny
MASTERTECHTIM
  • MEMBER
Does this engine have a supercharger?
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Saturday, February 28th, 2009 AT 7:44 PM
Tiny
BANDITGTP
  • MEMBER
Yes it has a S/C. It's a 99 GTP.
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Saturday, February 28th, 2009 AT 8:22 PM
Tiny
MASTERTECHTIM
  • MEMBER
This car runs on very high fuel pressure when hard accelerating. There is a resister located behind the passengers side headlight that reduces voltage to the pump when running at idle to reduce fuel pressure then when accelerator is pressed fast the fuel pump relay bypasses the resister and gives full power to the fuel pump which pushes 90 psi. This is normal. However it is not normal for 90 psi when at idle and running. Does the fuel pressure stay at 90 at all times? Even at idle when hot?
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Saturday, February 28th, 2009 AT 8:26 PM
Tiny
BANDITGTP
  • MEMBER
When I checked it today, I checked it when it was cold after first starting and after the car warmed up and the idle slowed down to 800rpm. Both times the pressure was still around 84psi. Even when I put the car ingear and the rpm dropped another 100rpm, the pressure was still at 80psi.
Do you know exactly where the relay is and what it looks like?
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Saturday, February 28th, 2009 AT 8:58 PM
Tiny
MASTERTECHTIM
  • MEMBER
It would appear that the fuel pump speed relay may be stuck on.
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Sunday, March 1st, 2009 AT 6:41 AM
Tiny
MASTERTECHTIM
  • MEMBER


http://www.2carpros.com/forum/automotive_pictures/139033_coil_10.jpg

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Sunday, March 1st, 2009 AT 6:42 AM
Tiny
BANDITGTP
  • MEMBER
I located the relay and I have attempted a few things to see if I would get any changes in the pressure.
1. I swapped the fuel pump relay with the fuel pump speed relay since they are the same part and there was no change in pressure.
2. I opened up the relay so I could bench test the relay to make sure it would switch between the 2 possible positions. It worked as it should.
3. I reinstalled it in the car while the cover was still off. I manually switched the relay while the car was running and the pressure went from 84psi while it was idling to 93psi-95psi when I forced the relay to the other position. I'm assuming this would shpw that it is not stuck open as I normally have not gotten a reading of 90+psi.

I also put my car on ramps so I could get under it to inspect the entire path of the fuel line. Everything appears normal. There are no kinks or other types of damage to the line that may be causing an obstruction.
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Sunday, March 1st, 2009 AT 10:35 AM
Tiny
MASTERTECHTIM
  • MEMBER
Can you check the voltage to the fuel pump on the gray wire while idling to see if there is the low voltage or the regular current of 14v. If there is 14v while idling with car running we will check for a short to power in the circuit, if the voltage is 9 volts then we need to find a restriction in possibly the fuel rail.
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Sunday, March 1st, 2009 AT 11:10 AM
Tiny
BANDITGTP
  • MEMBER
Where do you mean to check the voltage? Should I check it directly by the fuel pump? It it accessible through the trunk?
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Sunday, March 1st, 2009 AT 11:26 AM
Tiny
MASTERTECHTIM
  • MEMBER
Right at the pump. Its accessable under the car. The harness goes on top of tank and I think it enters from the rear of the tank. Grab the gray wire
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Sunday, March 1st, 2009 AT 4:59 PM
Tiny
BANDITGTP
  • MEMBER
I got to the wiring harness right before the pump. I got 11v on the gray wire while the car was running and at slow idle. The engine was running at 800rpm. I was testing this by myself so it was a little hard but as soon as I turned the key to ON, I think I was getting 12.5v on the gray wire.
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Sunday, March 1st, 2009 AT 7:12 PM
Tiny
BANDITGTP
  • MEMBER
I got the quick disconnect on the fuel return line seperated. This is the one directly on top of the motor. I tried to blow some compressed air (40psi) into the line to see if there is a clog. It doesn't appear to be pushing the fuel in the return line back to the tank. Does anyone know if there is a trick to it? Does the key have to be turned to ON? If this does point towards a clog, where could it be and how can I get rid of it?
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Monday, March 2nd, 2009 AT 1:10 PM

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