98 GTP losing fuel pressure

[fro10987]

Hello everyone, I hope someone here can shed some light my way. I have a 98 GTP 140,000 miles no mods just the little wives car. I am battling a fuel problem that just wont go away. It all started she drove to work running great came out at end of day would start up and run for 3-5 seconds then die. I checked the fuel pressure at the fuel rail and watched as the fuel pressure dropped rapidly. So I replaced the fuel pump, and fuel filter even drained the tank and towel dried (just in case of contaniments) no help at all. I then replaced the Crank Position Sensor and wooo whooo it will start and run smoothy except for the fact that the fuel pressure gradualy drops from the startup pressure of 40-44 psi 10 psi (before stalling) in about 30 seconds. The odd thing is if I pump the throttle a little the fuel pressure will rise and the car will stay running. Just out of curosity I put the old fuel pump back in the tank and it does the same thing. I do not get any codes or lights that I could hope to follow up on. I did make a boo boo when I originally removed the fuel tank, there is a EVAP hose that connects to the middle line of the sending unit that then connects to a "t" (one of those great GM plastic vaccum type "'t's") which connects to the other sending unit type thing in the tank and the charcol emmissons canister. I broke this T and since GM wants $125.00 for the line set (cant relace just the "t") I wanted to make sure she ran before I dumped money out for a overpriced piece o plastic. Since I am not getting any codes and this broken piece is emissions related I am doubtful this is causing my problem.

ANY idaes or help would be appreciated.

[Carwhiz]

check the pressure regulator

[BNaylor]

Fuel pressure regulator is a possibility. Remove the vacuum line to it and see if there is any fuel in it indicating a ruptured diaphragm.

However, also you could have a bad fuel pump resistor which is a common problem with GTPs. Tecnhically, the L67 GTP has a two speed fuel pump. The fuel pump resistor is part of the hi-speed fuel pump relay. There are two relays in the engine compartment fuse box. Fuel pump relay and fuel pump hi-speed relay. So I would check that out.

See link below:

http://rmcgp.clubgp.com/fpresistor.html

In most cases, the symptoms were that the car will start as normal, idle for 2-3 seconds, and promptly shut off.

[fro10987]

Thanks guys,
I will check the regulator and try bypassing the relay.
Keith

[fro10987]

Okay I tried jumping the fuel pump relay, no change. I pulled the vaccum line from the regulator "actually a T vaccum connector that also leads to something at the other end (I believe it has to do with the air intake) of the fuel rail on the same side of the engine (regulator on passenger side front of motor other thing on drivers side fornt of motor) and I did not see or even smell any gas of course the hole is only 1/8" so kinda hard to see into. The car has not bee up and running on the street in about 2 weeks but I have been running in the garage and assume that would be enough to see or smell gas there if it were present....right? Since the regulator is $50-60 at autozone and needs to be ordered I did not just go ahead and replace it yet. If you have any other ideas I would be very happy to check them as I said before I am just running out of ideas. I will probably stop on monday and order the regulator or try to find one in stock somewhere.

[BNaylor]

Originally Posted by fro10987

Okay I tried jumping the fuel pump relay, no change. I pulled the vaccum line from the regulator "actually a T vaccum connector that also leads to something at the other end (I believe it has to do with the air intake) of the fuel rail on the same side of the engine (regulator on passenger side front of motor other thing on drivers side fornt of motor) and I did not see or even smell any gas of course the hole is only 1/8" so kinda hard to see into. The car has not bee up and running on the street in about 2 weeks but I have been running in the garage and assume that would be enough to see or smell gas there if it were present....right? Since the regulator is $50-60 at autozone and needs to be ordered I did not just go ahead and replace it yet. If you have any other ideas I would be very happy to check them as I said before I am just running out of ideas. I will probably stop on monday and order the regulator or try to find one in stock somewhere.

Yeah, the fuel pressure regulator can be expensive. The adjustable FPR like the Casper I have costs alot more.

If the diaphragm is ruptured you will most likely have fuel in the vacuum line from the FPR to the throttle body. Are both the fuel pressure and return lines to the fuel injectors rails OK?

Out of curiosity what is the fuel pressure reading at ignition to on but engine not running. This is the fuel pump prime mode which lasts about two seconds.

Another way to test the FPR is start the engine (hopefully you can keep the engine running long enough). Record the reading. Then disconnect the vacuum line at the FPR while engine is still running. Put your finger over the hole on the disconnected vacuum line when you do this. Observe reading. If the difference between the two readings is greater than 7 psi then the FPR is bad.

[Carwhiz]

fuel pressure regulator is a common part of these cars. price is $105 from GM pt number 24506988

[fro10987]

The prime pressure is 50 psi then drops to 44psi after 2 seconds or so if I leave the guage on and the key on the pressure holds at 44 psi if I turn the key off it stays there also. I tried disconnecting the vaccum line while it is running there really was not much of a difference that I could tell. the only way to keep the car running is to "pump" the throttle. I guess I will just have to order a FPR and give that a try unless there is something else that you guys may know of. I dont remeber if I mentioned that as soon asthe car stalls the fuel pressure is right back at the 44psi.

[7futures]

I have a 98 GTP with 140 K miles and am have the same fuel pressure drop problem. I have changed both relays, installed a new fuel pump, changed the fuel pump regulator, and still the same problem. Can anyone help??
God Bless, Gene

[BNaylor]

Originally Posted by fro10987

The prime pressure is 50 psi then drops to 44psi after 2 seconds or so if I leave the guage on and the key on the pressure holds at 44 psi if I turn the key off it stays there also. I tried disconnecting the vaccum line while it is running there really was not much of a difference that I could tell. the only way to keep the car running is to "pump" the throttle. I guess I will just have to order a FPR and give that a try unless there is something else that you guys may know of. I dont remeber if I mentioned that as soon asthe car stalls the fuel pressure is right back at the 44psi.

You can try the fuel pressure regulator (FPR) but I am not convinced it is bad based on the work you have done to date and your symptoms but you never know. As least you could rule it out.

At fuel prime prime the initial spike at 50 is good and the 44 psi with ignition left on is good. This is what I get on my GTP with engine running and the vacuum line removed - 53 psi. That is with my adjustable fuel pressure regulator set for stock settings. I show 19 - 20 in-hg of engine vacuum at the vacuum line that goes to the FPR. At any time with engine running and vacuum line off/plugged to the FPR do you get fuel pressure in a range of 48 - 55 psi?

The most prevalent symptom of a flaky FPR is hard starting and long cranking period before the engine fires up. The primary job of a FPR is to maintain fuel and pressure in the injector rails to help engine starting and not allow fuel to bleed off to the return going back to the fuel tank.

In your first post you mentioned a problem with evaporative emissions lines. Exactly what is messed up? Although you are not getting any DTC error codes or SES/CEL light an evaporative emissions problem cause cause the engine to stall, especially if there is a vacuum leak. Also, what condition is the fuel return line at the fuel pump pod? There are three lines at the fuel pump pod. Fuel pressure, fuel return and tank pressure going to the pressure switch used to test evaporative emissions.

Have you checked into the possibility of a bad MAF sensor? Try cleaning the hot wire elements with electrical contact cleaner. You can disconnect the electrical connector at the MAF to see what happens. The PCM will recognize the input missing and switch to pre-programed MAF/VAF tables based on engine rpm and TPS setting.

How about cleaning the throttle body with carb or throttle body cleaner and don't forget the IAC valve to include seat at the throttle body.

[BNaylor]

Originally Posted by 7futures

I have a 98 GTP with 140 K miles and am have the same fuel pressure drop problem. I have changed both relays, installed a new fuel pump, changed the fuel pump regulator, and still the same problem. Can anyone help??
God Bless, Gene

I see you decided to join the bandwagon.

What are your fuel pressure readings and what driveability symptoms?

It must be frustrating changing parts like the fuel pump, relays and fuel pressure regulator only to find out you have the same problem. Not to mention a waste of money.

[7futures]

At initial prime, the pressure is at 55. Then after 2-3 seconds, the pressure drops to 10 and the engine stalls. by pumping the gas it does run longer but it runs rough starving for fuel. I spoke to a tech that works at a local dealer, and he said he would have to trace the problem on a schematic.

[BNaylor]

Originally Posted by 7futures

At initial prime, the pressure is at 55. Then after 2-3 seconds, the pressure drops to 10 and the engine stalls. by pumping the gas it does run longer but it runs rough starving for fuel. I spoke to a tech that works at a local dealer, and he said he would have to trace the problem on a schematic.

In your case it looks like the problem will need to be troubleshot because it may be a wiring problem. I see the relays were changed but how about the fuel pump resistor. The second relay is the fuel pump hi-speed relay and the fuel pump resistor works in conjunction with that relay. You can perform a visual inspection by removing the passenger headlight and check the condition of it and the wiring. Also, you could try a bypass procedure.

The wiring diagram the tech is talking about is no mystery and consists of one simple page out of the Grand Prix service manual.

[fro10987]

In your first post you mentioned a problem with evaporative emissions lines. Exactly what is messed up? Although you are not getting any DTC error codes or SES/CEL light an evaporative emissions problem cause cause the engine to stall, especially if there is a vacuum leak. Also, what condition is the fuel return line at the fuel pump pod? There are three lines at the fuel pump pod. Fuel pressure, fuel return and tank pressure going to the pressure switch used to test evaporative emissions.

DOH!!!!
The line that is broken is the middle line that would be the tank pressure that goes to the evap system. the line leaves the fuel pod as a small 1/4" line then "t's" into a larger 1/2" line that goes to the other pod in the tank and to the evap canister. I had asked the GM parts guy about it and he told me it would just throw a code, but if this keeps pressure in the tank which in turn allows the fuel pump to not run at full tilt 100% of the time that would explain why when I "pump" the throttle it will stay running as the PCM is probably telling the pump to get with it and send some fuel. I suppose I will try fixing what I broke then look for others if not the problem. bad thing is they only sell that "t" thats broke as a line set of all 3 lines @ $110

If you would like I can email you the electrical fuel schematic I got from alldata. For what it is worth I think 1 of my main problems from the get go was the Crank position sensor after replacing that I at least get fuel pressure other than the prime and whatever the car could vaccum out of the line before it died.


Good thing working on cars is fun.......right!!!! I sure do miss my old Nova and chevelle.

[richtazz]

This problem sounds like a bad Fuel pump resistor to me, one of the parts bnaylor mentioned that I don't see either of you (fro10987 or 7futures) mentioning testing/bypassing yet.