I've got a fuel pressure problem that I'm trying to diagnose.

When I first turn the key to the 'on' position (without starting the engine), the pressure comes up to 45 psi (Ryan, if you're reading this...it's 45, not the 42 I was babbling about). As soon as the fuel pump stops priming, the pressure drops to 18 psi within two seconds. It will hold at 18 psi for several minutes.

I have tried clamping off the return line to see if the problem is the pressure regulator. The problem remained the same, as the pressure quickly rose to 45 psi while the pump was priming, but would then drop to 18 psi as soon as the pump stopped.

I pulled the fuel rail to see if the injectors were leaking. None were. Also no leaks where the fuel injectors connect to the rail--everything was dry.

I clamped off the supply line to see if that would change anything. Same problem--pump primes to 45 psi, then drops to 18 psi within two seconds of pump stopping.

I suspect a problem with the fuel pump--perhaps a check valve or a split line inside the tank. That the pump instantly primes to 45 psi makes me suspect a bad check valve. If anyone has any ideas here, I wouldn't mind hearing them before I drop the tank and replace the pump.

Hey Norfolk!

If you clamped off on both the supply and return lines of the fuel rail and it still dropped either you did not get it clamped off well or there is still something with the injectors. There is no other place for the fuel to go.
Did you also check the cold start for leaks?
Did you also check for fuel in the regulator vac line?
Sure your gauge is good? (that would be a strange problem)

Good gauge--my other V6 Fiero holds at 45 psi after the pump stops priming.

No cold start injector--I've got the 7730 ECM. The bolt I used to plug the hole in the fuel rail shows no leak.

I'll go try to clamp the lines harder to see if that makes a difference.

quote Originally posted by go_home_red: I clamped off the supply line to see if that would change anything. Same problem--pump primes to 45 psi, then drops to 18 psi within two seconds of pump stopping.

Given that the Schrader valve is on the fuel rail, how did you manage to measure fuel pressure with the supply line clamped off?

If your return line was properly clampled the fuel pressure SHOULD rise above the regulator pressure ASSUMING the pump can obtain more pressure than the regulator is set at. This SHOULD be the case. If the pump can't then there is never any fuel being returned via the return line and you will have vapor lock problems after a hot soak park.

1 - recheck the plugging of the return line. Perhaps your not getting the return fully pinched off

2 - take off the fuel line and rig it up directly to the gauge. If you are only getting 45 psi with this test your pump/tank plumbing is not up to specifications. It will cause operational problems.

It would be of interest what caused you to look at the fuel pressure. What symptom are you troubleshooting.

Yet to be addressed is why the fuel pressure is dropping off so quickly. If the pressure the pump can achive is just 45 PSI from test 2, I think it would be very safe to assume there is something wrong with the pump or plumbing inside the tank that is causeing BOTH the low pressure and fast bleed off.

Always have an appropriate fire extinguisher handy when working on fuel. You don't want to lose your Fiero, garage, house, family, life because of a fuel leak.

You have an interesting case, and I like your troubleshooting thus far.

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quote Originally posted by Marvin McInnis: Given that the Schrader valve is on the fuel rail, how did you manage to measure fuel pressure with the supply line clamped off?

I'm speculating that he hotwired the pump, clamped the supply line, and then removed the pump hotwire

I agree, if both lines are clamped tightly, then there should be no loss of pressure. It makes me think the clamping isn't tight enough

[This message has been edited by phonedawgz (edited 07-19-2010).]

I think its time to drop the tank, only takes 30 minutes. I would say that the pump is bad. If the line was split then I don't think the pressure would spike like that.

I'm cool with dropping the tank if its going to fix it. I'm not convinced that dropping the tank is going to fix it. This still could be a bad regulator if the clamping procedure wasn't enough to fully block the flow.

I must not be completely blocking off the lines. I'm using vice-grips to clamp the rubber sections near the compressor, and I've got the vice-grips set as tight as I can get them to go. When I clamp off the supply line, you can notice a difference in the tone of the fuel pump when it primes, but it still manages to shove 40+ psi into the rail. Not that it stays there. When I clamp off the return line, it makes absolutely no difference.

I'll try disconnecting the return line and plugging the hole it connects to on the fuel rail.

I agree the pressure drop off has to be a problem with the pump or tank plumbing.

I'm not sure I'd tear it apart. The problem your talking about I just don't see how it could be caused by the fuel pump allowing the fuel to leak back when off, since the fuel pump is on the entire duration, and you have the correct fuel pressure the entire duration.

The pump/plumbing isn't up to spec. That could cause problems in the future. I think it is a very remote chance of causing your existing problem.

Any chance you can swap parts back and forth on the two V6's to troubleshot? Both running 7730's? Swap the ECM's between the cars.

EGR valve pintle is clean and not blocked and moving the way it should? It's fully closing?

Swap the throttle bodies between the two, leaving the IAC and TPS connected to the bodies so they get swapped also?

Recheck the timing.

[This message has been edited by phonedawgz (edited 07-19-2010).]

quote Originally posted by phonedawgz: I agree the pressure drop off has to be a problem with the pump or tank plumbing. I'm not sure I'd tear it apart. The problem your talking about I just don't see how it could be caused by the fuel pump allowing the fuel to leak back when off, since the fuel pump is on the entire duration, and you have the correct fuel pressure the entire duration. The pump/plumbing isn't up to spec. That could cause problems in the future. I think it is a very remote chance of causing your existing problem. Any chance you can swap parts back and forth on the two V6's to troubleshot? Both running 7730's? Swap the ECM's between the cars. EGR valve pintle is clean and not blocked and moving the way it should? It's fully closing? Swap the throttle bodies between the two, leaving the IAC and TPS connected to the bodies so they get swapped also? Recheck the timing.

My other Fiero has the stock ECM setup, so ECM swapping is a no-go right now.

The EGR valve is closed--blocking it off doesn't change any change. If the EGR wasn't closing, I would think the problem would remain even after the ECM goes into closed loop.

Swapping throttle bodies yields the same results. No change.

Timing is good. Can't tell you how many times I've checked it, except to say "many, many, many times."

Also changed the fuel filter. Didn't make a difference.

IIRC the pump has a check valve to prevent fuel from flowing back into the tank. If the check valve is not working correctly your fuel could be going back in that way. Also the fuel line or pulsator between the outlet of the pump and the steel fuel supply line could have a small leak allowing nearly full pressure but then letting some pressure out until the pressure is low enough to stop pushing out through the leak. If you have someone help you you can block the return line, prime and quickly block the supply line before the pressure has a chance to bleed off. If it holds, the injectors are holding and something in the tank is leaking, if not, one or more of the injectors are leaking.

Charlie

quote Originally posted by cptsnoopy: IIRC the pump has a check valve to prevent fuel from flowing back into the tank. If the check valve is not working correctly your fuel could be going back in that way. Also the fuel line or pulsator between the outlet of the pump and the steel fuel supply line could have a small leak allowing nearly full pressure but then letting some pressure out until the pressure is low enough to stop pushing out through the leak. If you have someone help you you can block the return line, prime and quickly block the supply line before the pressure has a chance to bleed off. If it holds, the injectors are holding and something in the tank is leaking, if not, one or more of the injectors are leaking. Charlie

Someone hasn't read the thread

quote Originally posted by go_home_red: I pulled the fuel rail to see if the injectors were leaking. None were. Also no leaks where the fuel injectors connect to the rail--everything was dry.

Did the car run ok for a while after the 7730 swap? Any chance you have a bad program on your 7730? Any good 7730 Fiero buds you know you could swap ECMs to see if the trouble sticks with the ECM or engine?

[This message has been edited by phonedawgz (edited 07-19-2010).]

^^^ true! ^^^

I tried re-reading it to see what I missed and just found it to be a little confusing. The only conclusion I am coming up with is the same as what you seem to be saying. It is in the fuel tank. I had the issue with the pump not being able to supply enough pressure to get some return line fuel flow and indeed had issues with vapor lock. It also caused a hard to start condition. As much as I don't like sniffing fuel, dropping the tank and fixing the dirty tank and fouled pump issues did solve my problems.

Good luck!

Charlie

I don't have a good way to swap the ECM right now, but I may at some point in the future.

The startup issue has been a problem since I installed the 7730. It is not a case where it ran well with the 7730 and then suddenly changed.

In the meantime, I'm going to measure the fuel pressue while the engine is running once I get the intake and valve covers put back on the engine--I need to tighten the head bolts one last time while I've got everything out of the way. I understand that the fuel pressure should remain at 45 psi when the vacuum line to the regulator is disconnected while the engine is running, and that it should be 45 minus whatever the vacuum level of the intake manifold is when the vacuum line is connected to the regulator. I'll post those numbers here when I get them in the next couple of days.

[This message has been edited by go_home_red (edited 07-20-2010).]

So any chance your chip is misprogrammed? Have you asked the programmer? Perhaps the chip didn't burn like it was supposed to. Any chance he coud send you a second chip to test?

I'm going to say that the programming of the chip is not the cause my startup issue. When it was programmed a second time, the problem remained unchanged. The car shook its head and said "look somewhere else."

I'll post again when I have the running fuel pressure #'s.

Up above you didn't mention the TPS. Also since during the first few minuites of operation the battery is being re-charged, any ground issues would be amplified. So have you looked into block to frame to ECM grounding?

TPS was checked. Works fine.

Wanted to update--I left the fuel pump as is, since the engine was seeing fuel pressure of 45 psi with the pressure regulator disconnected.

Replaced the fuel injectors with a "rebuilt" set of Multecs. Car now starts right up and idles great. Haven't sent the old injectors anywhere to have them checked, but suspect 1.5 years of sitting--coupled with being GP Sorensen (not that great) injectors to begin with--meant that they weren't flowing well/evenly/whatever.

Thanks to everyone for your replies.

Still have those injectors around? If you do, and your open to it, I'd be willing to hook them up to my junk engine's injector lines/fuel pump and actually see how they are spraying, just to see what's happening.

It sure sounds like you have the problem figured out. Cool cool re that!