low fuel pressure, im at a loss, need suggestions. - Page 3 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 05-12-2008, 10:53 PM   #21
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Quote:
Originally Posted by Technical Ted
The MAF frequency was 0 at WOT? It should be in 8 -10,000 range maybe higher.
well, the last run i did, it was unplugged. so the pcm may go to a default setting, but the live data only sees the sensor output.
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Old 05-13-2008, 03:04 PM   #22
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Let'* check a couple more things & then try to come to a conculsion.

See what the MAP (not MAF) sensor voltage is with key on engine off & again at idle.

Check the long & short term fuel trims at both idle & 2,500 rpm while in park.
Let'* also get the trims at or near WOT while driving & make sure what the MAF frequency is at or near WOT.
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Old 05-13-2008, 04:18 PM   #23
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Quote:
Originally Posted by samueljackson
ok, i ran it with the maf unplugged, and it does change a little bit. it runs better, and the vacuum is closer to 20in/hg. more like 18. but the fp only improved slightly as well. with the fpr hooked up the fp it went to 41 psi. if i accellerate slowly, it will only climb to 44-45 psi. if i go to WOT, it drops from there. my low was 35 psi.

and if i have the maf and fpr unplugged, the fp idles at 45-46 psi. but it will drop to around 35 psi if i accelerate hard.

so if i can see the vac climb back to zero, and build boost, and my fuel pressure still drops, does that mean that the pump is weak? am I delaying the inevitable, and do I just need to change the pump?

but what about the maf? does that test above indicate that its bad? when i was doing my scans i was told by Ryan at gm tuners, that the maf reading should start at 2000 hz at idle. I was reading low, around 1750 as a low, but it was fluctuating. and my scanner data said "Gr/*". to be honest im not even sure what that means. i think its gravity per second. but does that equal Hz?
Ok, first of all let me explain how the FPR works on these engines. With the fuel pump running and no vac or boost going to the FPR (such as what you have when you unhook the vac line from the regulator), you should get the spec fuel pressure - which it sounds like you have @ 45-46psi. If you hook the vac line back up to the FPR, vacuum should act on the FPR and lower the fuel pressure -- which it sounds like it does in your situation. So that part is working like it is supposed to.

Now under boost, boost pressure should act on the FPR and raise fuel pressure above 45-46psi; should rise by approx 1psi per pound of boost. This doesn't sound like what is going on in your situation.

The No.1 cause for what is happening in your case (fuel pressure falling off under boost) is caused by a weak fuel pump -- not being able to keep up with the fuel flow demands of the engine. OTHER POSSIBLE CAUSES for this could be a restricted fuel filter or kinked fuel line. SO I would try replacing the fuel filter first and checking your fuel lines before replacing the pump. But odds are the problem is either a weak pump or a clogged filter.

Concerning the MAF output frequency seeming very low, can you tell me if you have removed the screen from the throttle body inlet? If so, reinstall it and see if the MAF readings and engine running improves. Let me know what you come up with.

-ryan
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Old 05-13-2008, 06:34 PM   #24
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Ted,
this car has no stock vac/boost gauge. so there is no map.

the STFT was at about 137, but would fluctuate and stay around high 130s. when i hit the throtle, it shortly went to 128 and stayed there. the LTFT was around 160 at idle. and didnt move much with throttle.

the maf peaked around 38,000. I went to wot, but i didn't fully get on it for too long, as i am trying to limit my WOT runs. and at idle it is around 1200.



Ryan,
i hooked up 10 psi to the regulator and the fp will get to my desired 55 psi. So, that tells me that the fpr is fine. I went over the lines and I dont see anything kinked. And I changed the filter when i changed the pump.

When i got the tb, it never had a screen, but i did find a screen from a L36 and have been using that in there. it fits very snug.

does this confirm that the pump is bad?
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Old 05-13-2008, 11:35 PM   #25
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Thanks for making me aware of the lack of a MAP sensor. Later years have two MAP sensors. One for the gauge & one that the PCM monitors. That might not have started until 96.

I checked the 99 FSM & I was correct that the MAF frequency ranges from about 2,000 at idle to about 10,000 at WOT. We need to know if your'* should be in that same range or not.

The fuel trims range from 0 - 255 with 128 being "ideal".
When the fuel trims are higher than 128 the PCM is seeing a lean condition & has increased fueling.
When the fuel trims are lower than 128 the PCM is seeing a rich condition & has reduced fueling.

Since your long term trim is staying the same & the short term is dropping its hard to believe it'* fuel starved. Let'* try to find out what the MAF should be doing on your year.
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Old 05-14-2008, 01:43 AM   #26
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Quote:
Originally Posted by samueljackson
Ted,
this car has no stock vac/boost gauge. so there is no map.

the STFT was at about 137, but would fluctuate and stay around high 130s. when i hit the throtle, it shortly went to 128 and stayed there. the LTFT was around 160 at idle. and didnt move much with throttle.

the maf peaked around 38,000. I went to wot, but i didn't fully get on it for too long, as i am trying to limit my WOT runs. and at idle it is around 1200.



Ryan,
i hooked up 10 psi to the regulator and the fp will get to my desired 55 psi. So, that tells me that the fpr is fine. I went over the lines and I dont see anything kinked. And I changed the filter when i changed the pump.

When i got the tb, it never had a screen, but i did find a screen from a L36 and have been using that in there. it fits very snug.

does this confirm that the pump is bad?
Sounds like the pump is bad to me based on your checks...

I would replace the pump and solve your fuel pressure issues before worrying too much about the fuel trims. Correcting the fuel supply issue you have is probably going to impact idle and part throttle fuel trims so there'* no need wasting time working on those until we get the known fuel supply issue corrected first.

In the meantime, could you refresh my memory what you are using for a throttle body/MAF sensor (is it stock to your engine? Is it ported? What vehicle is the MAF from? etc)?

-ryan
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Old 05-14-2008, 07:36 AM   #27
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the TB and MAF are stock to 94-95 L67s. and my engine is a 93. The tb is not ported.
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Old 05-15-2008, 03:49 PM   #28
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Get us those MAF numbers when you can. I'll check the FSM 'range' when I get home or test my 95 (can't test the 93 at this time, half the motor is in pieces).

Do you have the 94/95 MAF installed? Do you have the 91-93 MAF still? Which FPR are you running? The 91-93 3.0bar, or the 94/95 2.7bar?

Yes, I believe your pump is not keeping up at the top end.
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Old 05-15-2008, 04:48 PM   #29
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Quote:
Originally Posted by willwren
Get us those MAF numbers when you can. I'll check the FSM 'range' when I get home or test my 95 (can't test the 93 at this time, half the motor is in pieces).

Do you have the 94/95 MAF installed? Do you have the 91-93 MAF still? Which FPR are you running? The 91-93 3.0bar, or the 94/95 2.7bar?

Yes, I believe your pump is not keeping up at the top end.
the maf reading at idle was only about 1200 gr/sec. i will wait until i have good fuel supply before i get a real WOT.

and I am currently running the 94/95 FPR and MAF. I also tried the 93 FPR and MAF, and no change in FP. and the readings were also low with the 93 MAF, but i dont remember exactly. i know it was below 2000 though.
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