low fuel pressure, im at a loss, need suggestions. - Page 2 - 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 05-11-2008, 06:03 PM   #11
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Does it idle smooth & at what rpm?

I'd like you to see if the throttle plate can be opened a hair without the rpm increasing & see if the vacuum drops. If the rpm increases before any vacuum drop then try again with the MAF & or the TPS unplugged. You may have to try all 3 combinations. No MAF, no TPS, no MAF & TPS. This could be interesting.
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Old 05-11-2008, 06:28 PM   #12
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at cold start ups, it idles high, about 15-1800 rpm. when the temp gets around 170+, it will idle closer to 1K rpm. but when i hit the throttle a little bit, it takes about 5 seconds to return down to idle.
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Old 05-12-2008, 04:34 PM   #13
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I tried my experiment on my son'* car & even with the TPS, MAF & O2 sensors unplugged it would still rev up when the throttle was opened. I was hoping it wouldn't feed it more gas without those sensors & the vacuum would drop. I'm stumped.
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Old 05-12-2008, 05:54 PM   #14
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if you look at the pic above, you can see that i have the ign wires running right next to the sensor wiring. is it possible that EMI from the ign wires are causing the injector pulses to act funny?
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Old 05-12-2008, 08:41 PM   #15
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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?
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Old 05-12-2008, 09:33 PM   #16
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I believe the MAF frequency should be around 2100 at a normal idle speed. I never got into the tuning much but I think the GR/sec is grams/sec. I think the PCM just reads the frequency & the GR/sec may be calculated using inputs from other sensors?? Do you know what the TPS voltage is at idle? Did you try swapping the IAC?
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Old 05-12-2008, 09:38 PM   #17
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i didn't mess with the iac. it is rainy here, so unplugging was easier, and became the first thing to try.

the tps is fixed at 0.41 V at idle i dont remember the wot but i think it was slightly above 4.0 V
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Old 05-12-2008, 09:46 PM   #18
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TPS looks great. What'* the MAF doing at WOT.
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Old 05-12-2008, 09:52 PM   #19
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Quote:
Originally Posted by Technical Ted
TPS looks great. What'* the MAF doing at WOT.
on the last run it was at 0.

on most of my runs, i have been mostly observing kr and my o2 mv. i honestly dont remember what the maf was at during wot. i will have to check tomorrow.
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Old 05-12-2008, 09:58 PM   #20
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The MAF frequency was 0 at WOT? It should be in 8 -10,000 range maybe higher.
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