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Old 11-24-2013, 05:31 PM   #1
jeffcoslacker
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Fuel pressure spec

Hey I'm wondering can someone provide a "known correct" fuel pressure spec for my '97 Lumina (3.1)? Looking around online has netted me everything from 30 psi to 35, 37, 41, 45...driving me nuts.

Here's what's going on. My car has 204,000 now, and has almost always run perfect. Yesterday I went to town and dropped off some movies, got back in the car and when I went to pull out into traffic it laid down and misfired and seemed like it wasn't getting enough fuel. I headed straight for home, it was idling poorly the longer we sat, breaking up when I first took off from a stop, it would run normally if I kept throttle opening very slight...above a certain point, the misfiring would start again. On the highway (cruise on at 65 mph) it would cruise fine until I hit a significant hill, then it would start breaking up, downshift, bellow through the intake but sounded "dead", like just air being drawn but not enough fuel...and lose speed despite the downshift and intake booming...I checked the airbox when I got home because I live in the sticks and sometimes thing nest in there...nothing. Checked my codes and had a PO300, random miss, but to me the misfire was very rhythmic, but "softer" than a dead hole...I was surprised it didn't pin it on one cylinder. When I drove it with the scanner on, everything looked pretty normal except the fuel trim was running in the 5-6% negative range, which I understand is the ECM trying to lean it out?

I put a new fuel filter on, because this one was many years old. No change.

Today I bought a fuel pressure gauge (finally) and it showed 35 psi with key on engine off. It drops when started, and has fast fluctuations when you play with the throttle. With the FPR vac line pulled off, it just stays at 35 constant, regardless of revving it or idling.

I remember putting new plugs in at 114,000, it has 204,000 now so I took them out and had a look...man, they were burned wide. Probably like .080-.090, I don't have a gapper that goes that high. So I put in a new set. Plug wires were new from chasing another problem a couple of months ago. Front o2 sensor is new by a couple of months as well, rear is factory, but seems acceptably active on the scanner.

After putting in the new plugs, I could tell right away it was almost back to normal. There seemed to be one very brief gag right as I smacked the throttle if I hit it hard from a stop. And if I wind it out hard, it would still lay down and not pull normally at the very top before redlining. But to drive it in a normal way, it feels perfect. Oh, also I put in a new timing chain set and damper a couple of months back too. The original was actually in good shape, but my balancer fell apart and I figured I might as well while I had it off.

I forgot to take the scanner when I took it out for test ride after the plugs, but hooked it up when I got back. It had a PO304, but I don't know if that was from before or since I put the plugs in. I reset and I'll find out tomorrow. I did notice the short fuel trim was now at 0%, long at 2%, and improvement from last time.

Things I'm contemplating...fuel pump pressure marginally too low, making the widely gapped old plugs lean misfire? Or perhaps one of the coil packs is having problems that made it hard to throw spark through the wide gap, so it's better now? Previous experience with coil pack failures have been profound misfire with codes for both cyls it fires...as I said, this felt like "soft" miss from one hole, but on hill-climb or heavy throttle from a stop it seemed like the whole motor was just laying down and not getting fuel...no profound shaking misfire like 1/3 of the motor not pulling, which is what I've experienced with them before....just kinda flat and dead feeling...

Thoughts?
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Old 11-25-2013, 10:47 AM   #2
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Re: Fuel pressure spec

Your max fuel pressure is very low vac. hose off, try and pinch off the return line and recheck max out put.
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Old 11-25-2013, 10:54 AM   #3
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Re: Fuel pressure spec

Quote:
Originally Posted by maxwedge View Post
Your max fuel pressure is very low vac. hose off, try and pinch off the return line and recheck max out put.
Agreed....according to Alldata your fuel pressure should be 41-47psi key on and eng off.
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Old 11-25-2013, 03:05 PM   #4
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Re: Fuel pressure spec

see if my issues relate. I chased a similar issue for months
http://www.automotiveforums.com/vbul....php?t=1091120
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Old 11-26-2013, 06:23 PM   #5
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Re: Fuel pressure spec

I'll try choking the return and retest. I should have known to do that, I've told people to do that before when checking the FPR. Duh. Thanks for reminding me.

I cleared the codes and drove to work, got a few more misfires but nothing like before, pulled hills well and only missed if I really hammered it.

I was kinda thinking back and it all started the next day after filling it with gas, and the temps were really cold that night. I'd managed to burn off 3/4 of a tank of fuel messing around trying to diagnose it, and threw some Techron in it when the trouble started, so I refilled it last night. Had a couple of hiccups when charging up the ramp onto the highway, less than 1/4 mile from the gas station, set PO304 and PO300 again...this morning it ran a little rough at first start after sitting out in the cold all night, but straightened out quickly. All the way to work and back it ran perfect, and set no new codes.

I suppose I coulda got a tank of shit gas, but it seems like even if so, it's trying to tell me something else that needs attention. I'll keep on it, and let ya know what I find...
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Old 11-27-2013, 10:34 AM   #6
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Re: Fuel pressure spec

I have a 97 Lumina 3.1 liter. Mileage at testing. Fuel pressure at rail. Key on engine off: 35psi, Engine Running. 34psi: Engine running vacuum line removed from regulator: 42psi.
Removed return line end next to fuel filter, connected clear plastic tubing and ran to fuel filler tank. Pinched off hose, pressure went above 60psi. That is completely pinched off to get that pressure. The pump was drawing 12.5amps , read at fuel pump relay.
This car now has 152,000miles on her and is running perfect. If you are close to these numbers I would say the pump is good.
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Old 12-08-2013, 01:14 AM   #7
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Re: Fuel pressure spec

1/4 coil pack is bad. Changed it yesterday and it runs well once again. I am going to replace the fuel pump soon, as I can't get more than 35 psi out of it no matter what, but for now it is not causing any problems with that output.

The coilpack had a half-moon shaped crack in the casing. No evidence of arcing but was probably sucking moisture with thermal changes i guess.
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Old 12-27-2013, 10:02 AM   #8
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Re: Fuel pressure spec

Sorry for the late response. Your answer went to my other e-mail and I don’t get to that one very often. Good to hear you’re found your problem. Hard to do without the proper equipment. Don’t forget when you change the fuel pump to be sure to check the fuel level sending unit. Probably wouldn’t be a bad idea if you put a new one in with the pump. Just don’t assume it will read correctly. I had someone change mine and the tank level has never been quite right since then.
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