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Fuel Pressure while running?


PAUL T

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I checked the fuel pressure yesterday on my 94 Concours (northstar) and noted the fuel pressure to be 42 engine off and 36 engine running is this normal? I could not find what the pressure should be with the engine running in my manual. I also separated the spark plug wires and I noticed the stumble at acceleration was better, not gone, but better. So I am leaning on a bad (new OEM) or arcing plug wire. EGR checked out fine, and the injectors were flowing o.k.. Still no codes lately. I am going to check the plug wire resistance soon.

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I have cleaned the phenolic plates, notches, Egr passages at the plates, run a wire down the holes from the EGR valve. I had the plates out 4 times to make absolutly sure I got them on right, the last time putting a little RTV to make sure. Visually checked the EGR operation and did the EGR test in the manual and it passed. I replaced the fuel filter, air filter, plugs (OEM), wires (OEM), cleaned the throttle body, visually inspected the FPR (no fuel leak), and visually inspected the MAP sensor (not unplugged). I did the injector flow test, all injectors dropped the pressure at the same rate. The only codes that have appeared intermittently is P071 and P074, not at the same time and not every time. I have been moving the plug wires trying to keep them separated and have noticed that it runs better for a small while then goes back to a miss. I am going to check the wire resistance when I get my hands on an Olm meter. I know the manual says keep the wires away from the coil base but the wire loom is attached to it. Any other suggestions will be greatly appreciated. After doing all of the above I have noticed the car being more responsive at the throttle and on the times I have been able to do a WOT, and it did not stumble, I really have to hold on to the steering wheel. When I did the fuel pressure test (engine off) the manual said the pressure should be 40-50 psi, but I could not find it for the engine on. Should I replace the FPR?

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I think the fuel pressure is supposed to be about 46-50 psi. The pressure should be the same running or not.

I would have expected a little higher fuel pressure also, but with my poor memory I can't remember for sure what a 94 was spec'd for.

In any case, for clearification , the fuel pressure at key on (engine not running) and idle is NOT supposed to be the same....

Oops, sorry. Guess I misunderstood some of your earlier posts. I think I am confusing "the same pressure drop across the injector" with keeping fuel pressure at the rail the same.

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So, should I replace my fuel pressure regulator? I also found that the cam position sensor was coated with oil and the wire possibly shorted. I am going to clean it. Is there an o-ring on it?

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I'd clean that connector first and see if it makes any difference. Not sure about an "O" ring. Someone else will have to answer that. Change the FPR if fuel is coming out of the vacuum nipple when the hose is removed while the engine is running.

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Change the FPR if fuel is coming out of the vacuum nipple when the hose is removed while the engine is running.

On a 94 Concours there is no vacuum line to pull off and you can not start it with the manifold cover off. I did check it by just turning the switch on while the cover was off and no fuel was present. I really do not think that the FPR is the problem because it does hold pressure. I keep getting a P071 - Intermittent MAP Sensor, code the last couple times I drove it. I guess I need to buy a vacuum gage and check it out. I also need to check the EGR tubes in the water pump housing. When I cleaned the phenolic plates the EGR hole in the cylinder head to the pump housing was completely plugged, I took a wire and cleaned it out but never took the plugs out to clean the tubes. I will let you know what happens.

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On a 94 Concours there is no vacuum line to pull off and you can not start it with the manifold cover off. I did check it by just turning the switch on while the cover was off and no fuel was present.

In that case your FPR is ok. Intermittent MAP sensor would lead me to thing the sensor is bad or the wiring/connector may be loose.

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No codes lately, but I did check the plug wire resistance, here are my results, maybe someone will be able to decipher them for me:

With the DVM set on 20K

#1 = 2.22

#2 = 5.5

#3 = 2.25

#4 = 5.56

#5 = 1.56

#6 = 5.1

#7 = 2.1

#8 = 4.25

The manual says values to be under 15000 olms, I am no electrician and I do not know what this is. Am I with in specs? I know Larry had trouble figuring his out on an earlier post.

Also, are the EGR and manifold cover gaskets reuseable? I have removed both while checking things.

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The readings you took are in k-ohms (since you had the meter set to the 20k-ohm range. I'd say your wires are well within spec..

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Thanks Kevin, I used to work with my uncle who was an electrician for Dupont. So my experience with a DVM is very limited.

Does anyone know about the EGR and Intake Manifold Cover gaskets being re-usable?

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There seems to be quite a few posts of late regarding missing and studdering...Does anyone think that the "winter blends " of fuel may be affecting our beloved Northstars??

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  • 2 weeks later...

O.K. here is an update.

I inspected the EGR and cleaned it and the tubes, replaced the gasket and it did not make any differance in the chuggle at acceleration. There has been no codes lately. I took a 450 mile round trip to Clifton Forge, Virginia and the car did good but still chuggled when we hit the mountains. Cruising at highway speeds it was smooth as glass, but accelerate and it chuggles. Gas mileage was 22 MPG.

I checked the fuel pressure "key off" and it went up to 45 and dropped back to 40 and held. Took it for a drive and the fuel pressure varied from 35 to 44. 35 when coasting and 44 at acceleration. During the chuggle at acceleration the pressure stayed at 44. At steady speed pressure stayed around 40 and on deceleration pressure dropped to 36. At idle, pressure was 36, after engine shut off pressure went to 42 and held, even after about 30 min.. I am not seeing anything wrong, what about you?

I am going to pull the new rear plugs and inspect them for carbon tracks next. I pulled the front ones and they looked O.K..

Chuggle used to be just on slight acceleration but lately it has been on almost acceleration. Anybody got any other ideas?

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Paul,

Your symptoms sounds exactly like what I went through about 2 months ago. It turned out to be plug wires in my case.

http://caddyinfo.ipbhost.com/index.php?showtopic=4068&hl=

Ranger, these are new wires that I have check the routing, the connections, the resistance, and all was fine. I also checked for arcing and nothing. Thanks anyway.

I did notice something though with the fuel pressure. After about an hour or so the fuel pressure started dropping. I checked the connection of the pressure gage and no gas was present so I eliminated the gauge. It was getting dark so I unhooked it and came in. That leaves me with maybe a leaky injector or bad FPR? How can I tell if there is a leaky injector?

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To check the injectors you need to pull the fuel rail, then turn the key on to energize the pump and pressurize the system. Then watch for a leaky injector. When the pump went on my '92 it was fine for about :15 min. After that it would fall on it's face only at WOT but would run fine otherwise. I'm sure it would have gotten worse if I had let it go. I once had a Blazer that would run fine for an hour then just quit. Restart and run fine. Pumps can go in many different ways. If the pressure is dropping after an hour I would suspect a failing fuel pump rather than an injector. Were you driving it when the pressure fell? Did you have a guage on it at the time?

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The pressure fell after the car was shut off for an hour.

Ok, that would mean that either the FPR is leaking, one or more injectors are leaking or the check valve in the fuel pump is leaking.

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Well I just changed the FPR and while I had the intake cover off I noticed a oily residue all over everything, I could not determine if it was oil or old gas residue. I could understand it being old gas residue but where would oil come from?

As far as the chuggle being solved, the jury is still out. I took it out for a drive after I replaced the FPR and it was better but there was still just a hint of a chuggle. I have not done an "idle learn" yet, I had to go to my mother-in-laws for lunch and when we got back the snow was flying. So I have not uncovered the car yet to do it. I will try it tommorrow and give my results then. Wish me luck, I want this to be fixed so bad.

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The residue you are seeing in the intake is normal. It is oil from the PCV and as Guru says "heavy ends of fuel" or something like that from the EGR. When the engine is shut down all that stuff just settles. He (of coarse) explains it much better than I if you do a search.

If you replaced the FPR then I doubt that is the problem. I wouldn't expect a leaky injector to cause that problem.... but, if a leaky FPR made it better, then maybe it is a leaky injector. The only way to tell if one is leaking is to pull the fuel rail, pressurize the system and watch the injectors.

I doubt that the idle has anything to do with it.

I believe you said that you replaced the plugs and wires. Have you tried swapping the coils one by one with a known good one? Maybe a coil breaking down under load? Though the more I think about that, it would get worse under heavy (WOT) load and as I recall that is not your problem.

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This sounds like a spark issue to me. There are lots of other possibilities, but you are quickly eliminating them. You said you replaced the wires, but did you replace the plugs at the same time. I have had carbon tracks on a plug give me grief, even after I replaced the wires. If the plugs are not new, I would take a close look at them.

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Plugs were put in same time as wires. If the FPR doesn't do it I plan on pulling all the plugs and checking them for carbon tracks and or hairline cracks.

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Well it was not the FPR causing the problems. If I had waited and checked this board I could have saved $70. Oh well, one more thing eliminated. My next move is to re-check all the spark plugs for tracking or hairline cracks. Then I think I will buy a coil and start switching them out and see if that makes a differance.

Thanks alot to all that have helped me so far. It has taught me well about the workings of this Engine and how easy some things are to work on. Such as the intake manifold, the throttle body, the EGR valve, the fuel rail, the waterpump, and the ICU and coils. It is all so complex at the same time, I am just at awe of the design.

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I may have really messed things up now! I changed the FPR out and all was well until today, The car had not been idling very well and it chuggled like crazy everytime I accelerated. I drove about 50 miles and shut the car off and there was smoke coming out from under the hood. It was not overheating, it was smoke and a plastic burning smell. The smoke was coming out around the purge solenoid or EGR area. After about a 10 minutes the smoke cleared. I called the Tow truck and went to my appointment. When I came back I started the car back up and drove about 50 yards to a good place to load onto the rollback. I really ran crappy, but it did run. I won't get a chance to pull the intake cover until this weekend. Anybody had this experience, and what should I look for?

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