1988 YJ fuel problem

[HEGO]

I have an 88 YJ with a 2.5L TBI. I am losing fuel pressure a few minutes after driving. I hooked a pressure gauge up to monitor the problem. it will idle at about 15 psi all day long but as soon as you start driving the pressure will gradually start to drop off until it stalls. I have replaced the pump, filter, injector, relays, I rebuilt the sending unit and the pressure regulator. I even tried hot wiring the pump straight to the battery with a switch. no matter what I do the problem occurs every time. i'm pulling my hair out at this point please help

 

[k-2]

2 thoughts - quick and easy
1) with KOEO observe If pressure drops after 1 minute - fuel pressre loss due to ?
2) Leave gas cap loose see if pressure stabilizes - a unvented tank ?

 

[bjscj7]

Not saying this is causing your problem but the way you describe I had very similar problem with a '90 YJ. After pulling my hair out I took it to an "old timer" independent mechanic who specialized in jeeps. He had the answer within 20minutes, ended up being something I never would have thought of - I had a mud in my tank. He dropped it, flushed it and problem was solved.

 

[HEGO]

sorry I don't know what KOEO stands for.
I'll try the loose cap idea although I have tested both of the roll over valves and they seam to be fine.

 

[HEGO]

I've hade my tank off several times to change the pump I don't thing I have anything other then gas and a pump in there but at this point anything could be possible so ill check it out.

 

[k-2]

KOEO - Key On Engine Off.

Also loosen the gas cap

Let us know

 

[HEGO]

with KOEO the jeep holds rock solid at 15psi for more then 2 minutes no problem.

Sorry I was out of town for a wile and couldn't try this until yesterday.

when you say loosen the gas cap do you mean wile driving or with KOEO?

 

[k-2]

15 and holding is good.
the loose gas cap is to confirm fuel tank is vented - easy fast test with it running drive and see if better.

Another issue for loosing power after a few moments is backpressure - A vacuum gauge, back pressure tester OR a noisy DIY just remove o2 sensor for a test drive ...

Let us know if these easy fast things pin it down

 

[k-2]

A thought - is the fuel pump a wire yourself aftermarket?

 

[HEGO]

the pump is factory in the tank the wiring was also factory. Although I did hot wire it to a switch pulling power from the fuse block but the problem occurs either way.

Also in reference to backpressure suggestion I am running without a cat. it was plugged so I replaced it with a straight pipe still running through a factory muffler tho.

 

[k-2]

Just a thought on the pump aftermarket wiring.
I need to reread the thread - you loose pressure after driving a few minutes - did you see pressure drop on the gauge OR it seams like fuel pressure drop?
any evidence of fuel in oil?

FYI: don't get mad at me just passing info - it is illegal to remove or render inop emission controls. nothing to do with state test.

 

[HEGO]

jeep is for off road use only.

I see fuel pressure drop on the gauge. It will start out strong at 15psi or more then after a few min of driving it will slowly drop off until it stalls.


no fuel in oil...the engine is new has less then 100 miles on it

 

[k-2]

have you tried with gas cap open - I m hoping for a quick easy fix,

 

[k-2]

start engine then What is pressure at @2k rpm?

 

[HEGO]

yes I have. but I will try again. I have also checked and tested both of the rollover valves as well as replaced all the rubber tubing that plumbs them.

 

[k-2]

My bad I missed the TBI in your post -
ignore step 3 use tach in vehicle

 

[HEGO]

Ok so here's the list of what I did last night following your suggestions.

1. at 2k rpm the fuel pressure holds at 51 psi

2. drove with O2 sensor removed. same problem occurs. jeep runs for a few minutes at 15 psi then the pressure drops off until engine dies out.

3. Once pressure started to drop I stopped and removed cap. No difference in pressure no sound of vacuum as cap was removed. (remember I have tested both rollover valves).

4. and lastly I can not adjust the pressure on the fuel pressure regulator per your last suggestion due to the fact that there is no adjustment screw on the bowl of the regulator. besides I have 15psi at start up like I should right?

 

[HEGO]

note: #1 should say 15psi not 51

 

[k-2]

what is PSI at 2k rpm?
Maybe deadhead pump pressure at filter - will isolate if in tank problem or other.

I have nothing else to suggest next I would get into tank and check pump assy.

 

[HEGO]

at 2k rpm the psi is at 15
what do you mean by deadhead pump pressure?