4.0 Fuel Pressure?

Rox&Mud

Rox&Mud

CTB
Joined
Nov 3, 2006
Location
Raleigh
I have been having some intermittent engine issues that only seem to occur while wheeling. It will start to run rough and may even stall out then it's hard to start if it even does. I have not been able to reproduce the symptoms except on the trail.

That said, I just checked the pressure again and have a question.

According the Haynes it should have 44-54 psi with the key on, running or not. With just the key on it goes up to 47 psi for about a second then immediately starts dropping back to about 10/15 psi, is this normal? If not any ideas? Also while running at idle and at normal operating temp the needle on the gauge starts to shake until I give it a touch of throttle then it evens out.

Any thought you guys have would be helpful chasing down this gremlin w/out just throwing parts at it.

Thanks
 
Not normal. Sounds like your fuel pump is dying. I had the same gremlin for about 8 months. Only happened on the trail and seemed temperature related. Finally dawned on me to check fuel pressure. I was at 30 psi with key on, engine off. Didn't leak down, but never got any higher. I replaced the pump with another used pump. I would hit 42-45 psi at initial key on, then it would settle around 35 psi and stay there. It runs 95% better and lasted through 6.5 laps @ DPG.

44-54 psi seems high. Seems like I remember the recommendation being 35 to 45 psi.
 
hadfield4wd said:
No it's not normal. Something is leaking down. What is the pressure while running?
Click to expand...
47 initially but then the needle on the gauge starts shacking very fast between around 45 and 50 but it's hard to say exactly.
cltdba said:
44-54 psi seems high. Seems like I remember the recommendation being 35 to 45 psi.
Click to expand...
I thought it was high too until I checked my Haynes manual. I was thinking fuel pump but wasn't sure. Would the pump itself be causing the leak down or should I look somewhere else for that?
 
Check your lines for leaks first before replacing the pump. I doubt you have any leaks, but it's worth the time to check. Yes, the fuel pump can be the culprit. Other candidate would be the fuel pressure regulator. What year 4.0 and application are we dealing with?

If you end up replacing the pump I hear people have the best results with the Bosch OEM pumps. I replaced mine with one of the O'Reilly Airtech (or whatever its called) pumps and couldn't get any pressure. Put in the Bosch pump from a spare tank and it worked fine. I believe both AZ and Advance carry the bosch pump, but you'll pay premium prices for it. Other option is the E2000, which is what I think the one I bought was. Fitment is a bitch. They give you rubber grommets to shim the pump, but it never goes in right. I'm a fan of OEM fit.
 
The regulator/filter is attached to the top of the pump. It is replaceable but for $118 delivered I can get the whole pump/regulator/filter etc from Parts Train. I'll try it, maybe it will resolve the issues.
 
Is your's a return fuel system? If it is the regulator will be on the fuel rail. If not it's on the pump.

The injectors could be bad as well. They will cause a leak down when the motor is turned off. They could be weak as well causing poor running. Is there a way to test yours. I know there is on mine. But mine is a '90 XJ with a Renix FI system. Every sensor can be checked with a multi meter.
 
Id pull the pump and check the straner pad/ fuel filter, had the same thing happen to a cherokee, filter on the pump was clogged , and made the needle on my fuel pressure tester flutter, then shortly after the pump died. Id replace it all, but if you dont have the $. Id check that straner pad first. Fuel filter is always the first thing you check. dont screw around buying a bunch of parts till you have tried the filter.
 
Sorry I haven't responded sooner, we were in URE for Thanksgiving. Got the whole assembly ordered last week. It only seems to do it while we're wheeling but it only ran rough a couple times this trip and it never stalled. I did have the opportunity to check the pressure when it started running rough. It had about 45/46 psi so the pressure is there maybe it's a lack of volume. When I gave it a fair amount of throttle for a few seconds it would clear and it also only seemed to occur when I started it.
 
check your grounds around your fuel pump. changing the fuel pump assy in a zj iisnt bad at all. it took me 2 hours outside on the road. it definitely sounds like ur pump is starting to get weak.
 
Back
Top