Jeep backfires when NOT under pressure

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Feb 19, 2019 | 08:05 PM

NWGeorgiaXJ

Thread Starter

Newbie

Joined: Sep 2018

Posts: 9

Likes: 0

From: Lafayette Georgia

Year: 1995

Model: Cherokee (XJ)

Engine: 4.0lt l6

Jeep backfires when NOT under load

So for the past week or so my Jeep, 95 xj 6cyl 4.0, has been backfiring pretty bad. At first it barely did it and only at high rpm, then it would do it anywhere between 2 and 3 thousand rpms. After that I replaced fuel filter, spark plugs, spark plug wires and fuel pump. None of which changed anything. Oh and I ran some sea foam through it. Then I replaced the fuel pressure regulator. At first it was running normal, actually better than normal. Then about a day later I would start it and it would backfire, then I’d turn it of and start again and it would be fine. So it did that for about a day. Now (the next day) it backfires no matter what if it’s NOT under load. If I’m going up a steep hill or if I floor the accelerator it runs fine. I checked the fuel filter and it’s clean, I checked the oil and it’s a smidge low but it’s a 24 year old Jeep so it leaks a little😂
Oh and the fuel pressure is around 30 lbs, I think spec is like 49.2 if I’m not mistaken. I had it stuck in the mud about two weeks ago but it was only up to the middle of the front wheels.
Anyways I’m at a total loss as to what to do. Ide really appreciate it if anybody has any ideas.
Thanks in advance!

Last edited by NWGeorgiaXJ; Feb 19, 2019 at 08:16 PM. Reason: Accidentally Put wrong word in title

Feb 19, 2019 | 08:18 PM

NWGeorgiaXJ

Thread Starter

Newbie

Joined: Sep 2018

Posts: 9

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From: Lafayette Georgia

Year: 1995

Model: Cherokee (XJ)

Engine: 4.0lt l6

The title meant to say “load” and not pressure

Feb 19, 2019 | 08:32 PM

gat

Senior Member

Joined: Mar 2016

Posts: 621

Likes: 9

From: Oklahoma

Year: 1992

Model: Cherokee

Engine: 4.0L L6 PowerTech (stock)

Change the cap and rotor for the distributor as well as the filter for air.
1994, I believe, is 31 p.s.i. with the vacuum connection on the pressure regulator and 39 p.s.i. without the vacuum connection on the pressure regulator. Doesn't your rail have a return gasoline line next to the input line? The +/- 49 p.s.i. is a few years later in model and only has one line into the fuel rail because the fuel pressure regular is on top of the of the pump and a part of the assembly.

Feb 19, 2019 | 08:47 PM

NWGeorgiaXJ

Thread Starter

Newbie

Joined: Sep 2018

Posts: 9

Likes: 0

From: Lafayette Georgia

Year: 1995

Model: Cherokee (XJ)

Engine: 4.0lt l6

Thank you so much. That clears up some stuff, I was wondering how the fuel pressure could be low after replacing almost the whole fuel system. I’ll pick up the parts tomorrow and let you know how it turns out!

Feb 19, 2019 | 09:23 PM

gat

Senior Member

Joined: Mar 2016

Posts: 621

Likes: 9

From: Oklahoma

Year: 1992

Model: Cherokee

Engine: 4.0L L6 PowerTech (stock)

Another good maintenance thing to do that is very easy is to spray the idle air control valve with throttle body/carburetor cleaner. That valve can be stuck partially open or over closed from junk building at that valve.
Also, one go with a tank based fuel injector cleaner wouldn't likely be sufficient to clean them. Do one addition for the next two to three tanks.

Feb 20, 2019 | 08:07 AM

SatiricalHen

CF Veteran

Joined: Oct 2013

Posts: 5,924

Likes: 203

From: Greenville, SC

Year: 1999

Model: Cherokee(XJ)

Engine: 4.0

Save the cap you take off of the distributor and cut a hole in it at the number one post, then roll the motor over to TDC cylinder one, line up the timing mark on the harmonic balancer with the 0 and then check to make sure the rotor is just past the #1 post. That's what was causing my issue for over a year and I just finally fixed it and it's wonderful.
http://cruiser54.com/?p=68