Alright, there isn't really any way to shorten this, so I guess I'll just need to give the lowdown. I can't really find anyone who has my same common issues either, that's why I think a new thread is in order.

Got a Jeep from Michigan about a month ago, and seemed to run like a champ other than a hard start. 98 XJ Limited with the 4.0.

Before I troubleshooted the hard start, I just gave it a new air filter, Bosch Platinum 2's, RP 10w-30 with a Napa Gold Filter, and I also fixed a melted wiring harness for the 02 sensors. It was melted around where the power steering pump is mounted, but against the engine block.

Seemed like for the next couple weeks after I changed all of that, every time I drove it, for about the first two miles I would just lose power going into second gear. After those two miles, it seemed to drive like a champ. I attributed that to needing that fuel pump more than ever, but I'm not so sure now.

Fixed the hard start with a makeover of the fuel pump assembly, starts first or second turn every time now. But, after it runs for about 10-15 minutes, it'll drop to under 700 rpms and chug like a dog.

Had a former Chrysler mechanic tell me over the phone to take off the TB and hit it with carb cleaner and replace the idle air sensor if it looked bad (which it did). Still idles like crap.

When it chugs and tries to die, the engine throws a code for bank 1 sensor 2 (P0138). Don't know if that's going to cause issues?

I know that some people have claimed that the Bosch plugs are no good in the 4.0, which may be where I start next. I had the Bosch four-prong no-gap plugs in my '99 and she ran like a champ, but I'm willing to try some Champions/Autolite's if it's a good possibility the Bosch's are the culprit.

Remember, it runs fine for 10-15 minutes and then totally sucks.

Any suggestions?

 

I have a 99 xj and I've had the same problem I've replaced every sensor, all injectors, all ignition components, fuel pump assembly, and it still does it ive tried everything but replacing my pcm I feel you brotha

 

Seems like these 2 statements are related. You said you fixed the wiring, bug did you put new 02 sensors in as well ? I would test the 02 sensors, burnt wires usually will short out something. Does it seem to run rich, black smoke when its acting up.

 

Even if it shorts, O2 shouldn't screw with the way it idles though right? And it ran alright before I did the plug swap and what not.

I'm working on it now, and I got it to chug again. I gave it the throttle for 30+/- seconds and it smoothed after that. Gonna keep playing with it.

 

Alright, new developments.

I went to the shop and messed with the jeep for probably 3-4 hours, and here's what I came up with.

When it chugs, if I go and select ANY fuel injector, unplug it, and plug it back in, the idle smooths out. No matter what I do, I can't get it to chug again after that until I shut it down.

That'e exactly what I believe it might be. I'm going to pick the Chrysler mechanic's brain on this a little bit and see what he has to say.

Does anyone know any other reason that it might smooth out like that after I unplug & reseat any injector harness?

 

My 91 was doing the same thing. the idle was very inconsistent, some times high some times low and sometimes not at all, I tried everything also. turns out, it was a bad Intake/Exhaust manifold gasket. after I changed it, the idle and power are perfect and I have had no other issues. Hope this helps.

 

Mine has a similar idle with the same code. Also sometimes when driving it'll want to die when giving it any less than full throttle. Found a similar post where someone had the same symptoms and replacing upstream 02 fixed it. I would at least try that first. Will be replacing mine soon so I'll see what changes. There is also a few other things that would cause a high voltage reading at the upstream 02. Don't remember them off the top of my head though.

 

Oops. Mine is p0132 which is upstream. 138 is downstream. As far as I know it's a wiring issue, bad sensor, bad CAT and what seems to be less likely is fuel pressure too high. Although if unplugging injectors fixes it maybe it is high fuel pressure. Running rich is what usually sets the code. Just need to figure out why its rich.

 

There's also a 15 amp fuse in the pdc for o2 sensor heater. Have heard if that being the culprit and being overlooked.

 

Alright all, some new developments.

I unplugged the O2 sensor from the harness, and I could not get the Jeep to chug like it did the other night.

Bought a brand new NTK and swapped it for the sensor that had the burnt harness (before the Cat) and it continues to chug.

I may crawl under there and see if I'm getting the outputs I should be with a multimeter. Anyone know of a site that would have those kind of specs?

 

"Had a former Chrysler mechanic tell me over the phone to take off the TB and hit it with carb cleaner and replace the idle air sensor if it looked bad (which it did). "
Find another mechanic.


Any codes, what's the fuel pressure in psi?

 

Easy turbo. He didn't say that, which I found out later. That was relayed by a friend on accident. It didn't look too good as far as buildup goes, so I replaced it. I called the mechanic the day after and he said I didn't have to replace it & that I should've just cleaned it.

Doing a clean run to see what codes come up, and fuel rail always checks out at 49 or so PSI.

 

I still think you would be best off finding another mechanic due too the fact that the answer probably lies right here in your own statement (due to a blown fuse):
"When it chugs and tries to die, the engine throws a code for bank 1 sensor 2 (P0138)."

https://www.cherokeeforum.com/f2/o2-...circuit-33042/

 

Oh no, I'm sure he's not perfect. He's giving me ideas over the phone so there's that. But yeah, I definitely came back here for a reason lol. I'll try out the link tomorrow.

 

I had a similar issue last year. Got very frustrated replacing parts on my 99 and it turned out to be the O2 sensor fuse under the hood. It was completely black and destroyed. Replaced the fuse and it was back to normal. Exremely cheap and easy solution. Good luck