BiplaneGuy

This is a strange one, but maybe enough clues?

Some weeks ago, while my daughter was borrowing the Jeep, she couldn't get it to start. Later, my wife got it going by holding the accelerator to the floor. After that it was fine.

A week later, it happened to me. It started, but barely, once I floored it, and ran rough, no power. Push down the accelerator, pop-pop-bang-bang, once I got it to get up over 2000 rpm or so it smoothed out and then ran fine. This happened again, but not as bad. I seem to recall something like this happening, once, last winter.

Today I was driving home with a borrowed trailer and it seemed rough, but at first I attributed it to the extra load of the [heavy] trailer. But when I stopped in my driveway, it was running rough, and pushing the accelerator it would go up to 2800 rpm and cut out, cut in again, up to 2800 cut out, etc. Connected the scan tool and got P3000 (multiple cylinder misfire), and the codes for #3 and #5 misfire, as well as a decreased cat efficiency. And it seemed to be in open loop per the scan tool and reporting low voltage (.02) from the O2 sensors. I shut it off and restarted the engine and then it ran fine with no problems, in closed loop according to the scan tool. Check engine light never came on though the scan tool said MIL status on, but it appears it's burned out as it doesn't illuminate during startup... otherwise I would have been looking into this sooner.

In most if not all of the times this has happened, the engine had been idling for awhile... my daughter was in a McDonalds drive-through and then shut it off, I was in a traffic jam that was so bad I shut it off, and today it was idling while I was jockeying the trailer into position to hitch it, then shut it off while I hooked off the chains, etc. I had suspected that low oil pressure (my oil pressure is very low due to the infamous cracked head / oil dilution / worn cam bearings problem, but it's been that way for a couple of years now) was causing the valve lifters to collapse, but that doesn't explain why restarting fixed it immediately.

I know lots of things can cause a misfire and many of them have been discussed here before, but this is the first I've heard of a restart immediately fixing things like that. There has to be a clue there... any ideas?

tech

It could be early sign of the ignition switch going bad or maybe a relay

'90Cherokee

My first thought was leaking fuel pressure regulator or injector causing the hard starts. That wouldn't make sense with it fixing itself with a restart, but that could have been a fluke. The regulator is easy to check, pull the vacuum line off and see if there's gas leaking into it.

BiplaneGuy

I don't know how the computer handles open loop. If an intermittent problem puts it into open loop and then goes away, does it stay in open loop until a restart?

I should have added the exhaust pipe is black, carbon indicating incomplete combustion.

4WD4EVER

Do you know the history of this XJ? Was it ownef by someone who did short trips around town frequently and rarely, if ever got the rpm's up above ~ 2,500 or so?

If this is the case, there is too much carbon built up on the exhaust valves and this condition will give the symptoms you're describing. There is a TSB for it from ChryCo.

To verify you have to pull the valve cover and rocker arms and look for a "bullseye" pattern on the tip of the exhaust valves. This pattern = good. No bullseye pattern = too much carbon and valves not rotating as they should at high rpm's.

BiplaneGuy

I don't know the history before I got it, but I replaced the cylinder head a couple of years ago. My own driving is a typical mix of local and highway driving.

Are you suggesting that carbon buildup is the cause rather than the symptom? Because excess carbon is itself a symptom of incomplete combustion for any reason, e.g. ignition problems. I don't see how it would cause the intermittent symptoms I've been seeing.

Do you have any more info on the TSB, or a link to it?

4WD4EVER

I believe you can find it here, or search, or call a dealer:

http://www.jeepproblems.com/tsbs/Cherokee/2000/

I don't know definitively that it is the problem, but, it is possible.

It may be a loose bolt in the exhaust (known problem on them).

It may be coils. I would test them. There is a procedure somewhere for it. Username tjwalker posted it.

I would recommend eliminating the usual suspects, but, I wouldn't be quick to rule out excess carbon.

I would help more but I'm getting ready to leave on vacation. Lol!

Here it is:
https://www.cherokeeforum.com/f2/tes...204328/index2/

4WD4EVER

Understood. Probably not the problem then. Have you figured this out? Curious...