XJ J.B.

Hello all -

One of my students was trying to get rid of her 2000 Jeep XJ 4.0, and because it was on the verge of getting scrapped, I decided to buy it from her. It's got 220k miles, it's 2WD with auto trans (that's why she couldn't find any takers for months), not a lot of things work on it, and though it was running, it had CEL on, and a few notable issues, like cracked PCV, leaky heater hoses, stuff like that. The last few months, I've done the basic tune up (oil, plug, air cleaner, fresh gas, of course), along with new catalytic converter (was clogged), injector (there was a leak on one of them), coil pack (I don't know if it was bad, but it had crack in it), PCV (front and back, along with the new hose on the back). And since they weren't too dear, the idle air control valve and the throttle position sensor. I reset the ECU (procedure that I learned from the forum), and since CEL does not stay on. However, it flashes, time to time, relatively randomly, but most often under constant, or heavy acceleration, flashes for several seconds, and that's it, it won't stay on. My OBDII code reader is not picking any codes (but that could be just the code reader, I noticed that it works on some and doesn't work on others). The car appear to run fine, no sign of smoke, overheat, or oil/coolant consumption. It revs nicely in neutral without flashing CEL, only flashes when in gear, under load. What could the possible causes of this? I'd really appreciate your input.

Thanks,

BlueRidgeMark

Hey, welcome, JB, and thanks for rescuing another XJ from the scrap yard!

Since you don't trust the code reader, maybe take it to your local Autozone and ask them to read the codes. It's free, and they might find something your reader is missing.

XJ J.B.

Hi,

Thanks. Yeah, I'm thinking of having the code read by repair shop or something. Does it still register the code even if CEL doesn't stay on?

BlueRidgeMark

It should store a code, which a good reader should be able to pull up.

XJ J.B.

Great. Thanks for the info, I'll go check on that. Still, if you have any thoughts, please let me know. The behavior of this CEL baffles me.

Spencer_P

Can you feel a misfire or rough rpm? Flashing CEL means active misfire.

XJ J.B.

Well, that's what baffles me. There's no noticeable misfire. Also, it revs from idle to redline smoothly when in neutral, and even in the gears, it is a bit more gruff feeling, but pulls well and cruising at highway speed is smooth and quiet.

As I write this, one idea came to my mind - I wonder if it has borderline low compression in one of the cylinders. Since it's not blowing and smoke anywhere, could be at the valve seat or something...

BlueRidgeMark

It's worth checking. Good info to know, anyway, as it tells you a lot about the health of the engine.

XJ J.B.

I finally took it to a local auto part store to have the code read. Nothing. I explained that CEL flashes occasionally under acceleration, but does not stay on, and he was, "well, that's bad," and that was that. I wasn't expecting miracle, and I guess I got exactly what I expected...

What would you think the best course of action here?

dave1123

You could have a burnt exhaust valve causing a misfire under load. A vacuum gauge will show a dip at idle when that cylinder fires usually. Check it out.

XJ J.B.

Thanks, I'll have a look at it!

XJ J.B.

Well...it's been a bit while. I took it to a local garage to see if they can pull the code, and still nothing. I showed the mechanic the flashing CEL under prolonged acceleration, but because there's no noticeable misfire (or any misbehavior), he just shrugged his shoulders and said, "well, it runs fine," and that was that. He did suggest replacing the O2 sensors, and though I was pretty skeptical what good it would do, I did replace both to see what it'd do. No change in the behavior. There was one video on YouTube on the crank position sensor that seems to have a similar behavior, so I wonder if that's what's next. I'm really baffled.

dave1123

An active misfire under load is usually indicative of loss of spark during high chamber pressures or a lean mixture. It can also be caused by carbon hotspots causing detonation. Why not give it the old steam decarbonization trick by slowly pouring water down the throttle body when it's hot and running at about 2 grand? Seafoam works, but not always. Water is cheap and works better.

Just for your information in case you didn't already know, that engine uses the "waste spark" system where the spark plugs fire on the exhaust stroke as well as on compression. They fire in pairs, 1&6, 5&2, 3&4. Only 3 coils in the rail. Recommended plugs are Champion RC12ECC or NGK ZFR5N.

XJ J.B.

Huh! I'd be honest, I didn't know steam decarbonization! Actually, I think I heard the CarTalk guys talk about it once. Anyway, I'm going to give it a shot. That seems cheap and cheerful. Should I just removed the tube from air box and either spray or pour a small amount of water in there?

IJM

Check out this thread:

https://www.cherokeeforum.com/f30/st...-4-0-a-226299/