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XJ runs like CRAP, advice?

Steve88XJ

NAXJA Forum User
When I hold it in 1st gear and give about 1/3 throttle, by the time it reaches 2,500 rpm it starts POPPIN like a mofo.

This is where I'm at:

Pulled CPS (it has 2,500 miles on it, but has shook out and may be damaged), cleaned and reinstall it.

Tried new MAP sensor, no change. I will return it tomorrow.

Re-routed the vacuum lines on the intake in a loop, as to eliminate any possible component leaks. Only components receiving vacuum were the brake booster and MAP, same problem.

I'm thinking my next step will be to replace the CPS. I have an old one (put a new one in when I rebuilt the engine 2,500 miles ago) that I think I'll try to put in and see if it fixes the problem.

Another thing to note is that it ran great until that CPS shook loose and came off. Ever since then its been giving me problems, so im PRETTY sure that would be the problem. I guess I just dont want to believe that the CPS would go bad just like that .

Any input?
 
Your plug wires may be out of order, or your plugs, wires, cap, or rotor may be bad. Sounds like it is one of those 4. Check them all.
 
Plug wires are definetely in order, I checked that. Just replaced the plugs, the wires, cap and rotor all have 2,500 miles on them, and the distributor has 10-15k miles on it.

It does it worse when cold than it does when hot, but it doesn't go away.

I should also note that the O2 sensor isn't that old either, and this thing passed CA smog a few months ago... before the CPS shook off.
 
Jon may have hit on the issue. You said that your CPS shook out. Do you mean it lost a bolt and got loose? One of the things I have found is that you really have to use the CPS bolts to get it to work right. The bolts form part of the magnetic path and substituting them can cause erratic operation.
 
Yes it shook loose, one of the bolts shook out. I replaced them with longer bolts and nuts because I couldn't get them tight, I guess the threads are jacked.

Magnetic path, or just ground? What if I just run a ground wire from the surface of the CPS to ground?
 
Magnetic path. The bolts direct the magnetic fields that are used by the sensor. I bet that is your problem. I probably have an extra set if you can't lay hands on some easily. I've seen this a couple of times. Ever wonder why they have that funny shoulder and shape?

I can't guarantee it but if you can lay hands on the bolts easily, its an easy thing to rule out.

Around here when we go to the pull-n-save, we always make sure we have spares on all the sensors. Just put them in a baggie under the back seat for emergencies. That little bag has made a lot of friends on the highways and trails over the years. I also enforces that law of nature that if you carry the spare, the original part will NEVER fail. Its called poor man's insurance. Its one of those biblical issues, It's better to give than receive.
 
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Jeep Cherokee said:
Does it only do it when its cold and then stop when its warmed up??
My ’91 4.0l does that. Runs great cold. As soon as it gets up to operating temp it misses, spits, sputters and will backfire under acceleration. Do you know what the cause is?
 
Steve --

Have you checked for proper indexing of the distributor? I just picked up on the fact you rebuilt the engine 2,500 miles ago. A lot of people have problems when the distributor has been out. Get it off by one or two teeth and, depending on which way, either it'll idle fine but not rev, or it'll run fine but won't idle.
 
Is it absolutely necessary to have the original bolts? I had an original and I couldn't get it to tighten on either hole because of the crappy threads on the bell housing... although I suppose the bolt threads themselves could have been crappy.

I still don't understand the whole magnetic thing, are the bolts themselves magnetic?

Proper indexing on the distributor? I dont know exactly what you mean by that. I had the distributor in for abuot 1000 miles and it was running fine, all my problems started after the CPS shook out.
 
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