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Replaced CPS Still stumbling bad


NAXJA Forum User
New Mexico
Is there anything else that could be causing my stumbling. I thought I had narrowed it down to the CPS. I changed it and it is still doing the same thing.

When I was replacing it I noticed that one of the bolts that holds the valve cover is broken off. How hard will it be to take this out?? Is it leaking and causing my jeep to run really rough and backfire???


Mike M., New Mexico
What kinda backfire ? out the throttle body or out the tail pipe?
How does it handle any kind of hill,does it get worse?
Did any of the suggestions mentioned in your last post help ?
Yesterday when I was changing the CPS I noticed one of the bolts holding the valve cover on is broken off. I was thinking I should take care of this before anything else, but I am not sure if it could be causing my problem. It is in the middle, close to the throttle body.

So should I take care of this broken bolt, then try to figure it out after that? Or is it not a big deal?

The jeep idles rough, backfires when I give it gas. Has power surges. I have no idea.....

Thanks Again,
MIke M., New Mexico
Sorry forgot..... The jeep is backfiring through the tailpipe. It seems to run smoother at higher RPM's. When idling it almost seems like it is idling to low.

thank you for any help
I would say if your not leaking oil around that bolt then it is not an issue related to your stumbling,but should be addressed eventually.Backfire out pipe should mean a rich burning problem,unused fuel combusting in the pipe or a timing issue.I am not that experienced with fuel injection or computer systems vehicles but your EGR valve could be suspect for the idle problem and a vacuum leak somewhere might make it run rich meaning your getting more gas than air in combustion chamber.It would be really helpful to purchase a good high impedance ohms multi meter and a vacuum gauge,you can do a lot of troubleshooting with these tools.I recently bought my 90' jeep and a manual is a must for any vehicle you own,saves you alot of money in the end.My preference is Chiltons for cherokee's,much more thorough than Haynes,I have both.FSM would be better if you can find one for your year.

Keep trying this board ,there is alot of knowledgeable people on here that know more about these newer motors than I do, but I will keep checking back to help if I can.
Wow, thanks for the quick responses and all the help. I don't think I have an EGR valve. I checked it out and I don't see it. I replaced the header not to long ago so I had everything out then and don't remember seeing it then either. I don't think there are any vacuum leaks. Replaced a lot of the hoses recently and check them all out. Doesn't seem to be any fresh oil where the bolt is broken (it's a little dusty and stuff).

I have some time to work on it now because I finished school friday! Took a calc final this morning at 7:30 and don't have another final till tuesday. I am going to clean the TB. Don't want to drive to the store to get the cleaner though...

Well, thanks for help.
I would pop it into the dealer and have them hook it up to the DRB for an hour labor.............can save you alot of money in the long run instead of tossing parts at it,yours is a Corporate Speed density system,you can see alot on the DRB ,can tell if it primary prob of a secondary prob..............I know most hate the dealer...................but might save you money in the long run.......Snap on scanners and such can only do so much which the independents use.......................DRB rules:D ....chime in here Daimler guys...................;)
I agree w. Rags. Get to the dealer and ask for an experienced early Jeep DBR guy and request the diagnostic 1 hr max. Have them fix it too if within the 1hr labor minimum. Shooting in the dark gets expensive. Greg
Timing is computer and sensor controlled if you jumped a tooth on timing gear computer might compensate to a degree ?On my chevys you jump a tooth you dont run at all.I replaced my t-chain on the jeep when I went through everything mechanical and there is a wear pad type of thing on the inside of cover that is suppose to let the chain rub on it instead of jumping gears.Do you have a strange rubbing noise upfront.

Also you might say whether you prefer to do your own repairs or let dealer do your repairs cause there are many resources such as this site to diagnose and repair your own rigs.jeepsunlimited.com also has alot of info.If you wish to do your own diagnoses correctly you need the proper tools and you dont need a 5000.00 computer either.Vacuum gauge,repair manual,Multi meter and fuel pressure tester will diagnose almost any problem.For around a 100.00 you have your own diagnostic tools.As for scanners if you got 500.00 there are 2 that I know of that are just as good as stealership diagnostics including real time diagnostics for all OBD ranges from 80' to present with expandability.
My uncle who is a mechanic came over today. We started checking if every cylinder was getting spark and when you disconnect the third cylinder the engine speed or anything doesn't change. When we plug it back in we hear a clunk in the engine. That same clunk can be heard when it is idling to. No strange rubbing noise comming from anywhere. Tomorrow...... compression test. Now that my uncle brought some of thses things to my attention I think it is something internal. The compression test will tell but it does sound like something with a bad valve or something like that.

At least I am getting somewhere. Thanks for all the help everyone. If the compression test looks fine I will have a dealer diagnose it. Other than that I plan on doing this myself with the help of my uncle. I, like most, don't like dealers too much either. Mine sells me the wrong parts on a regular basis. I think I know about the 4.0 than their parts guy.

What Kind of numbers am I look for on the compression test?

Thanks again for the help.
You have to do all the cylinders on the compression test. I like them all within ~10-15lbs of each other. Is that bad cylinder plug sparking or not. Could be a bad wire, bad plug, bad cap. A good top engine cleaner will remove carbon build up from the top of the cylinder domb and from around the valve seats, takes about 45 min to do it.
If the bad cylinder is low, squirt some oil into it, if the compression goes up then it's rings, if it stays the same then its valve related.
I had a similar problem on my 89. Had a bad miss at idle but would run good at highway speed. It would also backfire intermittently. After a compression test, I found that the #6 cylinder had no compression. I did the oil squirt test and still nothing. I removed the head and found a large chunk of the exhaust valve was gone. I took the head in to get a valve job and found out that the seat was cracked and was unrepairable. $300 later, I had a new cylinder head and now it runs great...
Well did a compression check and everything looks ok. Back to the drawing board. Going to give it a tune up today. See what happens. All cylinders had right around 115 pounds. Is this pretty good? One had like 112, or something little lower but they are all very close to 115.

New cap rotor and wires (plugs are less than week old) We will see what happens
Perhaps not likely, but ensure the obvious.....are the plug wires hooked up in the correct order from the cap to each plug?

If you get nothing on that compression check here's an idea....

So it runs rough on idle, surges, backfires, all that, right? Does the exaust smell extra rich? You might have a fuel injector that is dribbling (not every exactly shutting off). Is it really hard to start up in the morning? Also, do a fuel pressure check, it should be around 30 psi. Chekc the pressure while idling, then shut it down and see the pressure. Whatever the pressure of the fuel rail when you just shut down, remember that number. It should hold that pressure for awhile. If that pressure drops down (say goes from 30 - 5 in a couple hours) and you smell no gas under the hood you have a couple options, either an internally leaking injector or a bad fuel return valve.

Sequoia - Oh, and with your symptoms if you are losing pressure I'd lean toward the injector.
Backfiring seems to happen with a lean condition. The rough idle, backfiring on accel. and the miles of vacuum line on an XJ seem to point toward a bad vacuum leak. I would check the line from the CCV to the intake, the fittings to the vac. canister in the bumper,to the axle vac. can and all those other numerous locations. If I am wrong about the lean condition and you see black smoke, check the vac. line to the map sensor on the driver's side firewall. It has a little black painted metal "roof" over it. A disconnected Map sensor makes you idle rich and stumble. If it is a stock 89 it does have an EGR. Look on the side of the intake facing toward the driver's fender about midway lengthwise the intake. I would check those connections. You should be able to check the basic function by blipping the throttle and feeling if the diaphram in the EGR moves.
Distributor Cap, Rotor, and wires. That's all it took. That was the first thing I checked they all looked good. Yesterday when we were doing the compression test one of them just cracked and fell apart. It was very brittle. That was probably the problem the whole time. Thanks for all the help.

XJ's RULE!!!!

BTW IT is a 91. No EGR.

Thanks again everyone.