95 shaking at idle

It's probably been covered before but search yielded nothing solid. 95 xj 4.0 sport 4x4 2 door. 178,600 on it with original motor. All of the obvious has been done. Motor mounts and trans mount newer. Plugs are newer and I pulled them out yesterday and they all looked good and gapped at .035. Cap, rotor and wires are newer. I change the oil every 1500 miles with 10w30. No odd noises or misfiring. Runs great but shakes at idle through the body and the steering wheel. Harmonic balancer looks good. Once I get on the gas it goes away completely. I have heard this is normal for this motor. No CEL or anything weird it just bugs the hell out of me. You can even see my 33's shaking at idle. I have had it for 2 years and now I'm just tired of it. Disconnected the 02 sensor and it idles the same. Absolutely no vacuum leaks. APN header. I loosened the motor mount bolts and rocked the engine then tightened them back up thinking the motor mount rubber was just tight from the PO install. The mounts are good trust me. Any help would be appreciated. Thanks in advance.

Our 97 idles rough, as you describe. I've checked compression on each cylinder, all good and well within spec. Have cleaned the throttle body, changed IAC, the coil, wires, distributer module. The next item on my list is to change out the injectors. Seems to be a difficult issue to resolve.

The Jeep in-line 6 cylinder was never known for its smooth idle. Yours is 19 years old. If it doesn't stall, it's fine.

You might try cleaning the IAC, also the
throttle body. I'm sure there's carbon buildup
after that many miles...

I've had a few things cause a rough idle, first is a dirty injector. Running a load of injector cleaner in with the fuel usually helps.

Vacuum leak, some of the vacuum connections are off center on the intake, So it is possible to get more air to some cylinders than others.

Weak spark, either all the cylinders or a couple, multiple reasons for this. Partially internally shorted coil, high resistance in the spark plug cables, low voltage going to the coil.

The hardest I had to nail down was the TPS ground. My guess is the high resistance in my ground circuit caused some timing issues.

What shape is your CAT in? A partially plugged CAT can cause this.

When my CPS was slowly failing the idle got bad before it eventually crapped out all together. It would miss a beat or two at low RPM's. It was likely missing a beat or two at higher RPM's also, but that was less noticeable.

Chasing idle gremlins can be really time consuming and frustrating, been there done that. Sometimes it is a combination of things. The I6 has the potential to idle really smoothly and is actually one of the smoother configurations in motor design. But everything has to be right before this happens.

I clean the throttle body every oil change. Did it thoroughly when I had the intake off to do the header. Did the IAC and lubed it. I soaked the injectors in fuel system cleaner when I had the intake off. Replaced the injector o rings. Put a cat on it last year because it failed smog and then passed no problem after. I do notice that when I drive it about 2 minutes and do a hard pull then come to a stop I can smell the exhaust from the back smelling kinda like burnt hair. I'm just not the guy to blow money on parts if I don't know the source of the problem. I will find out what is causing the problem. Definately not a vacuum leak. I'm a mechanic at a local dealer and know quite a bit, not everything though. If your not learning something new everyday then what's the point of living. Definately some good info on here I'm just not too familiar with these motors temperaments. I have not changed the coil and it needs to be done anyway so I will start with that. CPS meaning cam position sensor or crank position sensor? I know crank position sensor as another abbreviation but everyone uses different ones apparently. Every once in awhile when the planets align it will get a fairly smooth in balance idle.

wschurfeld said:

I clean the throttle body every oil change. Did it thoroughly when I had the intake off to do the header. Did the IAC and lubed it. I soaked the injectors in fuel system cleaner when I had the intake off. Replaced the injector o rings. Put a cat on it last year because it failed smog and then passed no problem after. I do notice that when I drive it about 2 minutes and do a hard pull then come to a stop I can smell the exhaust from the back smelling kinda like burnt hair. I'm just not the guy to blow money on parts if I don't know the source of the problem. I will find out what is causing the problem. Definately not a vacuum leak. I'm a mechanic at a local dealer and know quite a bit, not everything though. If your not learning something new everyday then what's the point of living. Definately some good info on here I'm just not too familiar with these motors temperaments. I have not changed the coil and it needs to be done anyway so I will start with that. CPS meaning cam position sensor or crank position sensor? I know crank position sensor as another abbreviation but everyone uses different ones apparently. Every once in awhile when the planets align it will get a fairly smooth in balance idle.

Click to expand...

I meant crank position sensor when I used CPS, I usually write out the complete "Cam Position Senor" when I mean the other CPS. I'm really a Renix person (I have a lot more experience in the early years than the later models) and have carried over many of the terms.

My 96 had a nasty idle and occasional serious random ignition issues. I thought the Crank Position Senor was failing again. It turned out that the Crank Position Sensor was covered in oil from a leaky valve cover gasket and that was causing my issues. I cleaned the Crank Position Sensor ( and replaced the valve cover gasket) and had no trouble since. I have no idea why the Crank Position Sensor being oil soaked caused any issues, but it did.

A lot of things can affect idle, been my experience if your idle is good the motor most likely runs at (or near) top performance through the whole RPM band. A rough idle doesn't really mean much by itself, other than being annoying, but is an indicator of overall performance, including mileage.

My issue right now is I have a smooth idle but it does idle rich for some reason. This disappears when I reboot the computer, but returns after a few days. Happens on a cold and hot motor and I haven't a clue why. My mileage has gone way down.

Is your bad idle the same on a cold motor and a hot motor? If it is, I'd lean towards something mechanical rather than sensor related, ignition or injectors. The coils in the later models seem to be a bit temperature sensitive and generally work worse when hot while slowly failing. Sensors tend to have more input (influence) on hot motor management than on a cold motor.

Have you done a fuel pressure check yet. One of the first things I generally do, not because it is a common failure, but because it is easy to do and eliminates a number of possibilities right off the bat.

You really want to Ohm test the motor to battery ground (motor to firewall strap). The grounds at the ignition coil and the grounds next to the PCM. Been my experience ground issues can cause idle issues that may all but disappear at higher RPM's.

I just had a thought, have you looked at the tone ring? Pretty far down on my list, but something to think about.

The CPS wires cooking in the exhaust manifold or the front (injector) harness cooking on the exhaust manifold where it goes down the front of the motor? Another one of the first things I look for, just so I don't waste a whole day doing elaborate tests before I check the basics.

Thank you 8Mud I really appreciate the input. I did check the fuel pressure a while back because of a hard start issue with a new battery and it turned out being my battery terminals. No visual damage or wear to them but for some reason were causing a hard crank issue. I cleaned all the grounds with sandpaper at the body and connector a few months ago. It does seem to get a little better every time I change the oil. I guess I'm going to attribute it mostly to the motor being older than dust. Idle is a little better when it's cold but not by much. I will pull out the crank position sensor and inspect it. I did put a new VC gasket on awhile ago so I'm sure there is oil on it from the previous leak. It will have a really bad start up idle for a few seconds after a long drive but I kind of narrowed it down to my exhaust being a little short because of cutting it to fit my shackles. Probably almost causing a vapor lock style condition. I need to put a new muffler on it and dump the exhaust anyway, I just really haven't had the time or money lately to handle that. I have fixed endless issues with this thing since I've owned it and is probably an indicator of the PO's treatment of the vehicle. But overall the motor and trans are in good shape. I've done about 3 fuel injector cleanings with fuel lines disconnected and the special tool with shop air that we use here. It's never made any difference at all. I've run and still add Marvel Mystery Oil at every oil change and it definitely quiets down the top end a lot. I guess it's narrowed down to a sensor issue or just age and mileage. I really appreciate the input. This thing is going up for sale pretty soon anyway. I go through cars like clothes. I buy them, fix most of the issues then sell them on. I guess due to boredom with them. Was never really a Jeep fan but am glad to say I have owned one and love the 4x4. It goes anywhere and works well but it's not really my thing.