96Co

First of its a 96 xj 4.0 with 80000 on the ol girl. The problem is the entire jeep shakes when its cold or hot and in drive, reverse, park and neutral. Things I have done so far. New fuel and air filters. New motor mounts and harmonic balancer had some wobble to it. Cleaned TB and IAC, changed the ccv elbows and hoses. Everything was either worn out and dirty but nothing helped. It idles at just under 1000 a bit high. I looked over the spark plugs and cap and rotor they looked fine but might replace them just in case. There is no check engine light or codes. Could it be O2 sensors? Cap and rotor/spark plugs? What else could it be?

andrewmp6

Does the engine shake at all rpms or a small range?

Klutch

The indicators strongly suggest a vacuum leak. Make sure your intake/exhaust manifold bolts are properly torqued and the gasket is in good shape. Check all vacuum hoses and the PCV. You know, the usual stuff.

tlang

80k? Man I wish my 2000 was that young at heart!

I like the idea of replacing the cap, rotor, wires and plugs mainly because it cant hurt. However a missfire wouldnt bring the idle up like that.

I like the vacuum leak thought. Do you have a vacuum gauge? Normal for a warm, proper running engine at idle should be between 20-24 in but one of the key things to look for is if that needle on the gauge is steady. If your engine is obviously runnung poorly the gauge might not be the best bet. You can either spray water or some sort of alternate fuel around the intake and all vacuum lines. If the idle increases or steadies out you have found the general area of the vacuum leak.

My apologies if this is all stuff you already know.

Good luck!

96Co

it only shakes at idle as soon as I give it gas it seems to smooth out. Is the best way to check for a vacuum leak is a smoke test? And were would I get a vacuum gauge?

tlang

Yes the most effective way to check for a vacuum leak is to use a smoke machine. Do I have one at the shop? Yes. Do I always drag it out and use it to check for leaks no not really. I generally use the propane or brake clean method. Please be careful I DO NOT recommend using the brake clean method. You can easily start a fire.

How rough are we talking? Like real rough or just annoying at a light?
Have you checked the tranny mount?
My first thought was motor mounts but you addressed that.
You can pick up a gauge at HF, they are a great diagnostic tool if you do a bit of reading up and experimenting.
What does it idle at when it is rough?

kdot

Possible torq conv that is warped? I had that issue when installing remanned renix, it would shake the entire jeep violently. but smooth out past 3k RPM's

96Co

bad enough that it isn't normal. It idles at just a hair under 1000 and that's when its at its worse. As soon as I touch the gas it smooths out and runs fine.

tlang

Ok well You can start by doing a power balance test. Unplug each injector while at idle and see which cylinder affects the least. Only unplug one at a time. Then take that info and narrow your search. I would first check the plug from the suspected cylinder they can tell you a lot about whats going on inside during combustion.

Do you have a compression gauge?

It could be a weak valve spring, carbon build up, vacuum leak or even spark leak. These are just the top few that come to mind.

kdot

+1

Vac Leak
Dead Cyl test
Power balance check

96Co

so just did the power balance test the first cylinder had no change. I couldn't get the 3rd off so I will check that tomorrow. When I unplugged the 5th It died is that normal? Another weird thing is I took the wire off the first cylinder and no change in rpm but the distributor started ticking even after I pluged it back in and then when I went to shut it off it was running at 750 instead of 1000 then I shut it off and started it was back at 1000. Thoughts?

freegdr

Plugs, cap and wires were last replaced ?

96Co

couldn't tell you, it was done by previous owner. I have only put two thousand miles on it. The wires look pretty new. I'm going to run to the auto parts store this morning and pick up all the stuff for it just to be safe.

tlang

The ticking in the dizzy could be spark jumping around or a bad bearing. Technically speaking, disconnecting a fuel injector should have no affect on the ignition. The spark does not care if there is fuel in there at all.

Do you have any codes or a check engine light on?

I am very curious to see what the #3 plug looks like, also I would want to know what the inside of that cap and rotor look like.

As for disconnecting #5 and it dying? Im not sure, from my experience that is not normal. My only thought is that you have more than one cylinder failing and when you pulled that one you then had three miss firing and it died, but this theory does not really fit.

What is your coolant level like?

As far as it idling at 750 then back to normal after starting, the only thing I can think of is something like what ford does. If they have a missfire in a cylinder the computer shuts off fuel to that cylinder until the code is cleared. The only purpose for this is to protect the cat. That is the name of the with OBDII. Im not sure on the logic for the cherokee. Im a just a technician that has been doing this for almost 20 years and I happen to love my cherokees!

Any good test is only such because it is repeatable. I try not to head to far down a diagnostic path if I cant reproduce the results from a given test.

Good luck andlet us know what you find, im now very intrigued.

96Co

I replaced the spark plugs this morning. All of them looked good and not really any different then the new ones but replaced them anyways. It still is running rough but idle is running right around 800 rpm. I wasn't going to spend 60 on a cap and rotor at the store so I just ordered one on line for much cheaper. I'm going to take it to the shop next week and see what they can tell me.