Chasing an engine shaking problem

[SurplusFan98]

Chasing some engine shaking issues on my 98 with the 4.0. At first it was severe and I was sure it was a motor mount. Found a disintegrated one on the driver's side, and replaced it with a used but good mount. On next run, the shaking was better, but didn't go away completely. I have other XJs in the family, and this is worse than any of them, so definitely not normal. The passenger mount looks good. I don't know how to inspect the trans mount without taking all that apart, but I'll do it if I have to.

When looking around for other sources, I noticed some wobble in my harmonic balancer. I tried to wiggle it with the engine off to see if it was separated, but couldn't. I can hear a little bit of of pulley chirp after I first start the engine, but can't tell where its coming from. Goes away after about a minute. Could this be the source of the shaking that I can feel in the cab?

Other things I've checked:

• Engine idles fine at the correct RPM. If there's a misfire, I can't see it in the tach.
• Serpentine is fully tightened. Checked it with a cricket tool.

Engine has 250k miles but seems to run pretty well.

 

[RCP Phx]

If you have any suspicion of the HB I would replace it. You should be able to do a valid inspection on it knowing the usual signs. Replace all the mounts with new stock ones but do the Window Weld mod.

 

[Tim_MN]

I would install new motor mounts and a new transmission mount. Consider upgrading to aftermarket. Then I would make sure all the tune-up parts are fresh and functioning correctly, including the coil. I would add a bottle of Techron injector cleaner to 1/2 a tank of gas.

The most likely root cause of the shaking is an engine mis-fire. The harmonic balance does not have enough mass to cause a noticeable shake.

 

[lawsoncl]

Probably not the source of the shake, but yes that balancer looks like it's wobbling. The outer portion should be even with the inner portion.

 

[Lyman]

Mine had a weird shake at idle and also a little stumble at high speed. The o2 sensors were originals at 185,000 miles so I figured they were getting lazy. I replaced both upstream and now have a smooth idle and the stumble at speed is gone too.