Excessive vibrating while idleing and driving.

CHOMPERZ

MEMBER
1984 JEEP CHEROKEE
2.8L
V6
4WD
AUTOMATIC

I changed my motor mounts this weekend. The old mounts were not connected to the rubber anymore. I got the new ones on and while I was bolting the one on the passenger side down I noticed that I had to force it to go into its place but only by about 1/4 of an inch if that. Now that the new mounts have been installed the Jeep is vibrating throughout the entire vehicle both while idleing and when driving it gets worse. I have checked my fluids, all read good. I checked to make sure my spark plugs were connected in proper sequence. Ive come to a point where I am unsure as what else it could be. I have searched all day up until this point. My brain is tired beyond belief. ANY help would be appreciated. I am in an area where I have no family and my friend that I know can only help me out very little.

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

Are you sure you're not overlooking a misfire? There's hundreds of pounds of weight on engine mounts, so prying one a little to seat it is common and doesn't cause a problem. Other than a misfire, there's two things that will cause a vibration. That's an imbalance with a rotating part, or two metal pieces rubbing together that transmit normal engine vibration into the body.

First feel the engine while it's idling, then while it's running at around 1500 rpm. If it feels smooth but you're feeling a high-frequency vibration in the seat or steering wheel, look for the two metal halves of an engine mount that are touching, or more commonly, an exhaust pipe that's rubbing on its hanger or touching the body.

If you feel a definite vibration on the engine, at idle, a misfire will be felt once for every two crankshaft rotations. Rotational imbalance will occur once or two times per crankshaft revolution. Given the recent service, my first though is did you raise the engine by jacking on the vibration damper? If so, it is likely the bond broke between the inner hub and the outer cast ring. At least twice per revolution the belts will pull up on the crankshaft pulley. Normally that makes a knocking noise, but if the outer ring slips, it can become out-of-balance. It depends on how the damper was made. Your engine is "externally balanced", meaning part of the crankshaft's counterweights couldn't be cast as part of the assembly, so small areas of additional weight were added to the vibration damper and flex plate. On most vibration dampers that weight is added to the hub, but if it was added to the outer ring, if that ring slips, the engine will have a very noticeable vibration. The pulleys on your engine bolt to the inner hub so there will likely be no noise if the bond is broken between the hub and ring.

The inner hub can break too from being used to jack up the engine. I had one like that with a miserable vibration. The overlooked clue was the center bolt kept coming loose, then it would make that double knocking noise. After retightening that bolt four or five times, we finally pulled the entire damper off and found the part that slides over the crankshaft snout was broken into about five pieces.