1991 Jeep Cherokee Trembling

February 21, 2007 12:44 AM

My 1991 Jeep Cherokee automatic seems to be "trembling" while it idles. The shaking decreases once it's moving, but persists- albeit in a much reduced state. On top of that, the engine just sounds like it's working harder than it used to. Help!

I recently had to have the steering dampener rod replaced because it blew out- causing the car to shake uncontrollably at the slightest bump on the road over speeds of 35-40 mph... this shaking threw off my alignment, so I got that fixed (new tires and everything)- and I think that shaking might be behind this recent "trembling." There's also sometimes a "thunk" noise when the car starts moving from a stop. I took the thing to the mechanic, who -strangely enough- said there was nothing wrong with it.

after turning on the car, the driver side starts to tremble- enough to slightly jingle my keys in the starter and make the image in the driver side mirror blurry. The passenger side is much less effected-- it feel like its coming from just behind and under the steering wheel. On top of that, the engine just seems to be louder and making a bigger fuss when revving up from a stop, and I feel like it's giving less power than it used to. Once the car starts moving, it lessens significantly, but you can still feel/hear it slightly.

Am I being paranoid, or does this sound like a problem to anyone else?

posted by Waffle_King to grab bag (7 comments total)

A steering damper/suspension issue shouldn't have caused any problem with your engine. That said, you have a 16+ year old vehicle. It's high time to be replacing rubber parts. One thing you might have checked is that things like the motor mounts, and transmission mount/vibration damper are in good shape. Basically, the motor mounts are somewhat flexible rubber/metal plate bushings that attach the motor to the frame of the vehicle, while providing some vibration absorption and compliance for torque change absorption. Motor mounts do "wear" in that the synthetic rubber they're composed of breaks down over time, with heat, oxidation, and mechancial stress. This will happen more quickly in a vehicle which has been used extensively off-road, where the forces the mounts are resisting are incrementally greater.

You could/should also have the engine ignition system checked, as various parts in the ignition system like spark plug wires, distributor rotor and cap, and spark coil could be contributing to mis-fire of one or more cylinders, leading to a rough idle condition. Most of these parts should be inspected/tested or replaced during a tune up, but verify that they will be replaced with who ever tunes your vehicle (as many shops charge extra for parts that need to be replaced). Your vehicle would probably also benefit from several tanks of Shell V-power or Citgo Techron premium gas being put through it, as these fuels have 5x the normal amount of detergent additive, and can go a long way to cleaning up dirty fuel system parts and intake valves.
posted by paulsc at 1:00 AM on February 21, 2007

I had a somewhat similar symptoms on my 96 Jeep Grand Cherokee (noticable low idle shake, seemingly labored engine below 30-40 mph, sometimes stalling at idle) and it turned out being something I think was called the 'transfer case', some sort of gear box between the engine and transmission.
posted by Kensational at 5:45 AM on February 21, 2007

My '96 Peugeot 106 does this on and off. Having the spark plugs cleaned/replaced seemed to help one time, but it comes and goes seemingly randomly. Better petrol seems to help too, but it could be confirmation bias.
posted by corvine at 6:07 AM on February 21, 2007

Ten bucks says you're losing compression in one of the cylinders. If you had a manual, you might also notice it can't quite hold itself on a hill in first.
posted by floam at 6:46 AM on February 21, 2007

Yeah, check the motor mounts, and then replace the spark plugs, the plug wires, the cap and the rotor, and also put some fuel injector cleaner in your tank for the next fill-up.

The motor mounts are hard, but the other stuff is easy to do... borrow the right size plug wrench, a gapper and a screwdriver and you're done in 20 minutes. Just make sure to keep track of which plug wire goes to which hole on the cap.

It shouldn't be the transfer case, as that's actually between the transmission and the rear differential. At idle, there shouldn't be anything moving in the transfer case at all. It's got two drive shafts comming out of it, one goes to the rear axle, and the other goes to the front axle. It's how the Jeep gets power to all four wheels - it takes the output from the transmission and powers the driveshafts with them (or just one driveshaft, to the rear wheels, if 4WD is off.)
posted by Slap*Happy at 6:58 AM on February 21, 2007

A vacuum leak can cause this symptom. Look for a disconnected or broken vacuum hose.
posted by Kirth Gerson at 7:46 AM on February 21, 2007

I agree with the others that it might be time to check motor mounts and give your cherokee a tune up. One other cause could be something loose in the exhaust system. Vibrations can be hard to pinpoint sometimes.

The 'thunk' you hear could be any number of things, among them: The slip yoke in your driveshaft binding, then releasing suddenly when you start moving (grease it where it leaves the transfer case); worn, or incorrectly-installed rear brake shoes moving around in the drums; or worst case scenario, your transmission (I'm assuming this is the AW4 auto trans) is not dropping back down to the first gear until you start moving.

Mine's 10 years younger, and does the same stuff - I just ignore it. Probably at my peril...
posted by M.C. Lo-Carb! at 11:46 AM on February 21, 2007