My 1990 Jeep Cherokee Limited idle speed fluctuates up and down..when I am driving at cruising speed the car begins to shake. When I am driving on the freeway above 55mph the car shakes constantly. I recently replaced the alternator, had a tune-up, replaced the radiator, replaced the water pump, and had a new computer installed. Any ideas on what could be wrong?

The most likely cause is a bad throttle position sensor (TPS). It's located on the throttle body. With the engine idling, grab the connector on it and move it up and down. If the idle fluctuates it is for sure bad. Otherwise you will need a scanner to properly diagnose your problem.

Sounds like a fuel filter. Question - When was last healthy tune up? Plugs, wires, All filters especially fuel filter? I'd start there and then run a good fuel system cleaner through it. Ut'll give you a good "ground zero" point if problem continues.

mine also will quit going down the road, and idles high at times, or idles low, like having a mine of it's own. possibly idle air speed solenoid (which i replaced on mine) and still does this. another tech said only factory solenoid will "talk" to ecm properly?

hey there, artismo here! I have a similar problem with the idle fluctuating, and the trans down shifting itself at light throttle, which is the lock-up in the converter getting the wrong signal ? from the ECM.

check the relays under the hood and all fuses

try the tps.worked for my 89

Sounds like insufficient fuel delivery. Likely problem is the fuel filter is plugged.

DROP DOWN YOUR OIL PAN BY UNBOLTING IT THEN TWO MACHINE BOLTS HOLD THE PUMP UP TO THE MOTOR.THIS IS A EASY FIX YOU CAN DO IT.

Since you have a Laredo this should be where it is going in and out of overdrive. The reason is because 60mph is right at the end of the power range before going into overdrive. The other reason on your idle fluctuation is due to the cooling fan kicking on and off. Higher RPM's occur as the engine is trying to cool back down after reaching optimal temp. I due have a partial solution if you want better performance out of your jeep and will help MPG. You can replace the spark plugs and air filter for both mpg and performance. Local auto store should be able to recommend platnium or iridium plugs for your jeep. But K and N is the best air filter to replace it with. It will cost around 50 dollars but it has a lifetime warranty and is cleanable. If you decide on new plugs make sure to spend a few extra bucks and get some with a 3 to 5 year warranty. Just replace them before the warranty is up. Stephen

check the ...PVC valve and the MAF sensor.