Start with a good tuneup and injector cleaning. A compression test would tell you the condition of th valves and rings.
The spark plug gap can cause the RPMs to fluctuate, while idling. The more probable cause of the fluctuating RPMs is a bad camshaft.
An obdII code reader will tell you why the light is on and might tell you what is wrong
The idle solonoid needs cleaning. Take it off of the throttle body and clean it and the inside of the throtle body itself
i would try a can of ether pray all vacuum lines one a time. when it ides up then know where the leak is.
It probably needs a tune up. My 1998 Mazda 626 was idling very rough, so I did a tune-up by replacing the spark plugs and wires. This made the engine run better, but the rought idling continued. I took it to my mechanic, and he found a vacuum leak somewhere, and $270 later, it idles smooth as glass
need to replace the intake manifold gasket
The "check engine" light should be illuminated. Start with investigating the code(s). They may lead you to the cause.
Loose motor mounts.
bad/partially blocked catalytic converter. - kind of a rotton egg smell, right?
There are many reasons why your car might vibrate while it is idling. It could be running too hard for example.
A car can shake because of a problem in the transmission although this usually is not the problem. I will assume it shakes while it is idling or just starting out. It can also shake due to broken motor or transmission mount(s), bad u joints, a loose harmonic balancer, an unbalanced driveshaft or problems in the rear differential.
Engine is idling too slow or the alternator is weak. Or battery is getting tired.
if while driving you need wheel alignment an/or wheel balancing. if it s just while idling it can be a few things like worn motor mounts clogged fuel injectors causing low or ramping rpms
Effectively all the fuel that it burns while idling. You are getting 0 miles per gallon while idling.
There are several things that can cause your Jeep to stall while idling. The timing may be off or your fuel filter may be clogged.
If the car dies while idling, the timing might be off. Other causes can include a clogged fuel filter or a bad spark plug.
The most likely cause is a faulty engine crankshaft position sensor.