Q: Engine vibrates at idle

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After starting my Grand Cherokee, it idles out to about 700 RPM. At that point, a pulsating vibration can be felt and heard in the front end of the vehicle, steering wheel, seat, etc. It goes in and out, but is consistently there when idling. If I rev RPM's just a tad, the vibration goes away. It does not matter whether the AC compressor is running or not. Motor runs good and I don't believe that's the issue. Any ideas?

My car has 44000 miles.
My car has an automatic transmission.

A: Hi there, thanks for writing in. I'd be...

Hi there, thanks for writing in. I'd be happy to offer my insight.

I'm curious as to your vehicle's motor size. There are four different motors available in your Cherokee, each with their own set of issues.

I would say that 700 rpm is to low of an idle. It is often not easy to find idle speed specifications on vehicles with a computer that controls the idle, and because I am uncertain of the motor you have, I can't confirm. In my experience, I would say 800-900 would be normal. Keep in mind, tachometers are not intended to be completely accurate.

Vibrations like this are normal when the idle speed is too low. Every car on the road will feel like it is going to rattle itself apart with a low idle. But, let's assume that there is another problem aside from a low idle. This could be a variety of things. I would first suspect a problem with an injector firing or a spark problem. Figuring this out is an advanced diagnostic that involves some creative thinking. The methods for figuring this out are dependent on the vehicle being worked on. The first step is to observe the misfire counters on a scanner. Keep in mind, a code reader from your local auto parts store is not the same thing as a scanner. If the PCM is recording misfires, we can isolate the problem to a specific cylinder and focus on it.

Depending on the motor, you can also test the sensors and test for vacuum leaks, each of which should begin by studying scan tool data. If you would like help, consider having an expert automotive technician from YourMechanic come to your home or office to inspect and diagnose this issue for you, and make any repairs as needed.

Hope this helps and best of luck!

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