1998 Other Jeep Models Car vibrates excessively at idle

V1ROTATE

MEMBER
1998 JEEP
V8
4WD
AUTOMATIC
84,000 MILES

I own a 98 Grand Cherokee w/ the 5.9L. Out of nowhere it started vibrating really bad throughout the rpm range. So I replaced both motor mounts and my transmission mount. This helped my problem big time as they were worn, but she still shakes at idle and only idle. I replaced cap, rotor, plugs, and wires--no help, also double checked firing order, all is good. I did a vacuum test and at idle I get a steady 17 inhg. I did a compression test and all cylinders read 150 to 160 psi. I used a stethoscope on the wiring to each injector and I could hear each one happily clicking away. I did a light cleaning of the throttle body and cleaned off the carbon on the IAC.

If you have any other ideas as to what could be causing my problem it would be extremely appreciated!

1 Reply

MERLIN2021

MECHANIC
17,250 POSTS

You may have a vacuum leak, get a can of spray throttle body cleaner, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! If that doesnt do it, you may need a new idle air control valve, it's bolted to the throttlebody.
IAC's dont clean up as a fix! If it's the problem, replacing it is the only fix.