I seem to having more that one problem with my 98 cherokee 4.0 I have posted in the Grand Cherokee forum but have had no responses. It has a bouncing idle from 1200 to about 800 RPM and every few minutes the rpm needle jumps really sharply about 800 rpm and back almost instantaneously , and sounds like its cutting out or stuttering while you rev it. I have checked for vacuum leaks and found none. There are no codes present however I cannot connect my scanner while the car is running. I have cleaned IAC and throttle body checked map sensor vacuum line, plugs look very clean, porcelain on them is still white and no signs of fouling at all. Wires almost new cap and rotor also, coil shocked the crap out of me so I think thats good. 40 psi fuel with key on and around 45 to 50 while running needle bounces very rapidly. TPS while unplugged will not throw a CEL ? Has .84 volts in idle and 4.84 at WOT I have unplugged the upstream o2 and no difference, took temp readings at cat. After about ten minutes at idle i read 435 pre cat about an inch and an inch post cat read the same 435. Please help fixing any one of these problems. Thank you.

 

Sounds like a vacuum leak... You can rent a vacuum leak tester from a parts store

 

I checked all the lines that I could see and sprayed a whole can of starting fluid around the intake gasket and every vacuum line connection I could see. Only ones I cant see seem to go under or behind the drivers head light and one under the battery tray somewhere ?

 

Could be a cracked spark plug

 

If a spark plug were cracked would it show different color electrode and porcelain ? All the spark plugs looked the same when I pulled them ?

 

Ok sorry for the late response but here is a video of the vacuum test. 98 cherokee - YouTube

 

Here is a fuel and tachometer video alsohttps://www.youtube.com/watch?v=nKm0mB16YM0