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Bumpy Idle, ways to narrow it down?


NAXJA Forum User
Thanks for coming to commiserate over engine troubles with me. I appreciate your help.

My poor lemon is a 1995 XJ 4.0 Automatic, and it idles like it has indigestion. While the engine is running, it feels like it "bumps" of stumbles a little inconsistently and shakes the cabin a whole bunch. The tach says it's idling at a bit below 1500 rpm, but if I throw a digital timing light on a plug it says my actual idle is around half that, between 720 - 740rpm. Odd, but probably a bad tach. Coincidence that the tach is showing almost exactly double rpm?

Either way, I just rebuilt the head and had the valves done. Compression is good. The coil, plugs, wires, TPS, IAC, Cap and rotor have been replaced while I was in denial over having a bad head gasket. I have checked extremely thoroughly for vacuum leaks and was unable to find anything. I did find that I had fuel pressure that was a bit high. I replaced the FPR and verified it was at 30psi at idle, and 38 psi with vacuum disconnected. After the engine is turned off, fuel pressure will slowly creep to 38 psi over the course of 2 minutes or so. I assume that's just the manifold slowly loosing vacuum, right?

Engine mounts have been replaced. Maybe that was a bad idea, they're not very soft.

Im really just looking to bounce some ideas off people and make sure I'm not seeing anything weird. I think the most likely causes may be the crank position sensor or the pickup coil in the distributor? I saw a guide on testing the crank sensor but nothing helpful for the pickup coil.

Anyways, let me know if there's any more information I can gather. I appreciate the help!
Probably due for new o2 sensor and check the wiring while you are at it. Speaking of wiring, double check the looms on the back of the engine, worn wires can cause weird problems. I had a wire wore through behind the intake and it would make the engine shut off whenever I put it in reverse.
The Jeep in-line 6 cylinder engines have never been known for their smooth idle. Fuel injection improves the lumpy idle, but you should see/feel the idle my CJ-7 has.

If the tune-up parts are fresh, the coil tests good, it runs well without stalling, and the gas mileage is in spec, it is fine.
Make sure the basics, plugs and wires and cap and rotor are in good shape.
unplug the o2 sensor and see if it runs better.