Okay so since I've owned this jeep for a few months I've had a rough idle. The truck has never stalled or thrown a code for a misfire but that's what it almost feels like.

So far I have changed the plugs, wires, I have cleaned the entire throttle body including the iac, and tps sensors. The truck has new oil and a filter in it. I've ran fuel injector cleaner through it. Ive check for vacuum leaks with a smoke test. Nothing has made a difference. The cap looks fairly new and the previous owner had changed it recently. I've also changed the primary and secondary o2 sensors because i was getting a code for the secondary when I first got the truck..

Soooo... no codes, I hate throwing parts at a problem but I know in my mind they are fresh parts now anyways and should be changed.

This is the big hint to the problem though. When I pulled the plugs to replace them. I checked their gap and they were all the same at. 035 however they all had a different burn look to them. One looked lean. One looked rich. One appeared to have some detonation on it which is very bad... but what gives.

Id like to think the cap is bad and they are not all getting the proper spark but idk. Anyone else have this problem or have an idea?


Also the truck has zero drivability issues it runs like a too other then the idle problem.. no cutting out. Its great.

 

Maybe try pulling the plug wires one-at-a-time to see if theres any that aren't firing.

 

check the engine to chassis ground wire. Its on the far rear of the valve cover. Its a flat ribbon wire that goes from the rear of the valve cover to the chassis. Take it off and clean and sand iit down, and all contact points it makes. If this ground is dirty it can cause misfires, stalling, rough idle and all sorts of issues. Hope this helps.

 

Finish your tuneup. No matter what the cap looks like. You cannot see things like small cracks in this hardware. Install new cap and rotor. Gotta rule it out.

Simple stuff first. ALWAYS. You do not want to be chasing your tail for $20 worth of ignition parts.

If after that, you still have issues, then run a compression test on all cylinders. It is a good "snapshot in time" of the internal condition of the engine. The compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

 

Thanks for the info guys. I'll look at the ground and just ordered a new cap and rotor. I'll keep ya updated.

 

I have the same problem I have changed everything the only thing that I have found is the distributor shaft has some side to side movement in it could this be it?

 

Cap and rotor changed. The rough idle is still there... ignition coil?

 

I havent figured mine out either about to send it to the shop

 

Think I'm going to pull the injectors and fuel rail when I get off work and see if there is a clog somewhere.

Does anyone think j should replace my iac?