I have a 1999 4.0 and it has a mis fire at idle but gets smoother after you give it gas. Cap rotor plugs and wires have been changed and still does it. What could it be?

 

valves not sealing-probably due to carbon would be the next thing I would suspect. Look into sea foam. I don't know much about the older jeeps but if you can switch around the injectors maybe try that first.

 

I think iv seen that stuff. Im in california and when it was in for a smog it didnt pass at idle only

 

Check the cat converter. Soynds clogged

 

Just noticed I read the year wrong (thought 1990) but info still applies. I've got a 2000 and seafoam has fixed a few slight misfires for me in the past. Big thread on it around somewhere but basically you just put a nail hole in the cap and let it trickle in the throttle body while idleing. Assuming it runs fine off idle this might fix it. if not move to the injectors-could have a few clogged. Injectors are easy to move around but you probably should buy new o-rings to be safe.

 

Ill do that. Thanks

 

THINK!

If an injector was clogged, it would still be clogged above idle.
If the cast was plugged, same thing - above idle would be worse!

Sounds like a simple vacuum leak - blow cigarette smoke in a hose to the manifold, fix wherever it comes out!
SIMPLE, EASY, FREE!

 

So where does the smoke go in?

 

The thing is I created a vacume leak and it ran better.

 

Well turns out it blew the head gasket out between cylinders.

 

huh; 2 &4 are not adjacent... is it blown at cyl 3 as well?

I was gonna suggest a bad cap as 2&4 are right next to each other in the firing order hence on the cap.... possibly cross firing?

 

Going from the the frount to back the compression readings where 160- 160- 50- 60- 160- 160. The head gasket was blown out between the 50- & 60 cylinders.

 

those would be pistons 3 & 4...