I'm having a random misfire at idle. The plugs are relatively new, wires are new, fuel pressure regulator is new. I'm going to pull the plugs and check the gap and inspect them. I have no codes. Any ideas?

 

how have you determined it IS a random missfire?

 

Because it misses at idle.

 

Have you done a compression check of all 6 cylinders lately?

 

Post the year, the engine, and any engine codes pulled from the computer.

 

I have not done a compression test because I'm not getting any signs of headgasket, head or block failure. (cracked, warped or blown headgasket.)

 

'92, 4.0, I have not checked for codes because my check engine light is not on. Yes, the CEL bulb is still in tact as well.

 

Still check for codes, despite no light. This is not giving you a run around, it's because it still might come up with something that might give you a clue. If not, well, then we proceed with the next.

When new plugs and leads, did you do a new distributor/rotor too, by any chance?

(and its spelled misfire not miss..

 

I did not replace the distributor or rotor with the plugs and wires. It ONLY misses at idle. It will also stumble when I give it gas from a stop or if I totally get off the gas pedal and press it again. As soon as the rpms rise above idle rpm, it has no miss at all.

 

How old is your fuel filter?
What I would do (good maintenance anyway): check for codes, clean IAC, throttle body/valve and take fuel pressure readings.
You can go ahead and finish your tune-up with a distr cap and rotor, and have that ticked of the list.

 

Air filter? Brand new
Fuel filter? Newer (a few months old at the most)

 

Tune up not complete on a 92 XJ until distributor cap and rotor are replaced.

Not saying that is your problem. But I am saying that you pick on low hanging fruit like this stuff to eliminate them as possibilities; especially so if they are due for replacement. Don't want to be chasing your tail for about $15 worth of parts.

It's a good idea to replace copper plugs every 30k, I go 60k on my wires, cap and rotor.

 

thanks for the smart *** reply...

I was asking so I already knew what troubleshooting you had already completed to come to that conclusion. maybe it isn't random cylinders.. maybe its only in one cylinder at random times.. this would lead to a possible bad contact in the cap and rotor

compression check...

anything under a load?

thats cool... good luck!

 

I wasn't trying to be a smart ***. I was driving down snow covered roads, trying to reply. I only have a misfire/stumble at idle. I have yet to notice any sort of problem under a load. I have not done a compression check YET. My problem is although random, happening multiple times. It will idle fine and then "cam". Then as I press the gas pedal, the Jeep will stumble and occasionally die.

 

sorry.. i misread and jumped to conclusions.. I'm the ***

anyway.. what roler suggested is a great start... i was feeling throttle body and IAC as well... if this was my jeep.. this would be my start..

And since it is only at idle.. but runs great with throttle applied.. its a good place to start

As was mentioned earlier.. cap and rotor are a must..

So... good wire contacts.. good plugs with proper gap, quality cap and rotor..

a bottle of strong injector cleaner couldnt hurt either.. i like BG44K... but its harder to get a hold of.. seafoam, berryman b-12, red line, chevron techron, all great products to get your hands on at any parts store..

the 4.0 has a "choppy" idle in my experience anyways.. mine runs smooth as silk down the road.. but kind of burps here and there at idle.. you can feel it when your sitting in the drivers seat.. nothing like a "cam" though.. as you mentioned.. thats not normal

a weak motor mount can also give you the sensation of a misfire is some instances.. but then again... doesnt really apply to a "cam" idle

do you have a fuel pressure tester to rule out the fuel feed as a problem?