Hey, tried a search but I didn't find anything that fixed it so far, so hoping someone here has had the same problem and found the cause.

1992 XJ 4x4 4.0

I have an intermittant misfire on cylinder 3 (counting back from the front). It's only happening on the one cylinder. Seems like it's better when accelerating hard, but cruising on the freeway or at idle and slow accel it's a real pain. So far I've replaced the spark plug and swapped injectors but no help. I listened to the injector with a stethescope and it's opening regularly even during the miss. I pulled the plug and visually verified spark. Also hooked a timing light on it and it is receiving a spark regularly during misfire. I'm pretty sure my compression is okay since when it does fire I can feel a noticeable lurch forward as I receive the extra power. Jeep ran fine three days ago (other than running rough for a minute from a cold start if it had sat all night) and then this started. I just can't figure out why it's doing it. It acts the same hot or cold. Haven't changed anything lately that should have started this. What are some things I should try or any solutions others have found?

 

Spark plug wire and distributor cap must be ruled in or out next. They can be responsible for a single cylinder misfire.

If still misfiring after you try above, run a compression test on all cylinders and see if this one is low.

 

X2^^

 

You said "I pulled the plug and visually verified spark. Also hooked a timing light on it and it is receiving a spark regularly during misfire."
Do I read this right? I'd try a new spark plug

 

After checking to see if the plug was sparking, I did re-install it.

Later on in the day I replaced it with a different plug. Still no change.

On my way to work this morning, it backfired and started running normal again afterwards but I can here a little clatter up there, so I'm thinking something having to do with the valves?

I'll try and get a compression test done today but I don't know if I'll have time.

 

I'd swap plug wires and distributor cap before performing a compression test.

Simple stuff first is always the best strategy, and failing to eliminate the above from the process of elimination here wouldn't be wise.

 

Negative on the wires or dist cap.

Found an old guy with a Sun oscilloscope thing. Looks like the inside of a 1970's era sci-fy movie. Dials and gauges and weird old-tv style screens with squiggly lines and *****. He says I'm sparking and pulsing when I'm supposed to, and told me my intake is probably sticking open. Thinking maybe I could pull my valve cover off and see it if it is?

Still haven't gotten to the compression test though. Mostly cause I don't have a compression tester and I'm broke. I think my brother has one, so maybe i can get up there today and borrow it.

 

Well, it's confirmed. Apparently somehow I have a most-of-the-time sticking intake valve. Just thought I'd put that up in case someone else is having the problem. Found a guy with a head for $40 that might work. Figure I'll slap that on and see what happens. Hopefully it'll last long enough to get this one machined.