Misfire <1500 rpm
 

My 93 4.0L XJ has had a misfire for some time now. I started to notice that it only misses under 1500 rpm's and sometimes it will run fine for a couple of days and not miss at all so it is intermitent. have replaced the cap/rotor, plugs/wires, seafoam in brake booster line/in tank, checked all vacum lines and fixed any leaks and cleaned pcv. Any ideas? I also have a diesel like sound coming from the engine under load around 3-4000 rpm's. The sound comes and goes as the rpm's increase. Could be detination and or realated to the misfire problem I guess. :help:

Blown engine? Have you done a compression check?

Not blown... I drive it 60 miles a day 5 days a week. It has been very dependable it just has this misfire. I havent done a compression check but it is an intermitent problem so that kind of rules out alot of internal problems right?

Yes and NO

Does it miss when cold? any valvetrain noise? My jeep use to have a miss/pop due to a dead lifter...put in new lifters and its as calm as can be.

replaced O2 sensors lately?

No but if I unplug it nothing changes. Should it be changed?

Bump...

honestly don't know for certain...
I do know my rig had a fair amount of stumbeling/hesitation (even with new coil/cap/wires/plugs)... I put a new o2's on and the rig runs smooth from idle up to 4000rpm (typically never rev it over 2800).

If the sound under the hood is like hearing marbles being shaken in a tin can, you are likely experiencing detonation.
I suppose you could try to "seafoam" the rig... sometimes detonation can be caused from a lean condition...
Mine used to detonate under load (long/steep highway hill climbs) but no longer does (again, new cat and o2's.)

I seafoamed it already. Im the tank the throttle body and in the brake booster line. I guess I need to try an o2 sensor. Wouldnt a bad o2 make it run lean?

As for the cat I deleted it... :head:

Bump...

since you changed your plugs this could be the problem. one of my buddies was changing his plugs like a month ago and noticed that if was misfiring like 4 days later. so he had me look at it. The dumb *** was tightening down a plug and do you know the rubber washers on the plug sockets? well it was stuck to a plug lmao. check for that and check to make sure none of your wires are crossing.(crossed wires will cause a short circuit) i experienced that last night with my buddies yota. GL with the fix im sure is is something simple like that.

It was misfiring before I changed the plugs. And it doesnt happen all the time so it cant be a crossed wire.

Bump :head:

ok so i talked to my dad and he thinks it is one of two things. either a, when you changed your plugs you forgot to check your gap width, and the plugs before had the same problem. The other option is that your distributor cap is cracked.

another thing, any idea what cylinder is misfiring?

No i'm not sure what cylinder is misfiring. I believe the plugs are pre-gapped and the cap is new. The thing that throws me for a loop and everyone else is that it is an intermitent problem. Some times it will go for a few days running smooth and some days it misses worse than others.:wallbash: