Stumble in all weather and misfire when humid/raining. Out of ideas.
 

1998 Jeep Cherokee, 4.0, automatic, np242 transfer case. I will do the best I can do describe the issue. I've tried taking videos but can't capture it well enough with the camera.

The Jeep has a stumble when it's idling. Barely shows up on the tach but you can definitely feel it through the seats. When it's raining outside, anywhere after 5-30 minutes of driving, the Jeep begins to shake and misfire. I can hear it backfiring through the intake under throttle (sounds like a pop pop noise). There is virtually no power or response from the motor until I floor it, and even then it takes a few seconds for it to clear up before I take off. It idles around 100 rpm and lurches up to 900 or so before falling back down to 100 when I am at a stop. When cruising and it starts to happen, I can put the pedal maybe 3/4 the way down to maintain some speed. The engine runs for a few seconds, loses power for a few seconds, and then runs again (sounds very close to a steam engine with the "chug chug" sound). I assume it is not getting spark because if it were getting fuel - even just a little bit - I would be getting some bad vibrations instead of nothing at all. This cycle repeats until I floor it so it clears up. I can restart the engine if I'm at a stop and it will run fine for maybe 15 seconds before it begins to happen again.

Here is a list of everything I have done in the past 6 months in no particular order:
- cleaned grounds
- cleaned battery terminals
- cleaned throttle body
- new ignition coil
- new brass cap
- new brass rotor
- new plugs wires
- new spark plugs gapped to 0.035"
- new Bosch 4 hole injectors
- new Mopar idle air control valve (this needed to be replaced anyway because the old one was seized - buy once, cry once!)
- new CCV elbows and tubing
- thorough inspection of all hoses and tubing
- checked for vac leaks with carb cleaner
- smoke test (made a smoke machine and did it myself by pressurizing it with an air compressor. Nothing professional but I think it would have caught anything major.)
- fuel pressure test: 49 PSI @ idle
- vacuum gauge reads (15 in hg hot idle, but it rapidly fluctuates. I assume due to the stumble when idling.)

I'll edit this list if I can think of anything later on. Just can't figure out what's going on. Might go buy a new crankshaft position sensor since that's the last ignition component I can think of that could be misbehaving. Thanks in advance for any comments.

Engine codes?

Do you have a splash shield?

No codes and the Jeep doesn't have a spash gaurd.

Check the connection into the coil. Might be flaky.

I will check that later today and report back what I find.

All good there.

Was it doing this prior to replacing all those parts?
I have a '98 and '99 and have ended up having random problems depending on the brand of coils and plugs.

I replaced a coil with an Echlin from NAPA. It was fine for a few months and then started giving me random power loss, erratic idle, especially when it got hot. I have had pretty good experience with other echlin parts, but definitely not with that coil. I had changed the original coil becuase it was old. I put the original back in and it's been fine ever since. That was a couple of years ago.

I also made the mistake of using Autolite plugs one time. HAd super erratic idle and lot's of missing at idle. I never guessed it could be brand new plugs. Went back to Champion and all the idle problems went away.

Yeah...No. Autolite plugs don't tend to work right in Mopars. Even as far back as the 50's they don't work. Our rule for plugs was AC in Chevys, Champs in Mopars, and Autolite in Fords. Problem is they were cheaper and more available, so guys used them.

I can't recall what plugs are in mine but I'm going to have to deal with this issue later as a more serious issue has surfaced.