1998 Jeep Cherokee, 4.0 inline-six, AW4 automatic transmission.

My Jeep has had this strange idling issue ever since I got a few years ago. When initially started with the engine cold, it idles fine. However, once it fully warms up, there is a slight hiccup in the idle where it almost feels like a cylinder didn't fire or something along those lines. There is no pattern as it just happens randomly. It seems to get worse on rainy days. I was idling in the school pickup line to get my sister when the Jeep started to shake like it was trying to dance along to the radio. It was idling at about 250 rpm and lurching up to maybe 500 before it would quickly drop back down. Despite the idle being so low with the occasional backfire, it refused to die. Here is a video.

After sitting there for about 5 minutes with no change, I decided to kill the engine and restart it to see if that would resolve the issue. It did, for about 15 seconds, before it started to lurch and backfire again (only did maybe 3 times). Out of ideas, I put it into drive and went on my way. With my foot nearly on the floor, the engine refused to exceed 1800 RPM, almost as if that's where the rev limiter was. A few minutes later, the problem seemed to have suddenly fixed itself. I was able to drive for about 10 more minutes until I arrived home with no issues. I put the pedal down a few times and the engine seemed to have no lack of power. Once I arrived home, I let it idle to see if I could recreate the issue but had no luck. There was a slight hiccup as always but nothing more. I thought I smelled gas out by the fuel tank but it could have just been me. I rolled under there and looked around but did not see any leakage.

This is not the first time this has happened. I was thinking of either TPS or MAP sensor. I know it is not the spark plug wires as they are new.

Speaking of sensors, a faulty IAT can do that, too. When the engine is cold, the PCM adds fuel. As it warms up, it starts cutting fuel. If the IAT thinks the engine is cold even after its warmed up, the PCM dumps more fuel and the engine runs rich, and driveability suffers. It could be the other stuff acting up, but the IAT is just as important. Any codes? Even with no CEL, it is worth a look. A loose or dirty connections could be all it is. Or an exposed wire. Check the IAT, TPS, and MAP to see where that gets you. A cracked ignition coil can do it, and even a loose cap. I had that issue on older Fords when moisture got inside the cap. Right after or during rain. But that was back in the points and condensor days.

Basically, look at everything for anything suspicious. That is one of the first things i do.

First off, if you have a check engine light...have the computer scanned for codes with either a code reader or a scan tool. Post the EXACT codes(s) here for comment.

You mention spark plug wires are new. FINISH your tuneup with Champion copper plugs gapped to .035, quality premium distributor cap and rotor. Then see where you are at.

The importance of a full tuneup cannot be overstated with symptoms like you mentioned. Humidity/moisture can have a big negative effect on "secondary ignition" which means and includes all tuneup hardware like plugs/wires/cap/rotor.

Simple things should be first. Always. Plenty of time to ponder over more complex possibilities if these items don't help.

Good luck and keep us updated!

I've got no codes. Cleaned the IAC. Tested the map sesnor and it works. No cut wires or loose connections anywhere. Spak plugs are new as well and I have good spark going to all of then.

I figured out I can actually view O2 sensor readings from my OBD II reader app. I am able to get a reading from the bank one sensor but not from bank two (I assume this is the downstream sensor). Could this be one thing that is contributing to my troubles? I don't think it has anything to do with the rough idle in the rain but it might have something to do with my 12 mpg.

Edit: They are in the same bank and are both functioning so it is not the O2 sensor. I sprayed the cap n rotor with water to try to recreate the issue but was unable to. I think I'm going to order a new one anyway because I don't know the age of it.

Finally had time to replace the cap and rotor. The old ones had corrosion on them, but there is still no difference in the idle. Makes me think it's a sensor/electric faulure somewhere since spark pkugs, ignition wires, and cap and rotor are new.

Very cold and icy out today. Started the Jeep up to defrost the windows and it ran fine up until it reached operating temperature. Started idling too low and sputtering. Restarted it and it went away. Drove done the road for 15 seconds and it came back, heard it backfire through the intake a few times. Turned it off, turned it back on, no more problems. I notice the rpms drop fast when I let off the gas to slow down.

I'm at a loss here guys, and by the lack of replies since my updates I am assuming nobody here has any ideas. I may try replacing the throttle position sensor if I can find one that doesn't get awful reviews.