EDIT: Problem fixed. See #6 below.

1999 XJ 4.0 4-dr 4WD. Engine is bone stock with 167,000 miles.

Symptoms: Cranks very strong for as long as I turn the key. At first, it wouldn't start, but after fix #2, this was resolved. Once started, idles rough/misfires and misfires during mild/hard acceleration, bordering on complete lack of power. No Check Engine Light. No scan codes (see below).

What I've done so far:

1. Checked fuel pressure. Tests indicate approx. 38-39psi with engine off, key to RUN. Pressure drops slowly with key to OFF, down to approx 30psi after two minutes. I don't have a reading with the engine running as it wouldn't start on the day I rented the pressure tester.
2. Re-gapped plugs to .035". Engine started right up! This seemed to clear it up immediately, but it lasted only a day and the problems returned, worse than before.
3. Cleaned Idle Air Control (IAC) Valve. No change. Engine starts, but idles rough and accelerates poorly/misfires.
4. Replaced distributor cap and rotor. Again, issues went away immediately, then returned. Engine starts, but idles rough and accelerates poorly/misfires.
5. Replaced spark plugs (Bosch Platinum) and wires. Problem completely resolved ... until the engine warmed up on a 10-mile drive and I was luckily to get home. It was running so rough that I thought the engine was coming apart and had barely enough power to accelerate up to highway speed. Idled very rough and needed to be feathered while stopped in traffic. Finally saw some scan codes after this drive: P0300-Random/Multiple Cylinder Misfire Detected, P0301-Cylinder 1 Misfire Detected, P0304-Cylinder 4 Misfire Detected. Cleared codes and ran engine. Idled extremely rough and eventually stalled after two minutes, but did not immediately throw any new codes.
6. REPLACED ORIGINAL MOPAR IGNITION COIL. This seems to have fixed it. After replacing the part, I drove it about 15 miles. No hiccups or stumbles of any kind, from cold engine to hot. Time will tell, but I'm hopeful this is it. I will update if the problem recurs.

Recent history:
May 2019 @140K miles: New Mopar crank position sensor (after multiple non-Mopar parts failed in short order)
June 2020 @150K miles: New Mopar camshaft position sensor/distributor pick-up coil (after multiple non-Mopar parts failed in short order)
November 2022 @165K miles: New Duracell battery

Any clues/suggestions? I realize this issue or ones like it have been addressed MANY times in these forums, but the answers and explanations are never clear-cut.

I'm getting a little tired of throwing money at it and replacing parts that clearly are not malfunctioning. I spent the money on Mopar parts a few years ago in hopes that they would last longer than the cheap aftermarket parts that I've tried. If this is the best I can hope for (under three years and less than 30K miles of service), what's the point in spending the extra money?

 

Have you tried a new coil ? it could be going bad

 

The coil was replaced with a Mopar part in June 2020.

EDIT: Sorry. I thought you meant the distributor pickup coil. No, I have not replaced the ignition coil. I suppose I could throw another hundred bucks at that, since it is still the original part as far as I know.

 

Probably seems obvious, but the ECU can be funny. You probably already tried resetting it though?
I would try sensors such as the TPS or the MAP. Most of my problems with rough idles have been sensor issues. For your model they are both right by the IAC you replaced on the throttle body.

 

Problem solved with a new Mopar ignition coil. See original post for details.