I've read a number of threads on this annoying and elusive problem, but none seem to fully reflect my issue and symptoms. Trying not to make this to long, but want to provide as much pertinent information as possible.

THE XJ BEAST IN QUESTION: 1999 Jeep Cherokee, 6 cylinder, 4wd
About 6 months ago I sold my 2000 XJ with 386,000 miles (reliable but had it issues) and bought a 1999 with 269,000. The 99 had been sitting for over a year, but started right up and had a rough idle from the start.

WORK TO DATE:

• New rear brakes (not idle related...more a stopping related thing)
• New exhaust manifold (major leak) & manifold gasket.
• New Cap, Rotor, Plugs (copper), Wires (old cap had deposits on pickups)
• Removed and cleaned throttle body and IAC (but made matters worse)
• Replaced IAC and throttle body gasket (used gasket sealer)

CURRENT SYMPTOMS:

1. Starts without a problem, never failed to start.
2. Rough idle that fluctuates a bit between 750-1000 RPM, you can almost hear the engine trying to adjust by adding fuel then leaning it out.
3. When coming to a stop the idle sometimes drops to 250 RPM to almost stall, and stay that way even if I shift it into neutral, until the car comes to a "physical stop", where it usually jump back up to 750 RPM.
4. Plugs are black (usually a sign of a rich mixture).
5. If I "stomp on it" is kicks down and takes off, blowing out some black smoke (clearing out the carbon buildup.
6. Since the tuneup (wires, cap, plugs, IAC) when I accelerate it sometimes stumbles once I get up to and try to maintain steady highway speed.
7. Accelerates smooth and with power if I jump on it, but stumbles/hesitates if I accelerate gradually.
8. Drinks fuel! Normally I can get about 290-310 miles per tank (depending on seasonal blend), but right now I'm lucky to get 180.
9. Sometimes it will run "pretty good" with minimal stumbling, until about 40 minutes to an hour, then idle issue will worsen some.

OTHER POSSIBLE CULPRITS:

• O2 sensors (two pre-cat, two post-cat)
• TPS
• MAP
• Camshaft Position Sensor
• Crankshaft Position Sensor
• TCM (Ouch $$)
• Transmission (God I hope not...)

MY THOUGHTS-NEXT STEP:

• Possible bad injector (been using cleaner with no success), Acts like one for the idle issue anyway, and the fuel smell and horrid gas mileage leads me to think it is one. And if I'm going to change an injector I want to change the fuel rail as it's looking a little corroded and worry injector work will damage it.
• May be a heat soak issue at some level, especially with the old fuel rail and small crack in fuel injector plastic outer sleeve (on the one I can see anyway).
• Also think this is not a single issue but a combination....but I could be wrong. I'm and mechanical designer, so have some technical savvy, but I am NOT a car guru by any sense of the word (i.e. not my forte').