I was running my 2000 XJ in the driveway to warm up the other day and noticed a very rough idle. I let it warm up to operating temperature and within 30 seconds, it stalled out. Tac drops and surges and when it got warmed up, the voltage started dropping and slightly surging with the tac. No check engine light and when i plugged in a reader I get no codes and no misfires. I looked on these and other forums and tried some things:

1. Cleaned the throttle body and IAC (these were pretty clean, as I've cleaned them a few months ago as part of a tune up)
2. Checked the O2 sensor fuse (it was fine)
3. Replaced the coil pack
4. Oil change (I needed an oil change anyway, and when i checked the oil I noticed it was a little low, so I figurd low oil could be giving me a rough idle.
5. Put some heat shield tape on the fuel injectors

After all these things, I ran the truck for a while to see how it was idling. Still rough, but not as rough as before. I drove it around for a bit and it was completely fine in drive and reverse. Got back home, put it in park and let it idle a bit, and it was still rough. By now the engine was approaching operating temp and the surging and dropping was getting worse. Saw the voltage start going up and down too and gave it a few minutes, but it never stalled this time.

So basically, I don't know where I should go from here... Crank position sensor? Fuel pump? Any advice or information is greatly appreciated! Thanks.

 

I am havinf very similar issues with a 2000 cherokee 4.0L.

Idles great when cold, but after reaching operating temp, definitely idles rough.

The misfire goes away just off idle and it runs with very good power. I have 2 of these, this one is stock, the other has 4.10 geaqr, 4 1/2" lift, 32's, new engine. The stocker engine runs better except at idle after warmed up.

I have done injectors, plugs, coil pack, Lucas fuel system clean, verified voltage to each injector.

165,000 miles, doesn't knock unusually....

suggestions from the experts?

 

This a shot in the dark but have you reset the PCMs memory for fuel/air?

It doesn't always work but it surely did help my rough idle