rough idle

[JEEPINIT]

96 XJ 4.0 167k miles has a little rough idle. Runs great on the road. no codes. Also cuts out and sputters at 4100 rpms at wide open throttle only. Let up a little and she'll turn 5k on the tach. New Autolite platinum plugs, new cap and rotor and new 270ohm per foot summit plug wires. Timing chain stretch? Idle air controller carbon build up? I thank anybody with any ideas.

[jeepsrock]

If these parts havnt been replaced in a while then i would look into replacing the TPS and IAC motor. If they are new clean the throttle body.

pete

[8Mud]

Just before the CPS went out in the wifes 96, the idle got pretty bad and the ignition would break up some at higher RPM's.
Vacuum leaks can mess with your idle. The vacuum taps on the intake manifold are on one end or the other mostly, so you can end up with lean and rich cylinders. If the vacuum leak is small, the IAC will compensate some, so the idle isn't excessively high. If it's a large vacuum leak the idle will often be high. Vacuum leaks rarley cause very noticable problems at higher RPM's.
Uneven compression will also cause idle problems, unburnt fuel and/or excessive carbons can mess with your O2 sensors.
O2 sensor can mess with your idle and hurt your top end. One could be flacky enough to cause problems, but not bad enough to throw a code.
Low fuel pressure will also cause the problems you discribed. As will a partially plugged Cat. I had to replace the Cat on three XJ's an 87, 88 and a 96 all in the 130-160,00 mile range.

[dirtymuddog]

yeah, i just cut mine off. but we dont have pesky emmisions down here. the tps helped fix my idle.

[JEEPINIT]

Thanx guys. I'll dig out the multimeter this weekend