fluctuating erratic idle

[ChipsXJ]

88 4.0 xj auto

idles at approx 750 rpm. the idles up to about 900 rpm then back down to 750, then back to 900. cycle repeats itself, and does it if I hold the throttle up (ie hold throttle at 1000 rpm, it will idle up to approx 1150 and continue that cycle)

replaced iac
replaced tps and adjusted to .829
d/c o2 made no change
replaced spark plugs

any ideas what else it maybe?

thanks

[Lou]

Had you tried measuring the voltage at the ECU(rather than the TPS) while this occurs? Measuring with the probes at ECU ground, and signal points.

[old_man]

I've seen this happen several times on different rigs. It has variously been a vacuum leak, a bad engine ground, a bad battery cable, and an intermittent C101 connector.

[dyna]

intake mani gasket leak or loose nuts/bolts.

[4xBob]

If you have air conditioning, then it cycles up the idle speed when the compressor cuts in. Since the compressor is also used on the defrost setting, it happens there too. Irritating- I hold the jeep with just enough brake to stop it, the AC cuts in, and it surges ahead a bit. I need a heavier brake foot.