90 4.0 running bad

[xj8_8]

My 90 XJ 4.0 idle will go up to about 1200 rpm and stay there until I shut off the engine and restart it then it will idle normal for a while, and slowly goes back up to 1200 rpm. It also acts like it is loading up for a few seconds when you are driving and let it return to an idle but smooths back out. At other times it has a off idle hesitation. It does not do these things while cold, only after it warms up. Here is a list of what I have already changed. Could it be the ECM if so where is it located on a 90? Thanks to any one who can help, I need to get this fixed..

cap and rotor
plug wires
MAP sensor
complete throttle body w/TPS and ICS

 

[ehingerce]

Sounds like the same prob. I had with mine. Took it to the dealer, he scanned it, bad O2 sensor humm!! who would have thought?

Good luck

 

[4xBob]

You state that it does not do it when it is cold - that's a clue. When cold, the engine runs in "open loop" which is programmed settings, rather than reading sensors. When the engine warms up, it switches over to using sensor inputs to regulate function - and your problems start. This suggests that it is a sensor problem. When I have had flakely high idle on my 1990 4.0, it has always been the TPS (must be adjusted to spec for 4.0 engines up to 1990). Could also be a vauum leak, or IAC getting weird.

 

[xj8_8]

I have not changed the O2 sensor yet but I have considered it. I changed the whole vacuum harness on the engine and the complete throttle body all the parts came off same year that runs fine. May have to try O2. Thanks,any more ideas??