I've had this issue for several years. Some time ago, while driving my '96 XJS, the idle suddenly rose to around 2k rpm, and stayed there. Long story short, I traced the problem to the Idle Air Control Valve. I had the ECM checked - no issues. Replacing the engine temperature sensor mitigated the problem for a short time. Also, replacing the Idle Air Control with a new unit did not help the problem. My solution has been to do a manual reset of the valve then simply disconnect it. Problem solved (sort of).

I'd like to really solve the issue. If I reconnect it, sometimes the idle just rises and won't return to normal, sometimes the idle fluctuates. I am sure this is a wiring problem. Can anyone point me in the right direction?

Idle Air Control - disconnected.

 

Have you ever had someone reset the TPS? That has typically cured all idle issues for me.

For some reason, any time I monkey around with IACV, a TPS reset is needed.

Lastly, I assume you have checked for air leaks. There's a gasket and an o-ring on that IACV, but since you've already replaced it, those should be fine.

 

Thanks for your suggestion... I will look into that! AIR LEAKS: yeah, I have been fastidious about that. I have stethoscoped the IACV when connected. It will hold the idle well when the car is first started, but starts to go whacky (I can hear the stepper working overtime) when the throttle is applied. So, your comment about the TPS is well noted.