So.. recently acquired a 96 XJS Conv with 43,400 miles. Having and issue with fast idle (cold start or warm) About 1500 rpm Seller noted that common cause was idle control or stepper valve. I replaced it today and the idle initially jumped to 2800 rpm After driving settled down to 1600 rpm. I briefly obtained a normal 700 to 800 rpm idle after removing the stepper valve, cranked the ignition which activated the stepper valve motor. Reinstalled the stepper, and reset the system by battery disconnect. But, the next time i started the car and drove, high idle has returned 1600 in park, and up to 2800 in neutral while driving. Suggestions? Seller insists it has to be stepper valve and possibly may need to re-flash the computer

 

A high idle is usually caused by a vacuum leak. That is pretty much all the stepper valve is. On my 97 XJR, the high idle was caused by a binding throttle cable, nothing else.

 

It's a well known issue with the AJ16 engine. What you need is a Throttle Adaptation, and you'll need dealer level software to do that. See this thread for an example: https://www.jaguarforums.com/forum/x...-death-179174/

 

Jaguar issued several TSBs for you to consider.

bob

 

Take it to any shop with Jaguar diagnostic software, they'll reset the idle for you.

Should cost you about one hour's labor, even though they do it in under 20 minutes.

 

thanks for the help 👍

 

Ive already thrown all the parts you can throw and was unable to correct the high idle. A trip to the shop had it good to go in no time. I've posted much of my investigations here. If you want to search and read, go ahead, but spoiler alert, a WDS/IDS/PDU was always the solution.

 

Agree with those above.

I'm afraid it's a settings issue and can only be correct by a "vintage" Jaguar diagnostic unit. The one I'm aware of is the Jaguar PDU unit. Although I think there are others.

The modern equipment is not compatible with these cars so not all dealers will have one.

Via one of those units it's literately a half hour job.

My dealer doesn't have one so they wanted to replace the throttle body, stepper motor, and most of the other parts at a cost of nearly £1000. Pretty unimpressed by their service, lol

So I just live with it. Car idles at 1,600 at cold and drops to around 800-1,000 once warm. I can live with that.

Annoyingly I had the issue previously and someone at the garage did "reset" it for me with no issues, but this time around, either I'm getting the wrong mechanic or someone who doesn't know anything about XJS's but it wasn't worth pursuing.

If I'm in the UK at some point in the future I'll try and book an appointment with a XJS specialist to get it sorted but take it from me, it's not mechanical. It's electronic.

 

The Autologic unit can do the necessary calibration.