I recently purchased a 1988 XJS with the V-12. The car appears to run fine but the idle is to high. 1,100 to 1,200. I believe is should be around 600-700. I notice a mechanical adjustment at the side and rear of each cylinder bank but I'd rather not make adjustments until I know for sure what I'm doing.
Any help appreciated.
Ken

 

Welcome to the forum Ken,

I've moved your question from General Tech Help to XJS forum. This is the place to post technical questions about your model.

Graham

 

Welcome to the elite Ken.
The idle speed on the V12 is adjusted and controlled by an Auxiliary Air Valve (sometimes called the Extra Air Valve). Do not touch the throttle linkage to adjust the idle until you know the AAV is working correctly. The AAV is situated just to the rear of the B bank inlet manifold, just after it, and is bolted to the head, with short right-angled 1 inch diameter rubber pipe leading to the end of the manifold. It is item 6 in this diagram:

On the inlet piece of the AAV (near what is labelled as 10) you will see a large nut. This is the idle speed adjuster, believe it or not! Screw it in, once the engine is warmed up, to slow the idle.
BUT, it is almost certain that yours is jammed, as they all do jam. It is not the adjusting screw that jams, but the mechanical part of the valve which is meant to gradually close off the air source as the engine warms up. New ones are available, and it is possible to rebuild them yourself, post for help on that.

 

Hi Ken

Here is a Photo of the AAV to go with 'Gregs' diagram



The AAV Auxiliary Air Valve on a V12 XJS showing the Adjusting Screw

If the Car hasn't been used for a while, then as 'Greg' said they gum up but the good news is that once you have done a few journeys, they can also free themselves off and start working again

 

Hi Greg Thank you for the quick response.
I found the adjustment screw. I just applied some WD-40 to the threads. I'll warm up the engine tomorrow and attempt to adjust. How much would I expect to turn to notice a change in the RPM? 1/8-1/4-1/2?
If the screw will not turn in would you suggest turning it out then back in? The car has been driven on 4 occasions about 50 miles since coming out of Winter storage. What is the target RPM for idle?
​​​​​​​Regards Ken

 

As mentioned, it's NOT the adjuster screw that jams but the internal mechanical components.

 

Correct.
Turn the adjuster screw all the way in. target idle is about 700 rpm in neutral, engine fully warmed up. If the screw will not go in, undo it and try again. IF it is all the way in and the idle does not change, then the unit is jammed open (very usual). You can try to rebuild it or buy a new one.
See post 9 in this thread for Grant Francis' rebuild document.
https://www.jaguarforums.com/forum/x...ebuild-119510/

 

Hi Greg
I have good news. After turning the bolt in one ratchet click at a time (total turn about one) I was able to drop the RPM from about 1,200 down to about 800.. I decided to leave it there for now till I drive the car more. Total mileage on the car is 72,300, but only about 75 after a 6 month storage. After a couple hundred miles I will try to drop the idle to 700 as you suggested. I was prepared to purchase a new Aux-air valve but it looks as if a have a reprieve. I'll keep you posted if anything changes. Thank you again for all your help.
Regards
Ken

 

I had the opportunity to drive my XJS today and the idle RPM is down to 700 without further adjustments. I did notice however the engine missing some on slow acceleration. May have to do some highway speeds then see what happens.

 

Ken
Have you changed the plugs, the HT leads, R&R'd the dizzy, checked the two coils are OK, cleaned out the fuel system etc etc?