XJS V-12 High rpms (3,500) in park!
 

Hello, I have been having issues with my 1992 Jaguar XJS-V12. When the engine is cold, it idles at around 1,200 to 800rpms in park and in drive at 750rpm. Once the engine warms up to operating temperatures, the car will idle in park between 2,700rpm and 3,500. In drive it will idle at 2,000, not pressing the gas. I have a local mechanic who could take a look at it for me, but I’m thinking it would be a smarter idea to take it to a Jaguar dealership, since they’ll have the correct diagnostic tool. Two issues I have with doing this are these, one the price, and second I would have to drive the car an hour and a half to the closest dealership. Frankly, I would rather fix the issue myself, but I have little knowledge with car engines. (though I would be willing to give it a try) I’m more of a diesel tractor and small gasoline engine kind of guy. Any and all help is appreciated!
https://cimg2.ibsrv.net/gimg/www.jag...1de694e71.jpeg

Sounds like you have a massive air leak. A systematic approach is best here. 1200 is about right cold, 750 or so is about right hot, just for reference. Start with the throttle blades. Remove the air filter and pop the rods off the arm attached to the blade. Look inside and see if there is any black gunk or oil. Clean this with a solvent, don't scape it. Then set the stop position to 0.002" of clearance. Then adjust the throttle rods to make sure they're not holding the throttle open. After that's been done on both sides, you're on to checking rubber hoses. Your idle speed is so high I think you should start with the big ones. The small right angle one on the AAV, right hand side rear, they crack and let air in. The two on the balance pipe at the rear, there's another one on the crankcase breather. Actually, the whole breather is suspect. Then on to the smaller vacuum lines. Replace them one at a time, because diagrams are basically non existent. Once you get through all of that, there is an idle control screw, basically just a bolt, on the side of the AAV. Don't be tempted to crank it down until you've checked all the possible sources of air leaks.
You may also have a stuck AAV. They're rebuildable or replaceable. It is a valve, the Auxiliary Air Valve, that lets in extra air when the engine is cold and closes gradually as it warms up. The more I think about it, the more that's what I think is going on here.

YES an air leak some where!

AAV and air leak first - beware of those rpm they're going to destroy that auto trans in no time at all it isn't designed to tolerate that without moving and it sure won't like being whacked into drive at those rpm either

A jaguar dealer today with the right tools and skills - wouldn't be holding my breath on that one.

Find this on the back of the engine - this is the AAV and they can stick open, if you're RPM are good cold but not hot this is the place to start.

Basic skills is enough to do a lot of repairs on these cars. One of the great things about old Jags is the internet support is top notch. Many here can and will coach you along.

A lot of dealers (and independents, for that matter) won't touch an old XJS. They're like a dinosaur to them. They may not have anyone who knows anything about 'em.

Learning to do as much DIY as you can on these cars is the best bet.

Cheers
DD

OK, you are NEW, and it appears somewhere in the US of A.

Welcome to the Forums.

To assist YOU a tad.
Your V12 is known in Jag Talk as a Mareli car. This is the engine management system installed. The earlier cars, up to about?? March 1989 we Lucas Cars, very different in lots of odd ways. NOTHING scary, just different.

I would be VERY cautious using that car in that state. As mentioned, thw Tarns will HATE you. The rear axle/suspension/cradle will HATE you. They simply are not designed for that potential abuse.

The AAV, already mentioned is way up there above #1 on my suspect list. I have attached a Doco of the very simple rebuild that anyone can do.

Check the throttle cable adjustment, at the centre capstan, maybe "Freddie Fiddler" has played, and it is wound up too tight. 10 seconds to check and adjust.

Read the Stickies at the top of this XJS Section, a good light read and getting to know the ins and outs of the V12.

They are NOT complicated, and only made that way by overthinking.

Ask if unsure of anything, we dont bite.