Hello All.
Need help on my ’89 XJS high idle issue.
First, a little background..
I recently purchased this muchly neglected 59,000 mile Arizona car and have been picking through issues one by one.


Initially the car ran rough, literally shook at idle, and would pop through the intakes when throttle was applied.
First things first. In effort to eliminate the rough running and popping, (these things are probably not related to high idle) I replaced ignition module & jumper harness, distributor vacuum advance, plugs, plug wires, had the injectors cleaned and flow tested, replaced the engine compartment fuel lines, fuel pressure regulators, fuel pump, fuel filter, cleaned fuel tank and sump, replaced various vacuum lines, broken wires and connectors.
Now it fires up quickly, runs smooth, has better power, idles rock solid, but at 1000 rpm when warm. Idle would not drop below 1000, unless of course when it banged into gear and I held the brake, then idle would drop to 900 or so.


After reading through many threads about high idle, I pulled the auxiliary air valve and had it rebuilt by the guy who advertises on Ebay.
After almost a month wait, and no explanation as to if the valve actually was bad, I finally got it back. Bolted it on the car (with a new elbow hose) and fired it up. The engine immediately soared to 1800 rpm, slowly came down then started to oscillate rpm wildly. I noticed the adjustment screw on the side of the newly rebuilt valve was backed almost completly out, finger tight at best. I ended up screwing it all the way in and only then did idle come down. However, it came down to a rock solid 1200. Higher than it was before.
As aside, it appears that the expert valve rebuilder used vice grips on the head of the screw and buggered up head of said screw.
Pulled the screw, applied thread sealer in hopes that’ll seal up any air leaks and help it from backing out on its own. Probably more money shot in the rear.
So, I’ve resolved to re-investigate potential vacuum leaks by sorting through the miles of vacuum lines, myriad of switches, valves and solenoids.
Anyone have any other ideas or suggestions?

 

Are the throttle plates adjusted to .002" and throttle bores clean?

Linkages synchronized?

Linkage bushings in good shape?

That's all good stuff to take care of and might solve the problem.

Also check for air leaks past the bolts that hold the air cleaner housings to the throttle body. (If you wanna know what a high idle is *really* like, just remove one of the bolts entirely!! )

But.....

All that said.....

I gotta wonder if the AAV was assembled in such a way that, when 'closed', too much of the air slot remains exposed. Hard to explain without a picture.

More later.

others will chime in and maybe have time to post a picture

Cheers
DD

 

I agree with Doug, the first thing to consider is making certain that your throttle plates are set to 0.02".

If your AAV was stuck, the prior owner or a mechanic may have opened up the throttle plates to deal with a low cold idle or misfiring. You may have some other air leakage or vacuum issue, but you'll want to make certain that your baseline is correct before searching those out.

Good luck!

 

Good advice guys.
Forgot to mention in my first post, I did check the air cleaner bolts and made sure they were tight.
I did have missing throttle shaft bushings so I put in grommets, which seem to work good.
I did spray some throttle body cleaner into the throttle bores and wiped them down.
BUT! I did not check throttle plate adjustment or synchronize the linkages.
I will research how to do this, make adjustments and report back.


Admittedly, I'm learning as I go since this is my first Jag and first time I put my hands on a V12!


Expanding on the suggestion that the auxiliary air valve rebuild may be out of range, using this theory, I completely blocked off the inlet to the valve and idle dropped to 900! This supports the idea that the rebuild wasn't correct, but as suggested, getting a baseline is the next logical step.


Again thanks for the quick replies!

 

There is a thorough how-to for adjusting the throttle plates and linkage in the Kirby Palm book. If you haven't downloaded it, you should. Tons of great info.

 

Adjusting the throttle plates is almost a first stab at a high idle. I actually used carburetor cleaner while running, and located a couple of vacuum leaks. The rubber hoses that join the metal balance pipe may be bad? Cracked from age? I also found that the small plastic elbow that connects the vent on the distributor to the same hose that feeds the AAV was completely cooked away. That didn't affect idle, but was one hell of a hissing sound I had to track down.

 

Hello All.


I followed the great advice here and adjusted the throttle plates, as outlined in Kirby Palm's book.
I'm happy to report that when warm, it will now idle at a much more reasonable 800 RPM (indicated)! Thanks Guys!

When started cold, it'll first go from about 1000 rpm and quickly build up to about 1200. It will stay at roughly 1200 RPM until the engine warms up, with the temp gauge getting close to "N". Then it will settle down to 800 or so RPM.
Is this normal operation?


Also, I noticed that if I free rev the engine when the engine is cold, it will pop through the intakes. Makes me wonder if I didn't synchronize the linkage correctly or set the throttle blades correctly?