I have been working on a 1990 Jaguar XJS Convertible v-12 that has an idle problem. It is a California car with CA smog equipment. It was sitting in a garage for the last 2 years. When it was started again about 5 months ago the V-12 started "hunting at idle." I have been working on it for the new owner and have not figured out the problem. The previous owner (a trusted family friend) said it ran fine up until storage. It now has fresh 91 octane gasoline.

The conditions which cause the idle problem are... The car is started cold in a 50-60 degree F garage. While warming up in "Park" it runs fine with 15" Hg. Vacuum. After 3 minutes and 20 seconds, while still in "Park" or "Neutral" that car will surge between 600 and 1500 RPM with a vacuum range of 5-10" Hg respectively. If shifted into Drive or Reverse the car will run fine (no surging) with 15" Hg vacuum again, so the problem exists only when in Park or Neutral. If the car is shut off and started again without cooling down it will have the same hunting problem. If left to cool off for several hours or overnight the hunting will take 3 minutes 20 seconds to start again.

I have been checking the various sensors (coolant, crankshaft, oxygen) with no problems. I have checked for vacuum leaks with a home-made smoke tester--no vacuum leaks.

Has anyone had similar symptoms? Any ideas?

Thanks for any help,

 

A couple likely candidates are a faulty AAV--Aux Air Valve

and/or incorrect throttle blade gaps (should be .002")

and/or dirty throttle bodies


Cheers
DD

 

I'd also want to be sure the idle switch on the LH throttle linkage is adjusted right, and the throttle potentiometer is not set too high at closed throttle.

Good luck!

 

+1 on the extra air valve. Take the left air filter out and you will see a opening where the hose from the aav hooks to the back side of the filter housing. When you start the car there will be alot of air thru the aav, then as it starts to warm up it should go down to nothing. When the car starts to surge at idle plug the hole and see if the idle drops and stabilizes. If you need to replace it, be sure and get a gasket also- the new part won't come with one.

 

Thank you for the suggestions.

I will take a look at that Aux. Air valve this weekend. The throttle bodies looked very clean, but I will clean them and check the throttle blade gaps. I did measure throttle position sensor with a scope and it was within specifications, but I will look at that idle switch again.

Thank you again for the help and ideas.

 

I'd check resistance on temperature sensor, the one that goes to ECU (left front bank area). If it warms up and goes open, you will have odd injection behavior.

 

Isn't there a high idle timer on these cars? That would explain the exact 3 min 20 sec delay prior to the problem.

 

Wondering if you ever found the problem?? I'm having the exact same issue.

 

I did solve the problem. It took me a couple weeks before I had time check all of your suggestions. I am sorry I did not post a response.

Thanks to your suggestions, the problem did turn out to be the Auxiliary Air Valve was stuck due to corrosion from not being driven for a couple years. I used this article for reference...
Jaguar Auxiliary Air Valve
and solved the problem easily.

Things I learned:
-The Auxiliary Air Valve CAN be removed without draining any coolant as stated in the article. I was sure coolant would leak, but it didn't.
-I tested the AAV on the stove in a pot of water and there was no movement at all--stuck due to corrosion.
-I soaked the piston with penetrating oil for several days. I still did not get movement.
-I then screwed in a long sheet metal screw into one of the holes that is visible on top of the piston. (If you look at the piston it has about 4-6 holes) Gently using a slide hammer the piston freed up with some more soaking. Then pliers worked for moving it until there was no resistance except the spring.
-I checked the AAV on the stove again and I could see it would allow too much air through, so I squeezed the bulb to move the piston higher in it's travel until I was happy with the result.
-I installed the AAV on the car. The car started at about 1200 RPM and then settled down to 650 RPM after a minute with a steady idle.

As preventative maintenance I thought it might be a good idea to pull off the top hose off the AAV every couple years and spray a little oil, just to keep it problem free.

Thank you to all that replied and I hope this will help the next person.

 

I completely rebuilt mine, using Volvo marine thermostat bulb. There are a complete unobrainium, so if yours fails, it is either used unit, or what I did.