I have been reading threads regarding idle issues and how they may be caused by sensors/electrical issues so I've secured a multimeter to try to troubleshoot my issue. My idle will sometimes stumble as much as 2-300 rpm. It happens when its cold but can stall the car out when hot and sitting in gear at a light. it seems to come and go and isn't on a regular schedule.

I've uploaded a video to youtube of my tach as the car sits in park idling.

Car is warming up but not hot yet in this. Would anyone be willing to take a look and offer a guess as to if this could be a sensor or something else? I've had this to multiple garages with no luck...

 

Three points:


1. Idle speed a bit fast at 1000.


2. Ignition irregularity usually not as smooth a drop in RPM's. But, it is an area worth checking out.


3. Yes, temp sensors a definite possible. others know far more in this area than I.


4. Dirty throttle body. That causes a lot of irregularities. clean ones do a lot.


5. And, an almost Jaguar and other car mantra. Clean tight grounds. there are a lot of them!!!


6. Worn and thusly wobbly distributor shaft bushings. Resulting in wandering dwell. Time for that old dwell meter ???


Carl

 

Thanks so much. I'll start with the throttle body I guess and hope I don't have to run too far through this list. Much appreciated

 

My car does that when warming up, I would suspect the AAV in this case.

I wouldn't worry about item 6 being a 1992 it will be Marelli and dwell is controlled by the ignition computer and with 70k miles the dissy should be ok.

 

Update: I cleaned the throttle bodies, which were filthy, and when I started it cold the idle was even higher, 1500 rpm. I decided that if it was the AAV I might be able to adjust the screw on the side of it. I turned it in and the idle dropped to 1000 and started cycling from 1000 rpm, up to 1500, down to 1000, every few seconds over and over until I let the screw back out. After that I let the car warm up to operating temperature.

At that point it idled as it did in the video, right around 1000 rpm with the occasional stumble and jumps. Interestingly, I tried to turn the screw in again and now it seems to have no effect, I can tighten that screw as far as I can and the idle just stays at 1000. I am assuming this means the AAV is bad and will need to be replaced?

 

You can test that by blocking off the hose to the AAV. Remove aircleaner cover (L/H) and plug the hole on the backplate, this will prevent any bypass air reaching the AAV and should result in a drop in rpm.

 

I did this and didn't get the drop in RPM. I've read the article on jag-lovers and in the XJS help book and it seems if this doesn't result in a drop in RPM my problem is probably not the AAV. Still, does it make any sense that my AAV can be good but my idle is still at 1500 RPM cold and 1000 RPM warm with the idle adjust screw turned all the way in? And it still stumbles and jumps as well?

Am I back to checking electrical connections and sensors?

 

It can't rev without air so if you blocked the AAV feed (it wasn't the breather hole you blocked was it?) and it was still high it is getting air from somewhere. Are the throttle plates and idle switch set correctly?

 

I blocked the hole that goes to a hose that leads to the AAV, not the bigger hole with the throttle body in it. I think that's the right one? I'll admit I am learning as I go here. There was a high pitched whistling from that hole before I blocked it that went away when I did.

Does this seem like it could be a vacuum leak from elsewhere then?

I am not sure how to check if the throttle plates and idle switch are set correctly, but I will look into it.

Thanks

 

If the AAV is blocked off and the revs are still high, you definitely have too much air getting into the engine. The Great Palm's book explains clearly how to set the throttle butterflies. The other airleak possibilities are huge. You just have to go extremely carefully round each of the manifolds checking that all the myriad hoses and blanking pieces are sound. Do not forget to look under the throttle body part of the manifold. Basically this means pulling off pipe or blanking tube one at a time and covering the hole with your thumb until you find one that reduces the revs.
My money is on the throttle butterflies being set too far open! The high pitched whistling you heard was probably air being sucked through the dizzy air circulation system which joins the AAV rubber pipe.
Greg

 

When i first got my XJS i had a problem with the idle changing as the engine warmed up so i started looking for vaccum leaks with no luck, one morning i started the engine and the idle started going up and down like crazy after some time and lots of reading up i couldnt work out why so like you i turned to this forum.

After a few replys one member asked if i had taken off the air inlet filter box while looking for leaks, as it happened i had the day before and didnt think this would make any difference but this turned out to be the problem with the eratic idle.

The screw holes holding the filter box on are not blank but open through to the intake and allow loads of air in, putting the air boxes back on stopped the idle jumping up and down.

This might not be your issue but i thought it worth a post as i was going crazy trying to sort out mine. As it turns out my idle control valve is stuck as well and this is why my idle changed depending on the engine temp.

Being hit up the rear twice in 6 months and having to deal with insurance companies has stopped me from being able to work on the car this summer and the weather has now turned and forces another wait for spring to continue any more work

 

Wrong engine, but I had a vacuum leak at my intake manifold. I was not able to detect it with the carb cleaner spray trick, but my mechanic sure did.

It was the third mechanic I took it to. For some reason the first two could not find the leak. The local mechanic didn't charge me a dime. The second one ended up replacing a whole bunch of other parts, that probably were imminent for change anyways.

The third was the charm. (Last two were Jag specialists, DC seems to have two in the area)

 

did you check and see if idle switch is working?
it's on the drivers side throttle arm. could not be sticking it in idle mode. I had that issue on my xj12. I needed to adjust it just slightly for it to engage. that stopped my slightly high idle and hunting.

 

I'll go with Greg on this, a leak or miss adjusted TB's.

Check the bushings in the throttle linkages are in good condition. These are available and replace any that are loose or missing. If they haven't been looked at they are most likely all missing.

Follow Palm's guide for adjusting the TB butterfly's.

On the Marelli car there are a couple pipes under the (from memory) RH TB with blanking pipes connected, make sure there are no open pipes under either TB.

 

1000 rpm and no effect on idle when adjusting the screw for idle

hmmmm... screams Vacuum Leak...

and the drop in RPMs... check the Distributor ROTOR if it has a cheap one they have a habit of breaking down and the spark jumps right through the center... to the distributor shaft... it might be starting to break down... I carry a spare in the glove box... ( have not had to use it yet but my XR5 did that and stranded me on the way to a wedding, had it fixed in 20 minutes when I figured it out.. )