I have found lots of info regarding high idle RPM, but very little about low idle, so I hope any here in the forum have some tips.

A bit history
The car is still at sellers garage, and I work on the car now and then to make it work so i can drive it back to my place.

what I've done.
Engine did not run on all cyls.
Done a compression check. (Lowest:11.4, highest: 13.2) and cleaning plugs. - No change

Made new injector harness w/new connectors.
- No change

Changed spark plugs.
- Dramatic improvement

It now runs nice and fine on all 12 cyls. (Plug cables are OK, and distributor cap seems to be almost new).

The only thing now is that the idle speed is about 550 rpm and up to 600 rpm, and drops to about 400 or so and sometimes nearly stops when put in drive or reverse.
There is no difference between cold and hot engine. I have tried to adjust the idle screw on the AAV several revs CCW, but no change.

Is there anything else to adjust?

oh, one little ting. When pressing the pedal, the engine RPM drops a bit before picking up rpm.

 

You can not tell CR from a compression pressure test, there are too many variables valve overlap, throttle position etc, what is important is that all cylinders are close in cranking pressure, and the cranking pressure will vary depending on the model of engine.

Make sure you have adjusted the throttle butterfly's correctly. Insert a 0.002" feeler gauge in TOP of the butterfly and adjust the stop screw to just touch the arm.

The AAV (Aux Air Valve) is down the back of the LH air filter there is a screw on the lower front - this is the idle adjustment.

This sounds like the bushes in the throttle linkages are either missing or badly worn. New ones are available do a search on this forum as it was discussed where you could buy them from in the USA, I ended up making my own from some Delrin rod I had.

Do these simple things first and report back if this does not fix your problem then we will look further.

 

Thanks.
I'll check the butterfly adjustment. did not know about that.

The linkage is firm and fine. I checked the linkage and the throttle pedestall, suspecting that the bellcrank traveled to much back (making the link go over center) in idle so that the link would close the butterfly's a bit before opening them, but that's not the case.

I'll report back when the butterfly's checked/adjusted.

I'm not sure what you mean by: "You can tell CR from a compression pressure test"
but I don't think compression has anything to do with this. The compression seems fine to me.

 

Sorry what I meant was you can not tell compression ratio (CR) from a compression test. Due to factors like valve overlap - if you decrease valve overlap the cranking pressure will increase, but CR will still be the same.

The most important thing is that the pressure does not vary by more than 5psi from lowest to highest.

Check the bushes where the cross rods connect to the linkages on the manifold that operate the throttle butterfly.

 

warrjon
You nailed it. Thanks very much.
I adjusted the butterfly and set the opening for 0.05mm. They where practically closed.
After adjusting, I have 790 rpm. Further adjustment I can do witht the idle adjustment at the AAV.
Also, the butterfly axel has a worn down bushing at the back end. In fact, the worn is so bad that the bushing is totally gone. Se photo.

thanks again for very good help.
Case closed!

 

This is what is causing the revs to drop when you put your foot on the throttle.

Glad you got it sorted so quickly

 

Hate to hi-jack but this excellent posting got me looking at my own "butterfly axle" (is that what it is really called?) bushings. Both are shot and need replacement. Anyone have a lead on where to get replacements and/or the part number? Thanks.

 

Part Number is C34388. It is available from jaguar dealers and all the usual parts suppliers.

 

Available here

Bush Accelerator Linkage - C34388 | Jaguar XJ6-XJ12 69-87, Jaguar XJS | Jaguar | British Parts UK