Good day all!

I have received a lot of help from these forums and their generous members providing insights for my adventurous undertaking of this XJ12 project I inherited. For those who have seen my posts and seen the story thusfar, the car is licensed in WA under "classic" registration and has been on the road now! It's been an arduous undertaking, but it's really coming alive. I have an appointment for a four-wheel alignment this coming Saturday, and then it's good to go on highways! I verified with the state that the car had not been registered since 1994 and have been in contact with the previous owner's family. My assumption that it hadn't been driven since the mid 90's was correct, it last ran in 1995 before I started on it.

Now, there is an idle issue that is bothering me. I do not have a vacuum leak, smoke checked and went around with some flammable carb cleaner; did you know there's non-flammable now? Ask me how I know... No vacuum leaks were detected. I rebuilt the AAV with the kit from Jag Improver, put the throttle butterfly valves within spec, and re-bushed all of the throttle linkages. What is happening is as I slowly release pressure off of the throttle pedal, say when I'm coming to a stop, the car stays in a high-idle condition, usually 1000-1200 RPM. If I sharply release the throttle or tap the throttle at a stop so the linkages all snap shut with the quick spring tension, then it idles correctly. I have extremely limited freeplay in the whole linkage system, well within my standards of acceptable. I've removed and cleaned the throttle switch (C43031) and did a cursory inspection of the circuitry inside seeing nothing concerning. I've been using this site as well to assist in some diagnosis on the switch: https://www.sw-em.com/bosch_d-jetron...estigation.htm

I truly suspect the switch to be my issue, but I'm also getting the impression that this Bosch switch isn't a common issue, especially one that has seen very little use and therefore little wear. I suppose what I'm asking is where might other issues be? I believe I've eliminated the mechanical possibilities. I can't seem to find any other posts on this forum or others with a similar issue. I know the upgrade for Bournes switches to the updated red Lucas switch is common, but the Bournes switch is a three-post whereas the older Bosch C43031 installed is a five-post. I have not checked into the wiring to see if I could upgrade from this bosch to the red Lucas, but I haven't eliminated that as a possibility.

Thanks in advance for any insight, here's the car out in the streets:

 

Lovely looking car. Grant Francis knows all about this version of the engine; but my amateur guess is that you have a linkage slightly jamming problem. The throttle rods from the capstan must have about 2mm of free play before the linkage starts to move the system. If I had to guess where the jam is, I would guess the butterfly bearings or the many pivots in the system, or weak throttle return springs.

 

I'm with Greg. I'd first verify that there isn't a mechanical issue with the linkages. The only thing I can add to his suggestions is to disconnect the throttle cable and turn the throttle capstan by hand and see what happens.

I had a "hanging" throttle on my V12 2-3 years ago with symptoms like yours. Long story short, it was the throttle cable. Off the car is was fine but, when installed, it would bind.

Great looking old Jag :-)

Cheers
DD

 

AND

Remove the 2 rods.

Note the "ear" of the capstan against the stop, good.

Now, slowly open the throttle, engine OFF, using the pedal inside the car.

Slowly release that pedal, as you would on the road, and then look at that "ear" again.
Is it against the idle stop?
If it is NOT, then the chances of that capstan casting binding on its internal spindle is the issue 99% of the time.

This requires the assemble OFF the car, the switch removed, then the spindle bolts on the top can be removed and the spindle withdrawn.

Alcohol time now.

You will easily see the workings and the spring, so spray some good quality lube ( I use INOX), and work the thing while yo drink.

Then lube that spindle unit, slide it home, refit the 2 small bolts.

Refit the switch, HEAPS more alcohol as you now reset that switch, fun will be had. I have a write up on that if you need it, just ask, and I will dig it out and attach.

BUT

If that capstan returns to idle with the test earlier, that capstan is deemed OK.
The issue is in the horizontal rods and ankle type bits at the rear of each manifold.
The incorrect throttle spring on each throttle body is also a common cause of this. More likely to jam a full throttle, but your issue I have also had with generic springs at that location.

Wait, there be more.

Throttle cable binding, NOT common, but its sat for a looooong time.

Throttle cable adjusted wrong, as in, NO slack at idle position, simple fix. Give it some slack, about 2mm roughly.

Bonnet insulation rubbing on that capstan, RARE with factory spec, but many owners have replaced this insulation with new stuff and it can be thicker, and will touch the tops of the 2 cross rods.

 

The throttle rods from the pedestal were both bent, I replaced them with new ones from SNG and made sure to allow for some free play when cold as they will expand with heat. I have purchased what bushings I could find to take up the slack in all of the throttle bodies and created bushings where there were none by oversizing holes and milling brass, lubricating all of that with Thixosyn2 as I have an abundance of it. I firmly believe it is not the linkages fixed to the throttle bodies. Throttle return springs appear original to the car and match, not unwise to think they're weak from age. Probably wise to replace them anyway.


The capstan against the stop is something I've noted, however I have not attempted throttle actuation with the rods off in the test you're proposing. The springs appear and feel the same on both sides, I do not believe those have been tampered with thankfully. Still, I should replace them. I just found part numbers for them on a different forum, so that's handy.
I really didn't think about lubing the throttle cable until reading these last night. I can't tell you how many cables I have to spray Kroil down in aviation and on my bikes, this should have been the first thing...I'll do that when I get back home...

 

Another thought.

Inside the cabin, look up at the accelerator pedal arrangement. Note the area where the cable attaches.

Memory, HAHAHA.

There is a foam type pad around that cable and when at idle position it seals against the bulkhead, thats the theory anyway, and it can become dislodged, and sit cocked up and prevent the actual pedal returning all the way to "zero".

I need a beer, good luck.

 

Good day all!

Well after a lot of poking and prodding, I did finally discover the issue.

Right where the linkage ends at the butterfly valve, there is a U shape where the butterfly has a protrusion that’s sits in the U with a pinhole for the throttle spring to hook into just outside the throttle shaft bushing. There was slight play within that “U” shape and not enough spring tension to fully seat the butterfly in that “U” when heat expanded the metal after running for a bit.

I pulled both throttle springs and there was a 4-5mm difference in length, where the right hand side was the one giving me issue had the longer spring. I bought 2 packs of varying springs and replaced both. No more dile issue!

 

Well done.

Simple cars these V12's HA.