Okay. When I started this saga, my '91 XJS was idling higher than I wanted, at about 1100RPM cold in park. It was also idling a little rough. I had decided that (among other things) I wanted to replace the ignition wires and clean & regap the plugs (or replace them if they looked awful). The AAV adjustment is turned fully clockwise. No noticeable impact on idle speed (or 'tickover' as I've seen it referred to here)

This weekend I did the ignition wires & plugs. The plugs looked *great* (but there were at least two with cracked insulators and I broke one. A fourth one I'm not sure whether it was already cracked or if I did it, but bottom line is I replaced four plugs with new exactly identical plugs). I also re-set the butterfly valves (0.002" with the throttle rods detached from the capstan) and adjusted the throttle rods for minimum "play" or slack - but verified that the butterfly valves are resting on their stops.

After doing that, the idle speed is worse (but it is smooth!!). Per Grant's directions, all along I've been trying to figure out where the vacuum leak must be. Well, today I thought I had found it, in the form of the rubber elbow between the AAV and the LH intake manifold. It had a large crack in it at the bottom (it actually split in two halves when I tried to take it off).

I also noticed this: an open joint on the hose from the air filter housing to the AAV (see below).



What is this supposed to attach to????

I have closed up the hole temporarily (I live in Virginia, so naturally I used duct tape!). I also used duct tape to hold the AAV elbow hose together & seal it for the purposes of a temporary idle test.

But now the idle is even worse! I'm looking at 1200-1300rpm

In addition to that, I noticed that I have a stray spark from the distributor cap to one of the cap holddown screws:


Hard to decipher, but that is the injector for 4B that you're looking past, down into the valley.

This sparking is pretty regular, but stops as soon as you open the throttles (but who needs to open the throttles when you're idling at 1300rpm!?!?!?)

How in the world am I getting a high voltage spark there???? That can't be a good situation, so I'm curtailing any consideration of driving the car until I can get that sorted....and I need help from this collective brain trust!

At a bare minimum, I need:

1) Part numbers for the two rubber hoses that connect to the AAV. (I have Grant's AAV rebuild procedure, but until the vacuum leaks are sorted I don't even know if that needs work or not, right?). I searched a few parts places (Coventry West, Welsh Enterprises, Jaguar and Land Rover Classic Parts) but so far I haven't found these parts....

2) I need to know what that side nipple on the long L-shaped hose (from the air cleaner to the AAV) is supposed to attach to - clearly, the other side of that should be feeding vacuum to something, right??

3) Given this bizarre state of multiple problems, where should I start? Replace all of the vacuum hoses? Track down the stray spark issue? What should my vacuum be at idle? I haven't measured it yet (not exactly sure where I should tap in to do that, either.

Now I'm wishing I could go back in time and have the slightly-rough-running but otherwise fun (and fine) to drive Jag......<sigh>

HELP!!!

 

EAC2654 is the balance pipe hose/s, you need 2, as best I can see.

It is VIN reliant to a point.

The AAV Inlet hose is VIN reliant, so last 6 of your VIN will sort that sucker.

The 90deg elbow on top of the AAV is EAC2655.

Is the AAV closing 100%, and I know I go on about that stupid thing, but if its not 100% closed at engine operating temp, you are chasing your tail. It matters not what the pot on the stove reveals, get the engine to NORMAL temp, use a mirror and look up its snout, and ensure that piston has risen to teh task.I have NEVER had one do that as fitted, hence in my write up, I "tap down" the Tophat about 2mm, and all warm and fuzzy.

Vac hoses, YES, they will need attention, and PLEASE only do ONE at a time.

Look at the rear of the A Bank Inlet manifold, you will see various vac hoses, and a blanking cap on a spare spigot, make sure all that is 100%.

If you have cruise control, plug the vac supply for now, as the bellows may be leaking vac.

Check and tighten the 24 nuts holding the Inlet manifold/s in place, they do come loose, and leak vac.

Remove the throttle rods, and ensure the capstan is actually ON the idle stop. Setting those rods requires a spacer between the stop and the peg, then when the setting is done, and the spacer removed, there is slack in those rods, NO slack, havoc can invade.

Ensure the throttle cable is NOT holding something partly open, as all it needs is 1mm to upset the beast.

SPARK:

A dud lead, ill fitted lead, cracked cap, etc etc. Must be found and sorted for sure.

 

Reporting back, after yesterday's marathon 10-hour tinkering session.

I found three confirmed air leak issues, with a fourth suspected.

1) The AAV is indeed stuck at least partially open. Rebuild in the near future.

2) The elbow connecting the distributor vent to the AAV inlet hose was *gone*, leaving an unrestricted opening to the AAV even if the airbox opening was plugged with a thumb (hence the lack of appreciable impact on idle with said thumb-blocking)

3) The elbow from the AAV to the intake was split.

4) The Supplemental Air Valve is also (possibly/probably) stuck open.

Current status: I have blanked off the AAV and the SAV so that neither can supply air to the inlet plenums. The Cruise Control bellows is brand new, so I don't suspect that is an appreciable source. I have started replacing some of the vacuum lines (the ones for which generic tubing is appropriate and available) with several of the other parts on order and expected to arrive today. In this condition, the car starts easily and idles at 700rpm, quite nicely. Not sure this would be the case if it were 25° F outside, but by the time we get to that I expect to have the AAV and SAV sorted.

The distributor cap will also arrive today, so I should be able to get that (and a few other things) replaced tomorrow evening.

 

Well found, and well done.

The AAV is the root cause. Rebuild i simple, and athe PDF attached will walk you through it.
That elbow is missing on 90% of the cars. It is not a VAC rea in the true sense, it is a draft air intake that vents the distributor.
That 90deg elbow split, mare common than Ronald at McDonalds.
That SAV I would leave it unplugged and capped for now. It is not needed really. I use them as the Extra Air Valve for all mine for cold start air with the AAV removed.

Keep at it, you are doing real well.

 

Good, good, good work!