I think that I have found the cause of my erratic idle! The fact that it runs better at the far lean end of the ECU idle adjustment range made me rethink things. I've tested the ECUs vacuum line before, and it held a vacuum. But today I tested it again and the ECU line bleed down quickly! This is great news!

I pumped this up furiously, but cut that part out because of the frantic flailing camerawork. I could never quite get it up to 20.

I tested the vacuum pump, just to ensure it was working properly.


If the ECU vacuum line is bleeding down, then the ECU will see that constant leak as a bigger throttle than there is and increase the fueling. A leaky ECU vac tube should make it run rich right?

Is the ECU Pipeline constructed like this...
crossover pipe
rubber line
steel line
rubber line
ECU?

If so, I plan to replace the rubber line from the crossover to the metal line with high temp silicone hose and repeat the test. Will I find the junctions easily once I get underneath?

 

Well found.

Yes, less vac = more fuel.MAP sensor. Mine was cracked, NOT leaking, but real close.

Also right on the make up of that line. The hose in the boot with the plastic capsule in it is a common leaker, and the actual end that plugs onto the ECU easily splits. That plastic capsule is also known to split, and I usually remove them, less joins to cause issues.

 

Great news. As Grant said, ditch that plastic canister thingy in the vac line to the ECU in the boot, if it is in there. Mine never had one from the factory, and I tried one just to see, and the car ran like a dog with it. Although unlikely, it is just possible that the gizmo in the ECU that reads vac is faulty, so if the new rubber bits do not help, that could be the cause too.
Your list of how it is constructed is correct. It could, repeat could, be a break in the rubber line from the crosspipe to the steel line under the car, or a crack/rust in the steel line.
Testing the vac line section by section is the best way to track it down.

 

Thanks guys! I'm so excited to find this problem with my XJS that it feels like Christmas Eve! Tomorrow morning I could actually see what this V12 smoothness I've heard tell of is all about!

Jessie is rust free so I'm expecking the metal line to be intact, but if not I'll run a new line all the way from front to rear. Here's hoping new ends do the trick!

 

So, this morning I began to replace the crossover to steel line hose. Interesting design choices.

Pulled the exhaust apart to open a gap a gap into the trans tunnel. Halfway up I found the junction. Clamped. Flat blade sceerewdriver drive. Drive head against the tranny. Ok.

Later I slipped the new line effortlessly into place, slipped it over the pipe and snugged the clamp. I choose to remember it this way.

The hoses, and plastic damper were all replaced with a single silicone hose.

Testing the new setup, I found it slowly leaked down. Disappointed. I replaced the vac pump to line junction and bam! Holds solid. Ok. So now I have no way of knowing if my initial test showed an actual fault in the system or a fault in my test connector.

I decided to try to increase my luck with a parts offering and swapped the CTS with one I had on hand that read closer to expected ohms. I think this pleased the jaguar gods because I wouldn't know for sure which fixed it if it ran well.

She fired right up and ran and ran very smoothly! I recorded audio of the startup that I'll share. After she warmed up I took her around the neighborhood. Smooth even creeping along. Smooth pulling hard. Smooth coast down. I'm not 100% convinced that it's perfect, but it's so much better I'm thrilled!

I'll take her out for dinner, I'll bring her inside too, and report back after a longer run.

 

Well done.

I will get that chain write up done soon, work called again (I am long retired), as some "kids" failed to attend once again, more hours for me, and Jag pocket money.

When you cruise, make it long, and give the engine a work out, basically to blow out the system. There will be heaps to blow out, so take your time, NO need for Italian Tune Up stuff, just a work out will be fine.

 

Well, first run out to dinner was really nice. I'm used to sometimes having to pop her into neutral at lights if she gets a little shudder. But that night, no shudder at idle. Power seemed significantly different but the smoothness of the delivery was most noticeable.

Then yesterday I had to help build some custom furniture and couldn't do a long drive. I did have time to address the exhaust issues that I noticed while I was under the car.

The long exhaust section from my cat to the forward muffler had been rotated incorrectly. That pipe has a very specific orientation. I did get the A bank exhaust adjusted to move it away from the transmission pan. It was previously touching it. Thats got to be bad for tranny temps and I occasionally heard rattling that I think was this pipe against the trans. I also fount the three-bolt joint behind the front muffler was up against the rear cage. A lot of big wrenches, MFH and I got the two pipes to match. Nothing bumping into anything.

I was planning to do a circuit of the Mecklenbugring. There's a outer loop around my city that is lightly trafficked, consistent high speed, and something like 65 miles around it. Nice afternoon run. Then I turned wrong while working on the furniture and hurt my lower back. Temporary delay.