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Although my car has been running great I've noticed an increase in fuel consumption the last few weeks....or maybe months. Truth is, I don't pay much attention to fuel economy; can't remember the last time I actually checked it. But I could tell that it was using more than typical based just on watching the gas gauge....and tallying my month gasoline bill !
Some very quick and dirty checking with my MityVac hand pump revealed that the vacuum advance canister was not hold vacuum and was therefore inoperative. This can definitely hurt fuel economy.
The item they sell is adjustable which according to them, is what Jaguar installed the the V12s early on. At some point they switched to the non-adjustable type, commonly known as part number JLM519. The written description says the they're calibrated for the HE V12. This makes me happy....as adjusting after installation wouldn't be a 5 minute thing.
Replacement is not difficult. I removed the cruise control servo and throttle pedestal for easier access and, since I'll be changing plugs at the same time, that stuff would have to come off either way. Then I removed the distributor wires, cap, rotor, and the thin plastic shield under the cap
The advance unit is fastened to the distributor with a tiny roll pin that requires a tiny drift to remove. Drift the pin out and then use some needle nose pliers to wriggle the actuating arm off of the peg on the advance plate. The advance unit slides out of the dizzy with a few wriggles. Replacement is simply reverse steps.
I'll add that both the cap and rotor were unusually difficult to remove....and this isn't my first go-round with V12s. I broke the cap trying to remove it. And I broke the rotor, too...but that was intentional. It's better to break it off than force it off by upward pulling---something you never want to do on a V12 as you can damage the springs to the advance weights. To hopefully ease removal in the future I dressed he dizzy shaft with some 600 sandpaper and perhaps will use a smear of silcone paste or similar on the new rotor. Same for the cap-to-cap gasket joint
Some pics:
This is the old unit. I've circled the torn area of the diaphram...which was as hard as a rock. I didn't intend to break the housing. I broke it during the "wriggling out" process; a bit fragile.
This is the new vacuum advance unit showing the tiny roll pin and tiny drift
New unit installed. Adjusting screw and locknut seen at the pointy end of the unit. For more working room I removed the plastic wiring guide and also removed the two screws on the advance plate so as to allow some rotation. This gives a bit more room to detach the actuating are from the plate
Had to order a new cap and rotor so it'll be a couple days before it is running again. New plugs to be installed tomorrow. Fuel economy should improve!
I've been meaning to update this thread but here lately I've been swamped with distractions.
The car runs well with the new vacuum advance. I haven't had the opportunity measure fuel economy but it stands to reason that I'll see an improvement. The engine certainly feels happier with a working vacuum advance.
I changed all the spark plugs while I was at it although, truthfully, they didn't replacing as they were only a few thousand miles old. I was happy that they were all exactly the same light grey/tan color.
The downside to all of this is that I apparently managed to bollixed-up my TPS. I've discovered that a slight jiggle of the TPS wiring changes the engine speed. I must've tugged a wire loose or something. More on that later after I've verified what's going on. Grrrr!
Doug,
When I had my S3 e-type, I distinctly recall that the vacuum capsule was a vacuum "Retard" unit and not an "Advance" one. That was on a 1972 model and was probably updated later on.
02-11-2021, 10:44 PM
Vacuum Advance Replacement V12
