Good evening. I’m a new owner of a 1989 Jaguar Xj12 V12 5.3. Very excited with the car, my first V12 (not the first headache, already restore a Porsche 928, Mustang), but this one caught my attention for the V12 and for having some historic relevance (a history for another time). Anyway, the car was in storage for the last 3 years or so, therefore bringing it back to life has being quite challenging. The car now runs, rebuild brakes, all (ALL) fuel hoses, and a electrical revision. But we still have a pretty low level of vacuum (11 to 12inHG). I’m about to remove all fuel injectors to look for leaks (I can “feel” that at least 2 cylinders are not working on bank A). My lack of knowledge in the “vacuum delay system” is making things even worst. I test a number of hoses to find “0” vacuum, but they are properly connect with new vacuum hoses. So, the question here, any guidance or links to help me understand the vacuum system peculiarities (I do have my car hood vacuum diagram), and how to track all potential areas of problems in those cars? Thank you very much and hope to be able to support others in the future.
Hi and welcome to you! I don’t have a V12 but do have three six cylinder Jags. Two have had difficult vacuum problems that are now fixed. For these I found the primary source of vaccum at the intake manifold and made up a short pipe to isolate it from all the vacuum services. This way you are able to work out how the engine runs just on its own. In each case the measued vacuum improved. Then a step by step process to add vacuum services to find the leak. Sorry to waste your time if you have already done these things! Paul.
Thanks Paul for the reception and the advice. Actually this is a very good tip, i’m planning to go in this route as well…engine alone, then add brakes, then the other systems. Trying to sort out all at once proved a difficult task. This is frustrating as in my Porsche 928 we have lots of vacuum systems, all problematics, but obcr you understand how to test its just a matter of replacing hoses and actuator$$$. Will show the progress while moving forward!
Diagnose and repair the two inoperative cylinders. Then check your vacuum reading again.
I can send you the S58 performance guide which explains all the various vacuum switches and valves. But…it’s focused on USA market XJS V12s. If yours is a Canadian XJ12 a lot of if will be the same. If not, it may only confuse the issue as there were several variations on the vacuum system depending on year and market
Cheers
DD
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There are two reliable vacuum sources for testing engine vacuum, Guiseppe; the vacuum nipples on the A-manifold and the vacuum hose at the fuel pressure regulator - they all have manifold vacuum…
If these indeed show 11 to 12" Hg you likely have an engine problem - manifold vacuum is an expression of engine power. One step is to check ignition timing - engine power will be down if it is out of wack. If indeed you suspect two or more cylinders out of action, the prime step is to run a compression test to possibly clarify the reason - loss of compression requires actions other than vacuum diagrams etc…
Frank
xj6 85 Sov Europe (UK/NZ)
Frank, thanks for the advice. I tested compression and seems ok (210 to 218). Didn’t check timing yet, and the tip on the vacuum ports to use is very usefull. Thoae 2 cilinders might have a bad seal ora clogged injector, will jump into that and report back.
Doug, thank you for the support and quite happy to receive a message from you. I read a number of your posts here before and helped me a lot already in other issues. You’re right, the next steps (today and tomorrow) in to check proper function of all cilinders and timing. I already tested compression, and found between 210 and 218 in all cilinders.
I’m quite confused by the country origin of the car, as it was originally sold to the England government but it’s a left hand drive (??? Is that possible???). There’s a plaque in the engine bay saying “federal” and a sitcker at the door, in arabic and english, pointing manufacturing at leiland factory.
Anyway, if you could send me this guide it would be very very usefull, thanks a lot . My email is gsmarrara@bol.com.br.
Disconnect all vacuum hoses and plug the holes as Paul said, including the vacuum advance.
Check if the vacuum advance capsule is not leaking. Keep in mind that some have a tiny hole and a deliberate leak. Best is to remove the distributor and see if it actually moves the rotor when vacuum is applied.
If it checks ok discard all the delay valves etc, plug it directly to the inlet manifold and see what the vacuum is.
Advance timing is also a factor, so better check that too. You might find out that the vacuum advance mechanism is actually stuck…*
The two cylinders not working would be the first thing to check, the compression is good but you must find the problem.
Two non operating cylinders will have for sure an effect o vacuum.
To check if the really don’t work undo the injector plugs (easier) or the spark leads and see if it makes any difference when you connect/disconnect them. On the V12 is hard to tell if any or which cylinders don’t work.
To be sure I would test both banks.
*And get the Book if you haven’t done already.
https://www.jag-lovers.org/xj-s/book/Jaguar.html
One more important information. I live in Brasilia (capital of Brazil) and the altitute here is 1000m from sea or around 3500feets , so i imagine that it would cost me around 3.5inHG in measured vacuum, right?
0m = 29.9 inHg
1000m = 28.8 inHg
Thanks Aristides, looks like you already have you fair share of chasing leaks, great tips, thanks.
Also, i saw a number of references to “the book”, now i found it. Thanks for sharing.
Quick comment, I can only imagibe hiw cool is to cross the streets of Paris and around France in a XJ12.
Best regards, Giuseppe.
Meraviglioso…!!
All the best and welcome.
Sonmething does not ring true, the compression numbers look good. As reported. But, inadequate to diagnose. If you wish, post the comprsssion value for each cylinder. Variation between each means a lot. And best, yet, test open throttle and wet and dry. compare.
The opposite of compression is vacum. so, a cylinder with good compression should read good vacum. A push pull relation.
Vacum best tested at the manifold. As various systems alter it for various purposes.
And, another thought. use two vacum guages. One for each bank.
A leak somewhere ?//
Carl
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Your compression is as good as it gets, Giuseppe…
Two cylinders misfiring may relate to
a. Crossed plug leads, particularly if the two are adjacent - it’s very quickly done whenever leads are dis-
reconnected, particularly on a V12. You should carefully check ignition sequence.
b. Failed plugs, plug leads or a distributor/rotor problem - check sparking?
c. Clogged or misbehaving injectors - you could verify the two suspects before a complete overhaul?
While your altitude would certainly influence engine power, I’m uncertain as to the exact reduction in vacuum readings - but assume you have done your homework. However, I’m also uncertain about the dynamics; manifold vacuum is created by the throttle’s restriction on air flow versus engine’s demand for air. As the engine revs up after starting, increasing power as it draws more and more air, until the vacuum increase prevents further increase of air ingress/power the rpms then stabilizes at set idle. In theory; it is the amount of air available rather than ambient air pressure that decides manifold vacuum - and ambient air pressure only comes into play at high(er) throttle settings…?
In any case; your manifold pressure is too low whatever the interpretations; engine is low on idle power - and, as Aristides says, two cylinders misbehaving is at least part of the problem. Incidentally; is the vacuum steady or does it ‘pulsate’ - and if so, how?
Crudely, when checking ign advance look for something like 5 deg BTDC at 800 rpms with vacuum disconnected. If it is around this, timing is unlikely to cause low vacuum - but, as others have implied; the vacuum advance set-up varies with country and model year, and is complicated even if everything works as it should. Which of course may need to be verified…
Frank
xj6 85 Sov Europe (UK/NZ)
Hi Carl, yes, I’m sure I have a leak somewhere, but will have to make sure all cilinders are firing properly, timing is correct, butterflies adjusted, etc.
For that reason yesterday we removed the injectors and ignition wires/leads for inspection and test. We should have new seals on them as well, what may be one of the sources of vacuum leak.
By the way juat ordered a second vacuum gauge to compare A and B banks and to validate the meter are correct.
Frank, thanks for such detailed guidance in that, really appreciate it.
I’m sure I have “leaks”, so I’ll work on this in the next days while to injectors and ignitions wires were removed for cleaning/testing.
About altitute effect on vacuum, this is something very discussed on the Porsche (rennlist) forums, but i like this guide specially:
This one too: Mastering the Basics--Reading A Vacuum Gauge | MOTOR
I have some small fluctuatuon in the vacuum gauge what may lead to worn valve guide (but to be confirmed when all 12 cilinders are working properly).
By the way, wish a blessed Easter for those who celebrate it and a good weekend for all.
Let me share a picture of car.
Best regards to all,
Giuseppe
Ps. By the way, will replace all tires, not sure to go with the white stripe sidewall.
Good idea to get a second gauge Giuseppe, I did the same as my original Lucas unit was bought by me about 35 years ago.
Don’t know if this helps, but when i first got my V12 it ran really rough and was only getting about 11-12 inHg vacuum at idle. I thought for sure bad compression or intake leak.
Turned out it was running super rich (due to ECU issue), and once i got fuel mixture correct, it idled at 18inHg. So just a bad fuel mixture can mess with vacuum.