Vacuum regulator. HELP from the guys in USA needed!

Can anyone living in USA help me to source from junkyard vacuum regulator EAC5157 item:4 in diagram below:
Part is no longer available new.
My XJS HE V12 is one of the first ones and it came without it from the factory. Quite quickly Jaguar realised that uncontrolled vacuum at idle is not a good idea and added this vacuum regulator.
My car never got this upgrade and my idle in certain conditions (with a.c. on etc.) is quite unstable so I would like to install one.
Problem is that for EU spec. cars Jaguar used different vacuum regulator and my car was imported from USA. so I can’t find the part I need it local junkyards.
Would really appreciate if someone could help me to source this regulator from USA spec. car.


For a quick fix, eliminate EVERYTHING on this chart, and connect the hose to the vacuum advance capsule directly to that port on the top of the throttle body, the one close to item 11 in this pic. You’ll have to cap off several vacuum taps, which you can do by jamming golf tees in the ends of some hoses.

The difference: With the scheme shown in the diagram, you’d have a fixed, controlled amount of vacuum advance at idle, full vacuum advance at all other throttle settings. With everything tossed over the hedge and the vacuum advance connected as I’ve suggested, you’d have NO vacuum advance at idle, full vacuum advance at all other throttle settings. Hence, you might need to readjust your idle speed when done.

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Hi Kirbert,
Thanks for suggestion, I know about this option but I’m not going to go this route. I just don’t like the idea of “fixing” things by dumping half of the components and calling it a day :slight_smile:
Also I can see at least few potential problems with your suggested option. Actually car has this mode as you suggest (no vacuum advance at idle) for the first 15sec. after engine start to help heating up the catalytic converters. But what will happen with cats if you run it constantly? With retarded timing you will always have unburned fuel in exhaust and if you are in open-loop will the cats survive or will they melt? Don’t want to find that out :slight_smile: Also I presume that fuel consumption might increase at idle… or I might be wrong on this one…

The car is running well and idle is not that bad, but I would like to make it better :wink:

I think that your suggested fix is a good option that will take you back home but I wouldn’t do it as permanent fix.




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Experiment as you wish; hopefully you’ll get the desired results…but I wouldn’t get my hopes up too high. “Unstable idle” might well have nothing to do with vacuum advance.

Before expending much effort on sourcing the regulator you might want to experiment by applying varying amounts of vacuum to the capsule and see if the situation improves. A hand held vacuum pump would be useful here.

Good luck


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Thanks Daug for good suggestion! Actually I have already done that and constant vacuum at idle really helps. Well I understand that HE will never have perfectly stable idle but at least it will be better :wink:

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If you’re happy, I’m happy :slight_smile:


What gives you this idea? Retarding the timing results in lower efficiency and hence more heat buildup, but it certainly had better not result in unburned fuel. If it did, it would most certainly not meet EPA standards.

Running it retarded means the exhaust is hotter, which helps the cats light off and also makes sure the 1-wire oxygen sensors start working. It shouldn’t harm anything, though, at least not unless your cooling system isn’t up to snuff. And zillions of other cars, including most American cars of the time, came with exactly this vacuum advance scheme from the factory. It’s Jaguar that had to be different and come up with the vacuum regulator idea.

Well you said it yourself Kirbert:

When you retard ignition point you are delaying combustion, so combustion will not be complete by the end of power stroke and some of it will take place when exhaust valve is opening so basically fuel will finish burning already in exhaust and this is how you get fast cats light off. This will also generate more engine heat. Although my cooling system is completely rebuilt and in good condition, there is no point of running engine inefficiently and generating excess heat.
By the way retarded timing scheme was used in EU spec. cars for first 15min to heat up the engine more quickly which caused really poor fuel consumption for short trips. But EU cars didn’t have cats. There must be a reason why this time was reduced to 45sec. for cars with cats.

But Jaguar V12 HE engine combustion was nothing like the other zilions of cars… With it’s weak and slow burning mixtures it was quite unique in it’s time. And what worked for other engines not necessarily will work for Jaguars HE engine.
Also after searching the web more thoroughly it looks like that idea of using vacuum regulator was borrowed from GM and it’s American cars:

While it might sound tempting to throw everything away and make the vacuum scheme more simple but I think it is always wise to question, analyze, and understand why it was done in such complicated way… I don’t think that Jaguar would have bothered so much if they could just plug vacuum advance capsule directly to the port on the top of the throttle body.


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Has anyone used this site for buying spare parts?
For 10$ it might be worth a gamble :slight_smile:

Same appearance but perhaps different vacuum values? I dunno, something to think about. As you say, might be worth a try


Jaguar seems to have been in love with complicated things, John, sometimes needlessly and sometimes of questionable rationale and results, IMO.

FWIW, without going into gruesome and needless detail, on my present and prior V12s I’ve experimented with different vacuum advance schemes over many years of ownership, including the one mentioned by Kirby…which I’m currently running and have been for some time. In reality, nothing bad has ever happened; no melted cats, no overheating…no bad result at all that I have ever been able to discern. We don’t have emissions testing in my area so I can’t comment on any potential or actual bad result as far as that goes.

And, it should be said, I’ve experienced no positive change in idle quality, fuel economy, or coolant temperature. Or engine power…at least not according to my butt-o-meter.



Hi Doug,

I did a bit of homework on these regulators…
Light brown colour used on XJ6 engine - limits vacuum to 6" Hg (EAC4399)
Brown colour used on V12 A emissions - limits vacuum to 11" Hg (EAC5157)
Green colour used on V12 B emissions - limits vacuum to 12,5" Hg (EAC4012)

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Can’t argue with someone wanting to set it back the way it was, or could have been.

Since I’ve already tossed it and have no intention of restoring the original vacuum tangle, I’m willing to go over the hedge and retrieve mine and send it along for the price of postage.

Mine is a brown unit from an 86 Federal car. Functions properly per my testing prior to tossing.

PM me with contact info and we’ll work it out.

I wouldn’t classify the Jag HE V12 mix as either weak or slow burning.

Pre-HE it’s a standard as dirt IC engine. The HEs use a stratified charge to allow for higher compression without knock and allow a leaner charge for better efficiency.

This doesn’t mean weak or slow-burning. Just controlled, cleaner, lean burning mixtures.

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But it actually is… :slight_smile: Here are few extracts from article about Jaguar V12 HE engine and it’s ignition by Roger Bywater:
-“Swiss engineer called Michael May was trying to interest the motor industry in a new combustion chamber design which combined lean burn and high turbulence techniques to achieve high fuel efficiency at abnormally high compression ratios.”
-“at part throttle cruise a substantial amount of advance was necessary to light the slower burning weak mixtures needed for best fuel economy”

Of course if you run it with full throttle, then it’s like you say “standard as dirt IC engine”


check into how the last years of FORD DENSO changed there vacuum system for V12 jag!!

Jag stuff was archaic, compared to how Ford did it, with help from Denso Japan engineers!

i kinda like my Pre flat chamber engine, very forgiving, i now use a generic Chevy advance curve , idle 900 rpm, 24* degrees , at 3000rpm 40* degrees BTDC, and 50* degrees at 4500rpm onward to 6500/7000rpm, rev limiter kicks in at 7000., i can not make it ignition knock, just runs terrible rough!

try that on your HE, HE engine NOT designd for performance , to be used as a smooth running ,quiet , economical grocery getter, like a nice modern 4 cylinder car.