On the http://www.jagweb.com/aj6eng/index.php there is an excellent article decribing the V12 vacuum ignition advance (VIA) system, VACUUM IGNITION ADVANCE / AJ6 Engineering. If I understood it correctly it doesn't exactly refer to my 1994 XJ12, but at one point the author, Roger Bywater, writes, "Indeed the first clue that an HE has a fault in the vacuum advance system is that cruise consumption drops from about 20-22 m.p.g to around 16 m.p.g." On my first 140 mi road trip I got 16.9 mpg driving a very steady 66-67 mph for about 130 of the miles.

Also, my idle is a bit rough. Elsewhere in the article it is suggested that idle condition is greatly influenced by the VIA system. In the exhaust pipes there is a moderate carbon build up that suggests an over-rich air/fuel mixture. This, I gather from the article, can result from an ignition that is not properly advancing through the speed range.

https://www.jaguarforums.com/forum/a...1&d=1307142540
There is one hose fitting on the forward end of the left-hand air intake that is plugged, see above picture. I am not sure where it should plug into. But, just aft of the radiator there is a metal tube with a rubber hose that is plugged as well. This hose looks slightly larger that the hose form the LH Air intake. See picture,
https://www.jaguarforums.com/forum/a...1&d=1307142553

Under and just outboard of the LH air filter there are a few gadgets that are related in some way to the VIA. On one of them there is a hose fitting that has no hose connected to it. See Picture,
https://www.jaguarforums.com/forum/a...1&d=1307142601

In the LH side of the engine compartment there is a placard that is related to the vacuum system, https://www.jaguarforums.com/forum/a...1&d=1307142515
On the right hand side of the placard there is a diagram for the layout of hoses on the aft end of the RH air intake. This is an exploded diagram of the RH air intake in the placard. These hoses are shown in picture, https://www.jaguarforums.com/forum/a...1&d=1307142568. Curiously, the placard indicates that the left-hand hose is blank. But in fact all four fittings on my engine have hoses connected. The top goes to the vacuum delay valve with the white band. The LH and RH fitting have hoses that go through the firewall, to the fuel and ignition computers. The bottom fitting has a tube that goes to the LH side of the engine. I'm not sure what it connects to. Also on the placard the hose on the LH air intake is noted. It should connect to the "Purge System," whatever, wherever that is.

Many of the fittings have hoses that, while they aren't lose as in falling off, are just barely snug. In https://www.jaguarforums.com/forum/a...1&d=1307142586 the underside of the LH air intake is shown. The closest two fittings have proper plugs, and the third has a hose that is connected. These are examples of fittings with hoses/plugs that are only just snug. The third has a nylon tie that improves it but still it's not hard to remove the hose from the fitting. How tight should these hoses/plugs be? When the engine starts and vacuum is applied are the seals then "made?" I am thinking that if the majority of the fittings are just a little lose the cumulative effect on the vacuum system could be significant.

This is my first post with visuals. I hope they help. I am posting the originals here(for some reason the text in the images doesn't appear in the above uploaded versions):

PicPaste - FrwdLHAirIntake.JPG

PicPaste - PluggedHoseAftofRadiator.JPG

PicPaste - LHAirFilterVacuumEquip.JPG

PicPaste - VacuumSystemPlacard.jpg

PicPaste - AftEndRHAirIntake.jpg

PicPaste - UndersideLHAirIntake.JPG

If you click on the image in the links you will get a larger view.

Anyone have an idea where I can find information about the VIA system for the 1994 XJ12? I have a few thousand mile road trip coming up in a couple weeks and if I can get the mpg up a bit it will really help.

Thanks in advance for any help.

Robert Booth
Seattle, WA, USA
booth.robert.e@gmail.com

 

Robert:
Sorry I can't be of more help. I truly know nothing about the 12 cylinder engine. Most XJ40 came with the straight 6; and I don't even know how many guys on this site have one with the 12. XJS's more frequently came with the 12. Perhaps you could cross post there and see if you can't get a few more opinions. 17 MPG for a 12 cylinder engine... Personally, I'm not sure how much more than that I would have guessed you'd get.
Cheers!
Scott

 

Thanks for the suggestion Scott. I did place that posting in the General Tech section and thought of placing it in other forums as well. I was a little concerned, however, that the moderators might take exception to multiple posting like that.

I will give it a try, however.

Thanks again.

reb

 

Your 6.0 should have Marelli igntion. The timing advance is electronic, control by a separate ignition ECU. The ignition ECU, though, does need a vacuum supply. On the XJS with Marelli igntion it's a red plastic or nylon tube. Do you see one of those, by chance? I'm not sure you will...but if you do, is is connected to anything?

One thing about the fuel economy. Bywater's figures are for imperial gallons...which are 20% bigger than USA gallons. Your 16.9mpg is 20.2mpg in Brit-speak :-). That's not bad for 318hp pushing a 4400 pound car.

Cheers
DD

 

Not to mention the oxygen sensor question in the "XJ6 XJ12" section :-).

The categories are confusing. You belong in the XJ40 section. I'll let one of the moderators comment on using the "General Tech" section. It seems to be a catch-all.

One thing, though, and I'm speaking just for myself...but I darn near skipped over your opening post of this thread. It was just too long for me to slog through. No matter, perhaps, because I can't offer much help at all on XJ40s anyway. (sorry) But ...you may get more/better answers if the questions are broken down into separate, shorter postings.

I hope your not offended by my remarks, as that isn't my intention.

Cheers
DD

 

No offense taken at all. I am totally new to this sort of forum thing.

I have started taking a broad brush approach to question posting as you noted. I have found the forum categories confusing. More so as I have read more and more in various ones. Questions seem to be all over the place.

Actually, according to one source, my jaguar is more properly an XJ81. Jaguar XJ - Wikipedia, the free encyclopedia. But even that seems equivocal.

Thanks again Doug for your input and assistance. Just to let you know. I have read your replies to many folks in this website over several forums. You are extremely knowledgeable and this place is very lucky to have you.

Take Care.

reb

 

We all agree

As moderators we won't take exception as such, but will tend to delete duplicate posts, leaving the most appropriate post in place.

However, if a duplicate has started to generate pertinent replies then we have the job of moving the thread and merging it into to the appropriate one.

All extra work, and we may well miss something, not to mention the possibility of other members also missing a reply that they may have wished to add to.

The Gen Tech section should really be just that, 'General', however it is also populated by newer members who may be unsure of where to post a question.

The main thing is that you post a question and receive replies as you have done here. Especially as you have a pretty rare car, with not that many V12's of this model on the forum.

At least, you didn't post into the X Type section, just because your car commences with an X

Settle yourself into the XJ40 and XJ12 sections and the great members such as Doug, Fraser, Grant, our tech's and all the others too numerous to mention will soon get used to seeing you around.

Your car is a somewhere between the two sections, engine XJ12, body etc XJ40.

One piece of advice I will offer, include your model in the title of any new thread, 94 XJ12 blah blah, this will instantly show the guys just what they are dealing with.

Welcome to the forum, hope that you can get your idle cured.

Please keep us informed.

 

WELCOME from downunder, enjoy your stay, AND the car, fun will be had.

Your rough idle is more than likely the dreaded "black goo" that builds up around the throttle discs. It is common on the 5.3 V12 and other engines.

Remove the intake trunking and whatever else is needed to gain access to teh throttle housings, take a rag and soak it with solvent (carby cleaner is what I use), and open the throttles by hand (engine OFF obviously) and wipe all that goo out of the housing. It will take some wiping, my X300 required scraping with a blade, to get it all off the housing bores and the discs.

Reassemble the bits and try the engine. I reckon it will be OK.

On the off chance it is not spot on, then a set of spark plugs correctly gapped, would be my step #2.

Dont forget the beer, for YOU, not the car hahahaha.

 

Hi Richard,
Thanks for the tip on using the forums and how to label my posts for most effective response. I will limit my posts to the two forums you have suggested unless they are obviously more relevant elsewhere.

Please don't hesitate to let me know if I am overstepping in any way. I am a fast-learner about somethings and will be happy to comply.

Thanks again.

reb

 

Hi Grant,
Thank you for the suggestions about solving my car's idle issues.

By "intake trunking" do you mean all the plastic intake bit and pieces from say the intake trumpets outbound of the air filters to the intake manifolds?

I've attached a picture to clarify.

Thanks again.

reb

 

robert,

NO, it has been a loooong time since I have seen a 6ltr, not mnay in OZ, and only 1 I know of within 200kms of where I live, so I forgot about the 2 plastic air cleaners. They can stay in place.

Undo the clamps just forward of the master cylinder to remove that rubber joiner hose, and remove the elbow that attaches to the throttle housings, and I have no idea how that is done, but it will NOT be rocket science I am sure.

I reckon some digging, swearing, drinking, more swearing, will take place to remove more than is required. Once you have done it once, the next time will be simple, and less stuff removed. I clean the bodies on "her" XJ-S, and my X300 at every oil change, so it really is a maintenence item that is overlooked.

 

Thanks for the pics , the location to your red oil filler cap is way different than straight six engines
I had not taken an attention to this detail