On the WELCOME TO AJ6 ENGINEERING / AJ6 Engineering there is an excellent article decribing the V12 vacuum ignition advance (VIA) system, http://www.jagweb.com/aj6eng/vacuum.php. 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, hhttps://www.jaguarforums.com/forum/attachment.php?attachmentid=10854&stc=1&d=13071425 68. 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

 

Oh those good old days when checking the automatic advance and retard was just a matter of seeing the bob-weights were free in the distributor, and vacuum advance was a matter of pulling of the manifold hose, blocking the manifold connector with your finger, and seeing if it made any difference. And best of all, so easy to set the spark timing yourself to suit the petrol by just slackening one nut: Good old days.
Leedsman.

 

My first car was a 1959 Fiat 600D. When I pulled the motor to take it to the HS auto shop to rebuild I hefted it out by hand and placed in a friends Volkswagen fastback from which we had removed the passenger seat. Returning it was just the reverse. Simplest thing ever built. I never really did get the starter motor to work off the lever between the seats so I push started it. I just had to careful to not parallel park. Although even then, I could get it out, it just took a bit, which irritated other drivers who were waiting on me. 16 yo. Not a care in the world. Good old days, for sure.

 

You never forget your first car do you? Mine was a 1939 Morgan three wheeler with a Ford-8 engine that my mother subbed me £70 to buy. It was horrible, and I thought it was great at the time. The chassis was cracked in two or three places, the brakes went out of adjustment every couple of weeks (cable brakes), the suspension was so hard it might not have been there at all -- and was responsible for the cracked chassis I think. But it got me around and was FREEDOM!
Leedsman.