Submitted by bsheridan@rest… on Fri, 08/09/2013 - 14:23

Fellow Jaguar Enthuiasts,

My 1986 XJS V12 is in the shop to replace the distributor vacuum advance and the mechanic is perplexed - I hope someone can offer a suggestion.

BACKGROUND:

I recently bought the 1986 XJS V12 which had been running great except it was running too Hot on the temp gauge. I took it to my Jaguar mechanic who has been specializing in Jaguar repairs for many years and is in my opinion VERY COMPETENT. He noticed the vacuum advance was missing from my distributor and said "that may be the reason my car was running Hot". We agreed that he would install a new fan clutch, new fan blade, new distributor cap and new distributor vacuum advance.

Yesterday the mechanic replaced the parts and attempted to set the engine timing but was unable to time the engine correctly. He called to say he needed to keep car overnight so he could pull out the distibutor and move it over one tooth so he could properly time the engine.

Mid-afternoon today my mechanic called to say he was still having diffulties.

PROBLEM:

1. The car idles okay without the distributor vacuum advance being hooked up, however it runs Hotter than Normal.

2. When the distributor vacuum advance is hooked up, the engine does not idle or run correctly.

3. My mechanic (although he is not asking for help) is perplexed and may be able benefit by some advice from some of you Jaguar engine experts.

PLEASE HELP! THIS REPAIR IS COSTING TWICE WHAT I THOUGHT IT WOULD AND IS GOING TO GET MORE EXPENSIVE UNLESS MY MECHANIC CAN COME UP WITH A SOLUTION SOON!

Thanks in advance! Bob

Submitted by stevejag@sbcgl… on Sun, 08/11/2013 - 14:43

Edited on 2013-08-11 14:45:56

Edited on 2013-08-11 14:45:08

Robert,

As Dick said, the vacuum and advance system on the V12 of that era is VERY complicated. Give your guy the chart below, It will help him figure out if the Vacuum Regulator is working right, it may very well not be. To top it off, all of that depends on the distributor timing being PROPERLY set as described on the tune-up label. If it's not right, the vacuum readings will never match the chart. Ignition timing very much affects manifold vacuum. All this is why so many V12's are never set up correctly.

Good luck,

Submitted by rcmaury@bellso… on Sat, 08/10/2013 - 11:48

The vacuum advance system is rather complicated on the early HE cars. Properly set up, the advance changes the timing 20 degrees at idle. This will make a cooling difference. At idle, the vacuum at the module should be 11 inches of vacuum. If more, then you have a bad control module or something is hooked up wrong. The timing is 18 BTDC at 3000 RPMs with the vacuum disconnected. These are specs to be set at. Once set, if the engine is not running properly, look elsewhere.