I recently purchased an '86 XJS V12 knowing it has problems. I assume it has a blown head gasket because of white sweet smelling smoke coming out of the exhaust. I have been told to check the trans modulator because if that's bad the trans fluid could be sucked into the engine causing the same smoke. I haven't had the time to investigate further as of now. Does anyone else have any other simple things to check before I dive in to change the head gasket.

 

Head gaskets on these cars is a bear! We sell a head puller tool for $395 plus freight. Shipping is expensive--this thing ways about 70 lbs.

Go to the local parts house and buy one of the little kits to see if you have exhaust gas in the coolant. Also, check the oil and see if it looks milky. Run a compression test. All of these tests will help determine if you have a blown head gasket.

 

I checked the vacuum modulator on the trans and that is fine. The oil doesn't look milky. It uses a lot of coolant and brake fluid but the brakes work fine, hmmmmm.

 

Nope, no other simple things to check, cuz I don't know what else to check. I bought an '87 XJ6 with the straight 6 cylinder and the head gasket went two weeks after I bought it, same thing....white smoke and it ran like there was "water" in the gas tank or lines, it ran real rough and had no power and had lots of white smoke, actually it is steam that is being burned of from what I understand. The XJ6 is good for about 100,000 miles before the head gasket goes on it, I don't know about the XJS but they are from the same manufacture and the engines are prone to overheating if not looked after I would highly suspect the head gasket. If it isn't the gasket then it wouldn't hurt to replace it anyway since it will eventually fail and you will have to replace it anyway. So??? No matter what....it's going to hurt.....the pocket book mainly....LOL
I am not laughing too hard since I just had all my wheel bearings checked, some needed replacing and one of the wheel bearing hubs was cracked and needed replacing.. OUCH......now that really hurt, my wallet is awefully scrawny right now.....LOL Sometimes buying an older Jag is like beating a lazy horse into becoming a race horse....LOL

 

My XJ6 "used" alot of coolant.....and I couldn't figure where it was all going, it was like the car was "eating" it~!~! I had to keep a jug or two in the trunk all the time and then one night I took it for a drive to a small city about an hour from where I live and half way there the coolant light came on..... so I topped it up and then half way back home it did it again.... I couldn't find a leak under the car, it wasn't dripping or wet anywhere and I was puzzled. So the next day I thought I would play automotive detective and went and bought a small bottle of coolant "dye" and put it in the overflow and ran the car, the next step is to buy a LED flashlight and shine it in and around your motor, the LED light makes the coolant glow an bright amber yellow green, so that was how I found the leak. The leak was so fine that when I was on the highway it sprayed out in a fine mist so before I stopped the car the engine parts and the engine bay itself dried it out instantly which is why I couldn't find the leak to begin with, the simple solution.....I already had a small hose in the trunk, the guy I bought the car off had a small box of hoses, I guess he was going to change them all and decided to sell it instead. I changed it in the parking lot of a parts store.....good thing it was summer here in Canada.....that's all I can say without being mean about the weather here. LOL>>>>
I almost forgot to mention, if there is small black "blobs" floating around in your coolant it IS a headgasket, check your coolant overflow, the oil will float around like small black beads floating on top the coolant....a classic sign of a blown head gasket.....the other sign......white "smoke" out the tail pipe, which is actually steam from coolant in the engine that has leaked passed the failed head gasket.

 

the usual visual things to look for in a blown head gasket is oil in the coolant which will float around like small beads of black crap on top the coolant, and the white "smoke" which is not smoke at all but steam.

 

wonder if you ever figured out the problem with your smoke- i had the same problem with smoke out of the left bank of my v12- turned out to be the reactor valve on the brake master cylinder was sending brake fluid to the intake manifold for combustion

 

How does the brake master cylinder send brake fluid into the intake manifold? Doesn't make any sense to me, never heard of such a thing, but if it can happen I'd hope this was the cause of smoke out of the engine rather than a head gasket if it was my car..lol..

 

If your modulator is dry and fine as you say, then brake fluid via the vac chamber of the booster is the next obvious cause. If there are NO external brake fluid leaks then usage of it can ONLY be via the booster, and white smoke (sweetish smelling) is the result.

The fact you state that it uses lots of coolant AND brake fluid, I reckon you got 2 seperate issues.

Coolant loss on a V12 is a common issue OTHER than head gaskets, and will take some finding, as there are hoses everywhere, and then the radiator itself is well documented as a leaker. Head gaskets on these engines are not a common failure item in the big scheme of things.

Hoses, cross pipe to water pump connector, transfer tubes on the top of each cylinder head, radiator banjo bolt, and its pipes, heater taps, are all common coolant loss areas on any V12. The list is LONG, and will grow as you reply with good diagnostic reports.

 

I've had the 'white smoke' beast and I lost a really good car because of mis-diagnosis by several garages.
The car was a 1977 Porsche 911s which started puffing out bits of white smoke.
It ended up by completely covering the village in a massive cloud of white smoke (I was living in a small village in Normandy at the time so some of the more elderly residents were getting a bit concerned to say the least).
After several discussions over the phone to garages in the UK I decided that I could not justify spending £4500 on an engine rebuild on an £8000 car.
Traded it in for XJR6 which was utterly awesome! If you've never had one you must buy one immediately! Even if just for a week.
Anyway, the 'blown' engine on the Porsche turned out to be a blown seal on the brake servo, not the master cylinder but the servo itself.
Because the seal had gone it was drawing brake fluid back into the vacuum feed every time that the brake pedal was pressed, a simple matter of Bopyle's law proving itself yet again. If the upstream side of the system is supposed to be a sealed unit and it is not sealed then Boyle's (sorry, is it Boyles or Boyle's) (or Boils?) law forces a suction effect where there should be a vacuum. The suction basically sucked brake fluid (only very small amounts) to be drawn back into the exhaust side of the combustion chamber (where the vacuum is drawn from). The brake fluid then got really hot and turned into absolutely enormous amounts of thick, white smoke.
I now know how battlefield smoke bombs work; every single round has a very small flat 6 engine with a buggered up brake servo in it. The loaders must be brilliant!
Anyway, check the servo seals before you commit to anything really deep and expensive.
Steve

 

Don't recall if your year had a vacuum "assist" for the power brakes, but if so, it could be a leak in the diaphram on the vacuum booster on your power brakes. If it is ruptured, it will suck brake fluid through the rupture and draw it into your intake.
Easy way to check is disconnected the vacuum line (if it is one that has the vacuum assisted brakes). The car will require a lot of leg pressure to stop, but if it stops using brake fluid, there is your issue. Brake fluid does emit a light colored smoke when burned.

 

Didn't read the entire thread (shame on me), but thought I'd put this out there. I travel extensively, and even in Houston TX where it doesn't get THAT cold.... if I've been gone for a couple of months in the winter, and fire up the XJS - there's almost always a period of billowy white smoke before things settle down. I understand it's moisture in the exhaust burning off.

 

once you have disconnected the vacuum line, take a moment to inspect inside of the hose- may not have to wait at all to see if consumption of brake fluid goes down- moisture on the inside of the hose may tell you real quickly...