Im the new guy on the block. I just bought a 1982 XJ6 Vanden Plas and it needs a complete restoration. But before I start with the cosmetic stuff I want to make sure its mechanically sound. Ive owned 2 Jags before, but both of them were XJ40's with the AJ6 engine. The XK engine is new territory for me.
The car seems to be misfiring. At low RPM's it sputters until it gets above a certain RPM and then it gets smooth. I can tell exactly the RPM range because the tach doesnt work.

 

Doesn't sound much wrong there, probably needs cleaning out etc. When was the last time the car ran ?

If you're doing a complete restoration, I assume you will be having the engine stripped and rebuilt. It is a lovely engine to work on, (I have rebuilt 2), but the XK engine was well past its best in 1982, and was still being made on machinery installed in 1948 that was 2nd hand !! Jaguar spent an awful lot of money installing machinery to build the V12, and then along came the oil crisis and killed off almost all the demand for it. This machinery could not be used to produce the XK engine, but was later used for the all-alloy engine fitted from the XJ40 onwards.

Key problem is that the 8L block, which is yours, suffers cracking between the bores which then results in frequent head gasket failures. Bores 1-2, 2-3 and 4-5, 5-6 get cracks. I wrote an article about this engine in the XK engines forum. The only known cure is to machine out the original steel liners, then machine a recess in the top of each bore, and fit "top-hat" liners that cover the cracks, (the liner lips almost touch each other).

Sorry this is so depressing, but that is the truth of the matter. A 7L block can be used instead if you can find one. They also crack, (3 out of 5) but some tend to survive unlike the 8L blocks of which 8 out of 10 crack. On the other hand, you might have one of the engines with machined slots between the bores. These were produced in the last few years of XK production as a cure for the cracking, but I don't know the year this feature started.

Other design feature that gives regular trouble due to neglect are the long head studs; (like most old UK manufacturers, Jaguar uses head studs not head bolts). These are about 12" long, passing through the coolant jacket to tapped (and blind) holes in the bottom of the block, inside the jacket. Jaguar did this to "strengthen" the engine, although it seemed to manage for many years with short studs. Being continually bathed in coolant, these corrode badly to the point where they snap in two, leaving the stub in the block and it is very difficult to extract these. So take particular care when taking off the domed head nuts before you pull off the head. Take all the core plugs out of the block and hose all the crud out before extracting these studs or the tapped holes will fill up and the head studs wont go back in fully.

Apart from this the engine is pretty robust, but clearly, as an engine first announced in 1948 as a 3.4 litre, it is not leading edge !! Jaguar tried to make it do a little tooooo much when it was obvious the V12 would not be able to replace the 6s. It took quite a long time to engineer the all-alloy 6 and the XK just had to soldier on until it was ready.

Have a read about it by an ex-Jaguar engineer: -

http://www.jagweb.com/aj6eng/xk-engine/index.php

 

Well the car still does run, but the previous owner said it started misfiring about a month ago and he thinks it just needed a "tune up". Im not as optimistic about the problem. Im going to have a look at the distributor and see if that could be the problem.

Im hoping to avoid an engine rebuild, but Im going to for sure clean it out.

Thanks so much for all the information! You have a great deal of knowledge.

 

I rebuilt an XK engine around a replacement (and uncracked) 7L block due to cracking of the old 8L block in 1993-4. At that time I was in my early 40s, but it's all a bit too much now, and I can now afford a newer car, (an XJ6 Sport of 2003, the aluminium-bodied saloon whch came to the US in 2004). I completely flushed out the new block and replaced all the head studs. Engine was still going strong when I sold the car in 2002.

By the way, check out the timing chain cover at the front of the engine where the coolant passage is located. Corrosion of the aluminium can cause leakage of coolant into the chain chamber and hence into the oil. If you replace the block don't forget that this cover forms part of the engine top where the cylinder head sits, and must be flush with the block surface. The final machining of the block top at Jaguar was done with the timing chain cover in position.

 

Hi Fraser Mitchell,
I just read that article of past Jaguar engineer. Whoo!!!
That's just unbelievable.
Did you found a same one on V12?
I can't search by myself, cause I'm french and, as the most of french people, my English is too poor.