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There was a hole in one of the fuel tanks
I didn't realise they had more than one! Could you cross feed like in a plane? ;)
I've heard about leaking fuel tanks in XJ-S's. Mental check list includes a good sniff around...
As I understand it your XJ6 would have had one of the last XK6 engines, replaced by the AJ6 (which powered the 3.6/4 litre XJ-S's). The straight sixes seem to have a good reputation for reliability, which might explain why some XJ-S owners report similar experiences to you (tales of the X-JS actually being more reliable than the sensible, modern daily driver) while others (V12 owners?) reckon it's very unreliable and there are lots of known V12 issues. Actually the guy I spoke to has a V12. Engine choice doesn't fix the rust of course...
So I reckon best plan is find a good AJ6 engined one and hope. All academic right now as building project means I don't have the money, even for a cheap one.
Fox-
One of my dads jags wasrestored by a classic car mag and the restorer (the company) still managed to fuck things up even when they should know what they are doing.
Personally I would buy the cheapest decent shell and fucked mechanical then get the car stripped. Shot blasted, then welded and painted. Then decent new loom, re fitted interior and building a new engine with much closer tolerances and using modern kit, such as brakes suspension and transmission.
lynx
I had a 1986 4.2 XJ6 (similar mechanicals I guess) for a couple of years 2011-13. Did 6,000 miles and not a thing went wrong, air con, trip computer, many electrics, all working perfectly.
There was a hole in one of the fuel tanks, but I just used the other one.
If you fancy one, get one. If it's a pain just get rid of it. It might be awesome...