With an experienced team and a lot of data.
Protect your 1977 Jaguar XJ-12 from the unexpected.
Jaguar rewrote its own book in 1968 – the new XJ6 was so good it replaced all the sedans which preceded it. The signature 4.2-liter DOHC six-cylinder engine delivered 180 hp and was matched with anti-dive suspension, power rack-and-pinion steering, automatic transmission or 4-speed with overdrive, power disc brakes and superior handling. Inside there were leather seats, walnut fascias and enough gauges for a light plane. An up-market sibling, the six-cylinder Daimler Sovereign, was introduced in 1969.
The first series XJ12 and Daimler Double Six followed in 1972, using the 5.3 –liter V-12 E-Type motor with four carburetors and standard air-conditioning but, sadly, only automatic transmissions. Fuel injection boosted the V-12’s 244 hp to 285 hp in 1975 in the Series II, and the collectible two-door coupe was introduced the same year, in both six-cylinder and V-12 configurations. Neither was very popular and the accompanying Daimler Double Six V-12 coupe is very rare, with only 407 built. There were 1,855 Jaguar XJ12 Coupes, meanwhile. The six-cylinder Jaguar XJ6 and Daimler Sovereign coupes were more successful, with 6,487 XJ6 coupes and 1,677 Daimlers finding owners.
A final redesign occurred in 1979 courtesy of Pininfarina, who raised the Series III Jag XJ6 and XJ12 roof at the rear and added a “kick” to the back fender. Quality control improved significantly at long last. The XJ6 was replaced by the XJ40 in 1987, but the Jaguar XJ12 soldiered on until 1991. Daimler Double Six models were built beside the Jaguars, with bespoke interior and vinyl roofs and the very last V-12 was a Daimler Double Six in 1992.
The 4.2-liter six is easy to work on, though inclined to cylinder head gasket failures, especially with air-conditioning. The Jaguar V-12 engine is more difficult, and the addition of catalytic converters to American cars in 1975 – literally beneath the manifolds – mean that underhood temperatures can be a critical issue. Hose and engine wiring longevity was limited to about two years between replacements. Additionally, the V-12 engine can slip its valve seats in the case of (practically inevitable) overheating, and don’t make the mistake of just repairing one cylinder head. Just like the XJS, the biggest single problem appears to involve the fuel system, with fuel vaporizing and over-pressurizing fuel hoses when the car is switched off after a hot drive, leading to engine fires.
The V-12 engine was such a good idea for this excellent design that it’s frustrating to deal with constant maintenance issues. Good Jaguar XJ12s are hard to find but avoid bargains like the plague. The XJ6 is almost universally a better bet, provided it is exercised regularly. In all, 352,025 XJ6s and Daimler Sovereigns were built, and all those exported to the U.S. were automatics, so an LHD 4-speed and overdrive car is virtually unknown.
There’s considerable cachet to the V-12, which is silky smooth when it’s in good tune, despite getting about 10 mpg. Jaguar XJ12 prices can be pretty low for a flawed example, but the wisdom of “buying the best you can afford” has never been truer.