The original Jaguar XJ6/XJ12 went through three revisions during its twenty-five years of production (1968-92). It was a beautiful design, but toward the end of its run became known for its unreliability. Jaguar sought to remedy these concerns by replacing it with the new XJ6, code named XJ40. The new car was almost an instant success. “Motoring Journalists of the time dubbed the XJ40 ‘the best saloon car in the world’ beating regularly in group tests competitors from BMW, Mercedes-Benz, Rolls Royce and the ever popular XJ6 Series III that it was designed to replace.”  Things went so well that “the XJ40 consistently outsold all its rivals in 1989 becoming the most popular executive car of the late eighties.”  What made it such a popular car?
The new design admittedly was not as striking as the series III designed by Pininfarina, but “important Jaguar elements were maintained, such as a low roofline and sweeping curves” . Two major difference involved the headlamps and fuel tanks. The previous XJ6 had incorporated four, round sealed headlights. While the new XJ40 continued this in the base model, the upscale models used the new one piece headlamps, which caused a bit of controversy within the Jaguar fold. The new fuel system incorporated a single upright tank which actually sat inside the trunk — a welcome change from the dual tanks offered by its predecessor.
Jaguar used a variety of engines in the XJ40. In the beginning Europe received undersized 2.9 slant six engines producing an abysmal 165 horsepower. Over time, these were replaced with larger engines, including the DOHC 4.0 liter six making 235 horsepower (pictured below). But what about a V8 or V12? “Originally, the XJ40 engine bay was designed not to accept any Vee engine, something the designers did to ensure that then owner British Leyland wouldn’t be tempted to use a Rover V8 to power the new XJ40.”  But things didn’t stay that way for long. After Ford took over the company, revisions were made to the engine bay to allow the use of a 318 hp V-12.
“The XJ40 was also the first Jaguar to feature what would become a Jaguar trademark, the J-Gate gear selector which offers better control of the automatic transmission.”  The pattern for shifting included normal selections, but placed second and third on the left side of the J channel. This allowed for “manual” shifting without the probability of accidentally placing the car in reverse. The electronically controlled transmission also included a “sport mode” option which made better use of the engine’s power when the driver desired a bit more of a peppy ride.
Although, “Jaguar’s developments in the area of occupant protection earned the XJ6 the title of ‘The Safest Car In Britain'” , “the earlier versions of the XJ40, still under the influence of Jaguar’s dark days, had their shares of problems” . “The early 1987/ 88 XJ40 models were the least reliable, with electrical problems such as erratic gauges, starter and fuel pump failures being very common. … The 1990-93 years were ideal years for reliability. Improvements were made to the power seat motors, improved sealing of the trunk (less leaks) and more improvements in the electrical system. … Overall though these cars have a very good record for keeping going with most faults being annoying and easily fixed.”  “In the end, the XJ40 proved a success for the company – but its true potential was probably unlocked after Ford took over the Jaguar and began to build in a considerable amount of extra quality into the product. As one Jaguar Daimler Heritage Trust (JDHT) employee recently commented: ‘The XJ40 only really reached full maturity after it became the XJ12 in 1993 – and that was because Ford scrapped the original XJ12 (it was planned for launch in 1990) project and started it again from scratch, making sure the quality was right at every stage…” .
“After a production run of eight years and 208,706 units the XJ40’s production came to an end in June 1994.”  “With an all-new replacement still years away, … Ford ordered the XJ40 to be facelifted and ‘retrolutionized’, reintroducing some of the style of the popular Series III. The X300, as it was known, was redesigned by head designer Geoff Lawson and was launched as the XJ6 for the 1995 model year” . Viewed from the side, the X300 still carried the basic shape of the XJ40, but included a newer front end with round headlights reminiscent of the original XJ6.
Jaguar XJ40’s are currently underappreciated in the auto market. According to Kelly Blue Book, a base 1990 model advertised by a private party should sell for $2200 to $3000. However, good models can be had for as little as $1800 with a little searching. Despite the reliability issues of the early models, the Jaguar XJ40 is a nice car which is still appreciated by those with both a desire for sporty performance and enough room for a family of five.
 1990 Jaguar XJ6 brochure
 1993 Jaguar XJ6 brochure
 1993 Jaguar XJ12 brochure
 Jaguar XJ40 by Minder Jaguar
 The Jaguar XJ40: Tips and Tricks from the Internet by Richard Hyde
 Retrospective: Jaguar XJ by Channel 4
 Back to the Future by Keith Adams
 Wikipedia’s “Jaguar AJ6 engine”
 “1994 Jaguar XJ12 Sedan” by Bill Russ
 “The Years 1989 to 1996” at Jaguar.com