The XJ6, using 2.8 Ltr (2,790 cc/170 in³) and 4.2 Ltr (4,235 cc/258 in³) straight-six cylinder versions of Jaguar's renowned XK engine, replaced most of Jaguar's saloons which, in the 1960s, had expanded to four separate ranges. An upmarket version was marketed under the Daimler brand and called the Daimler Sovereign, continuing the name from the Daimler version of the 420. The "XJ" designation was from the car's code name during development, standing for Experimental Jaguar. The car was introduced in September 1968. Power assisted steering and leather upholstery were standard on the 2.8 Ltr 'De Luxe' and 4.2 Ltr models and air conditioning was offered as an optional extra on the 4.2 Ltr. Daimler versions were launched in October 1969, in a series of television advertisements featuring Sir William. In these spots, he referred to the car as "the finest Jaguar ever". In 1972 the option of a long wheel base version, providing a modest increase in leg room for passengers in the back, became available.
Normally known simply as the "Series II" (pronounced Series 2), the XJ line was facelifted for 1973. A 3.4 Ltr (3,442 cc/210 in³) version of the XK engine was available from 1975. The XJ12 (and Daimler Double-Six) version, with a 5.3 Ltr V12 engine, was again part of the line-up, along with long-wheelbase models and the XJ-C coupé, now considered a collector's item due to its rarity. These Series II models were known for their poor build quality, which was attributed to Jaguar being part of the British Leyland group, as well as to problems inherent in the design of certain Lucas-sourced components. Visually, apart from the longer wheelbase and available "XJ12" badge, the Series II cars are differentiated by a smaller grille. 91,227 were produced, 14,226 of them with the V12 engine. Though worldwide production of the Series II ended in 1979, a number were produced in Cape Town, South Africa until 1981.
In late 1979 the XJ was facelifted again, and was known as the "Series III" (pronounced Series 3). The long-wheelbase version of the car incorporated a subtle redesign by Pininfarina, externally the most obvious changes over the SII were thicker, more incorporated rubber bumpers with decorative chrome only on the top edge, flush door handles for increased safety, 1 piece front door glass without a separate 1/4 light. The grille had only vertical vanes, the reverse lights moved from the boot plynth to the larger rear light clusters and a revised roofline with shallower glass area. There were three engine variants including the 5.3Ltr V12, the 4.2Ltr straight 6 and 3.4Ltr straight 6. The larger 6-cylinder, and V12 models incorporated Bosch fuel injection (made under license by Lucas) while the smaller 6-cylinder made do with carburetors. The smaller 3.4Ltr 6-cylinder engine was not offered in the U.S. The short wheelbase saloon and coupé had been dropped during the final years of the Series II XJ. The introduction of the Series III model also saw the option of a sunroof and cruise control for the first time on an XJ model.
Throughout the 1970s Jaguar had been developing "Project XJ40" which was an all-new model intended to replace the original (by now Series III) XJ6. Due to problems at British Leyland and the fuel crisis, the car was continually delayed. Proposals from Jaguar's in-house designers and Pininfarina were received. Eventually, it was decided an internal design would be carried through to production.
This car was finally released in October 1986 with (in European markets) controversial square headlamps on all but the lowest specification; these were a lingering feature from the 1970s development. The car was considered more evolutionary than revolutionary, and had to fight off a new competitor: the recently enlarged BMW 7 Series (E32). While the British press favoured the Jaguar, the XJ40 tended to lose comparison tests run by German publications. Only six-cylinder models were initially offered: a 2.9 Ltr (in Europe) and a 3.6 Ltr. The V12 (XJ12) and a long wheelbase model, including a high-roofed Daimler Majestic model (reviving the model name of 1958-1962) and destined for official use (one was used by the British prime minister), were again delayed, being launched at the very end of the XJ40's life.