A car that might have been called the 'XJ6 Series IV' was given the project name 'XJ40' and is now commonly known as that. Intended to replace the original XJ6 work began early in 1972, and the XJ40 went through an incredible gestation period of around 12years and was finally launched in 1986. In part this was due to technical problems, and led to the development of the stop-gap XJ Series III to be carried out until the XJ40 was ready. However, the Series III more than filled the void for the unready XJ40 and was a real sales success especially in its final years. Many lament the XJ40 for being 'un-Jaguar', but it was developed to be easier and cheaper to build than previous Jaguars because it would otherwise have been too expensive. The V12 engine could not fitted into the XJ40 engine bay until the car was re-engineered into the XJ81 in 1993; it was said that XJ40 designers deliberately gave the XJ40 a narrow engine bay for an inline engine so that British Leyland could not force the Rover V8 engine onto Jaguar. The XK6 engine was never fitted to the XJ40, replaced by a new AJ6 unit in 2.9-litre ohc and 3.6-litre dohc 6-cylinder form, later 3.2-litre and 4.0litre versions. In 1994 the X300 XJ6 replaced the XJ40, but in spite of its looks underneath much of the engineering was from the XJ40.