QUESTIONS AND ANSWERS
Jaguar World's technical advice service
V12 Concern
Q As Asa US owner of an early V12 Jaguar XJ-S, a recent video caught my eye. I don’t know if you follow ‘Harry’s Garage’, but he posted this recently: https://youtu.be/ DQdheMlLmB8.
It covers the necessity to totally replace the original fuel injection and ignition system in order to simply achieve a reliable car, consistently running on 12 cylinders.
Is the original set up really that unreliable and do we really need to spend a pile on a new system? I have always held out hope that someday soon the world will understand just how fantastic V12 engines are, and they will be hugely sought after – V12 XJS’s commanding E-Type prices.
Forgive me, but a nicely sorted V12 XJS convertible is so much better than an E-Type in modern motoring conditions... BUT, I suppose if our XJSs are worth $150k… $10k on a new FI system will be ok?
AWe do indeed follow the progress of the ‘Harry’s Garage’ XJ12 Coupe YouTube channel saga – two of the contributors to this column, Tom Lenthall Ltd and Painting Classic Cars have been responsible for the mechanical and bodywork restoration respectively.
A full answer to this question would require a lengthy feature, but intrinsically the ‘problem’ only applies to the early D-Jetronic fuel-injected engines fitted to Pre- 1981 XJ-S’s and Series 2 XJ12 saloons. The later Digital P-Jetronic system proved to be much more robust and relatively troublefree, hence the XJS convertible you mention is unaffected.
We can divide the analysis into two distinct areas, ignition and injection:
The most unreliable aspect of these models is the ‘Opus’ ignition - also fitted to the earlier V12 carburettor cars (for which the ‘opeless’ joke was coined). It was always a problem in period, Lucas itself introducing a number of revisions. In addition to amplifier and coil failure, it was common for the ferrite rods in the timing rotor to ‘drop’, resulting in loss of active cylinders.
We recommend replacing it with ‘Lumenition’ or SNG Barratt’s ‘constant energy’ systems as a matter of course.
The D-Jetronic system is now at least 43 years old and has many agerelated modes of potential failure… chief of which is the injectors, which can stick after periods of disuse and are hard to overhaul. See the April 2023 Q&A for a full explanation.
OBD diagnosis was a distant dream in the ’70s, and very few specialists now have access to the ‘Lucas Epitest’ which was the diagnostic system of the era. But most faults, including those mentioned in the video as occurring after restoration, can be traced to brittle wiring or corrosion of terminals and circuit boards, and perishing of vacuum hoses and diaphragms – such as those found in the distributor advance module and manifold pressure sensor (locat
