De Marelli ontsteking kent een paar specifieke problemen, waaronder het lopen op slechts zes van de twaalf cylinders. Je hebt in feite twee 6-in-lijn motoren en elke bobine stuurt een van de twee cylinderbanken aan. Onderstaand een paragraaf uit de XJS-bijbel van Kirby Palm die gratis te downloaden is vanaf Jaglovers.
Speciale aandacht voor de in rood afgedrukte opmerking!
CATALYTIC CONVERTOR MELTDOWN/FIRE (ONE-BANK FAILURE): While any misfire is a cause for concern with catalytic convertors (see page 315), the Marelli ignition system provides a unique and especially threatening possibility: running on six cylinders. While the various Lucas ignition systems were single 12-cylinder ignition systems, the Marelli is actually two separate 6-cylinder ignition systems. Hence, it is possible -- probable, even -- that sooner or later one of these ignition systems will fail while the other continues working. Since each half of the ignition system operates one bank of the engine, one entire half of the engine, along with its dedicated catalytic convertors and oxygen sensor, may see no spark all of a sudden while the other bank continues to run normally.
The V12 can be accurately described as two six-cylinder engines; each bank has the same firing order and the same natural balance as an inline six-cylinder engine (note: conversely, a V8 is not two four-cylinder inline engines). Hence, the V12 is actually driveable on six cylinders; rather than a “misfire”, it runs quite smoothly, and drivers who have not read this section may decide it’s not too serious and press on. According to LaRue Boyce, symptoms to look for include: “Loss of power, more gas smell but no noise, just the no power feeling. Oh, the loss of vacuum also causes the transmission not to want to shift. You will know when it happens, there is no power, just enough to get you to 45mph on flat ground.” Julian Mullaney says, “When my car started running on 6 it was very noticeable. I thought that the tranny was slipping at first because I had to give it so much throttle to get it moving.” Note that, with the typical failure mode described below, this typically happens all at once; there is no intermittent operation where it runs well most of the time and drops to low power on occasion. Once a bank shuts off, it’s gone for good.
Considering that people have been known to drive with seized centrifugal advances, inop vacuum advances, kickdown switches that don’t work, one of the two intake butterflies disconnected, or various other problems, perhaps it shouldn’t come as a surprise that some people would continue to drive with the engine only running on half its cylinders. There are lots of people driving around every day with their dashboards lit up like Christmas trees, and many drivers seem to feel that such things as idiot lights or drastic reductions in power are cause to visit a mechanic within a month or two, whenever they get some time or money to spare. Unfortunately, the Marelli-equipped Jag V12 appears designed to remove such drivers from the gene pool!
Running a non-cat V12 with no spark on one bank fills the inop side of the exhaust system with fuel -- a disaster looking for a place to happen. Since the XJ‑S exhaust system loops up and over the rear suspension, it should be able to hold several gallons of fuel before it starts pouring out the tailpipe! Since there is no spark in that bank at all, the owner might just get away with it -- but he’d better hope his ignition system doesn’t mysteriously start working again! The same goes for a cat-equipped car that had an ignition failure when started from cold; the cats won’t work when cold, so the fuel in the exhaust system on that side may never be ignited.
If the car is equipped with catalytic convertors and they were hot when the ignition failure occurred, the cats will burn the fuel as it arrives, not only keeping them at operating temperature but getting considerably hotter in a big hurry. What’s worse, if the car continues to be driven, more throttle will be applied to get any speed out of it, so even more fuel and air will be dumped into these cats. The inevitable result will be a cat meltdown within seconds. What’s worse, often the cherry red-hot catalytic convertors will ignite something, perhaps by melting through the fuel hoses that are above and not too far away. The car is immolated so fast you’ll barely have time to pull over and get out.
You don’t want to be driving on six cylinders, not even for a minute.
If you were dozing when you read those last few paragraphs, pinch yourself, clear your head, and go back and read them again.
DISTRIBUTOR ROTOR PROBLEM: Unfortunately, a one-bank ignition failure turns out to be a common occurrence on Marelli-equipped V12 Jags. Although anything that kills one side of the Marelli ignition system can cause the car to run on six cylinders and threaten a cat fire, there is one possible cause that clearly outweighs all others in terms of frequency of occurrence. Randy Wilson: “The most common failure, the one that kills the A bank, is the center post of the rotor burns through, allowing a ground path from the rotor contact straight to the distributor shaft.” This characteristically results in a fire or meltdown in the right side cats only, since it is always the A bank ignition system affected. For a photo of a cut-away rotor that suffered this failure, see