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XJS Enthusiasts' Club Established 2005 |
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Tales
from the Cobblers Workshop:
Changing Plugs on a V12 Jaguar engine
We were supping real ale in our local; I took the view that if I had to spend time with Big Al, the soporific effect of alcohol could only be an advantage. It also helped that he was buying. After some boring preliminary talk about gardening, the conversation turned to Jezebel, my 1986 V12 cabriolet. I proudly explained that I had detected a misfire, not an easy thing to spot on a V12 engine. Big AL ever suspicious, demanded to know how I had verified the miss. I explained how I had held a banknote over each exhaust, and by watching it flutter in the gas stream had made the diagnosis. The passenger side had exhibited the misfire. Big Al remained unimpressed by this display of technical brilliance. ‘Of course you realize it could be an injector not working, or a spark plug or even a valve, he particularly relished this last horrifying possibility. They are the most likely suspects. You know how to check the injectors don’t you? ’Yes I do’ I said, but nothing stops Big Al and he told me anyway. ‘If you use a stethoscope or just hold a screwdriver against the injectors with the engine running you should hear working injectors clicking, take care not to hold anything conductive against the plug leads you don’t want to fry your brain cell.’ With this witticism he spluttered into his beer. He can be a hard man to like. Because the cabriolet is a hobby I didn’t hurry to attend to its misfire, and was astonished when Big AL appeared with a new set of plugs and presented them to me as a gift. ‘It takes a good mechanic about two hours to fit them to an H.E. engine’ he advised. I discussed this gift later with my wife. ‘I told you he is a very generous person I don’t know why you persist in pretending to dislike him’. I retreated to the garage and examined the plugs they were stamped Unipart and had the part No. GSP4763 and were obviously new, it later transpired Big Al had bought them on Ebay at a bargain price. Although it is considered inadvisable to look a gift horse in the mouth, I took the precaution of checking the Number with my friend at the auto factors. He confirmed they were correct for an H.E. engine. I rewarded him by buying a new set of plug leads.
It took a threat
from Big Al that he would personally advise and supervise me if I needed
assistance, before I started to try and cure the misfire. Whenever I lift the bonnet I experience a feeling of awe. Managing to overcome this feeling, I used a cheap motoring stethoscope to listen to the fuel injectors, they were all clicking satisfactorily. Had they not been operating correctly I would have inspected the clips that connect to the injectors as it’s not unknown for them to suffer from poor or broken connectors. On a car that has been standing, the odd one can stick closed. It is worth applying a brief shock of twelve volts to see if it can be moved. This can restart a reluctant injector. . I knew from hearsay that the plugs were inaccessible but had not realized just how inaccessible. I began by removing the cruise control bellows, three bolts plus the control cable taking care to retain the connector that grips the control cable. The next job is to move the compressor, in order not to depressurize the air con system I placed a piece of wood above the radiator, removed the two 13 mm bolts at the front of the compressor and then the two 17 mm bolts at the rear. At the front there is an earth lead held by one of the bolts don’t forget it when replacing the compressor. It is then possible to lift the compressor and rest it on the piece of wood. This is just to allow access to the two front plugs. Jaguar supplied a special plug spanner with a swivel head for this job, however when I tried it, it seemed on the point of breaking the rolled mills pin it uses in its construction. This brings up the first snag. The plugs were very tight in the head. I returned to the house and sipped a beer as I recalled stories of stripped threads.
Fortified by
alcohol I returned with new determination and with a long lever applied
pressure to the No. 1A plug, it yielded and I soon had it out. Upon
examination it showed that it had been fitted using an anti seize
compound. I was in luck. The last set of plugs had been fitted with
care. One further point it’s not a good idea to remove the plugs until
the plug recess has been thoroughly cleaned out. I use a vacuum cleaner
with a small nozzle and it was amazing how the debris collects. Before
you use the vacuum cleaner check there are no petrol fumes you could
suck into the vacuum motor. On the H.E engines the plug gap should be set to 25 thou and it is worth checking. I replaced No 1A plug after checking the gap and wiping Copaslip on the threads and taper seat. There are no washers on these taper seated plugs. I also replaced the H.T lead. Although it is back breaking work leaning over the engine bay all went well until I reached plug No.3A. The plug spanner extension fouled the distributor body. I thought I might have to remove the distributor for a while. Eventually I realized that the plug spanner could be used with a ring spanner on the flats at its end. See the photograph. This allowed me to take small turns at a time to remove it. Partial success six plugs changed six plug leads and the compressor replaced, I forgot to replace the earth lead and had to remove one of the bolts again to replace it. When I tried to stand up I realized that jezebel had had the best of the encounter. Six plugs removed and replaced with new leads, enough for one day two and a half hours gone never mind, the six rear plugs will probably be easier. I went indoors for a hot Radox bath. After leaving the bath it looked as if I had used Redex.
The
next day I returned to complete the job. I started by marking all the
wires that go onto the coil. This was later proved to be a wise action,
when a wire came adrift as I unbolted the coil and laid it aside. It
then became obvious that I would have to remove the throttle pedestal.
The front two bolts are reasonably accessible and hold two clips that
locate the loom in place. They were the same size as the coil 11 mm
spanner required, alas upon trying to remove the rear bolts I couldn’t
reach them with any spanner I had to hand. I looked through all my tools
and finally gave up. It might be possible if the oil pressure sender is
removed, I didn’t wish to do this. I took a trip into town and purchased
a cheap set of crows foot spanners. Even with these I had difficulties.
Fortunately I managed to release the bolts with the spanners tipped at
an odd angle
The next day Big Al called round to see how things had progressed. I was able to report that I had taken four and a half hours to change 12 plugs and leads. When we inspected the plugs that I had removed one was found to have a cracked insulator. If you are going to do this work on a pre H.E. it is much easier as the plugs are at a different angle and more accessible. They use more conventional plugs with washers instead of taper seats.
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Copyright@2005, XEC |
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