The independent engineer who does my car was scratching his head over a not very old, 4-litre I think, Jaguar XJ6 the other day. He said it 'wasn't running right' and would also run with three of the plug leads disconnected. He demonstrated.
The engine idled very quietly and was clearly not in a bad mechanical state. It continued to run perfectly at and above an idle with any or all of the three rear plug leads disconnected. Pulling off any of the front three leads resulted in the car missing as one would expect.
I thought it might be severely coked up and the explanation might be red-hot carbon around the exhaust valve seats in those three cylinders. But he said the car would start and run from cold with those leads disconnected.
Mystery. Any ideas?
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There's only one possibility that I can think of. For some - as yet unknown - reason, it simply wasn't running on the three rear cylinders. When any of the running cylinders was disconnected, then it made a difference - as it should.
Modern idle speed control means that it can be very hard to feel a single cylinder misfire. Three cylinders missing would actually be harder; the idle speed would still be exactly normal and it would still have "firing balance." There were systems that used to do this for economical town running - V8s that dropped to four at low speed for example. Reportedly; it was difficult to feel the difference.
A quick check with a laser thermometer soon shows which cylinders are working; non-running cylinders have cold[er] exhaust ports. Believe me; on a V12 Merlin it's the only way to find a missing cylinder.
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Didn't some Vauxhall Senators with 3.0l V6 engine's deactive 3 cylinders for economy whenever possible?
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But would they be all three cylinders at one end of a straight six? One would expect them to be spread out or even to alternate.
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The three cylinders at one end of a straight six fire at even intervals (that is why you find two three-into-one exhaust outlets, and why two carburettors were often found to work better than three. It seems as if the rear three cylinders were doing nothing at all.
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Still strikes me as odd. Wouldn't there be a bit of a heat gradient down the engine?
Does anyone know if any recent Jaguar straight sixes have been wired to run like this?
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>>...A quick check with a laser thermometer soon shows which cylinders are working; non-running cylinders have cold[er]exhaust ports. Believe me; on a V12 Merlin it's the only way to find a missing cylinder.
Well how did they do it in 1940 then?
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If you can see the exhaust stubs, the patina on the outside changes when they are up to temp. It's hard to explain, as they don't quite glow, but dead cylinders show up from the exhaust stub appearance. Of course, this is only obvious (for a spark plug defect) when the magneto switch is on 'left' or 'right', not 'both'. It's almost impossible to pick a bad cylinder with both mags on. The drop in revs on left or right from the both position is the best diagnostic tool. Good fitters can diagnose how many plugs, what's wrong with them etc just from looking and listening. Many of my Air Force instructors were ex-RAF fitters, and they passed on to us all this useful information. Laser thermometers? Good leather gloves and feel the stubs after the engine stops.
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A quick check with a laser thermometer soon shows which cylinders are working; non-running cylinders have cold[er] exhaust ports. Believe me; on a V12 Merlin it's the only way to find a missing cylinder.
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Oh, I don't know, couldn't you grasp the exhaust pipe firmly in your hand?
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The last jaguar straight 6's were the AJ16 which were a development of the AJ6. These are engines designed in the late 70's, they don't run on 3 cylinders unless there is a problem.
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There were many V12 jags that didn't have all twelve going at the same time.
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AFAIK the V12s ran all their cylinders, and didn't switch off a bank at low speeds. There were three versions of the V12 all with the same basic design. The 5.2, 5.2 HE and finally the 6l before the engine was phased out.
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I didn't say they switched any cylinders off,did I???.Most had at least one cylinder misfiring if not more.
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misunderstood sorry
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Its a bit of a shot in the dark, but didn't some big Jag engines suffer with a similar issue to the BMW Nikasil problem? I'm sure I've seen a couple for sale on ebay stating they require new engines.
Has the mechanic carried out a compression test?
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The engines that suffered Nikasil problems were the early AJV8's from 1997-2000 (most of these have been replaced for free by Jag as long as servicing was done at a MD). The straight 6 (AJ16) never had this problem, and is an intrinsically very strong engine.
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I didn't say they switched any cylinders off,did I???.Most had at least one cylinder misfiring if not more.
Just shows what sort of people must have been driving them.
Actually in New York in the early seventies my impression was that a lot of cars were only firing on five or six.
But then American V8s sound like that anyway.
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One of the first MK VII Jags I bought sounded very rough, with a real vuh-vuh-vuh sound. Fairly soon after buying it I checked the carburettor float levels. The rear carb float level was very low. Correcting that brought the missing cylinder back to life. I suspect that the price I paid was related to that unresolved problem. It never missed a beat for the rest of the time I had it.
Talking about V12s and missing cylinders; The later (around 1989 on) XJ-S cars had Magneti Marelli ignition, which had a distributor like two stacked six cylinder ones, each firing one bank. They had a tendency for the HT voltage to burn through the insulation in the cap and short out the HT to one bank (usually the left one). This caused a drop in power, and a lot of unburnt fuel going to the catalytic converter, which then became a very expensive pot belly stove. Result was often an engine fire, and sometimes the whole car was destroyed. The good news is that the Series 3 XJ12 saloons never got the Marelli ignition, and kept the Lucas Digital P system, which didn't have the same problem, until the end of production in 1992.
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