Sounds like you have some carbon buildup on the piston, but running for a few minutes after the spark plug lead was disconnected would be a miracle. How far was the lead away from the plug? At least 1"? Maybe to grounding attachment, connected to the bail cable, isn't working. It holds the wire from the coil and grounds it.

AZ is not the "normal." I'm aware that the car has been to Jaguar service several times. The dealer, however, is 1.5 hours away, one-way, so a routine battery replacement is reasonably obtained locally. When the battery issue continued after the first replacement swap, I advised having the car checked for electrical problems ... but per what I was told, the last swap with AZ, the battery ~6 months old, was leaking acid, wouldn't take/hold a charge. Perhaps an electrical problem causing the battery to overheat? The car isn't driven much, he has a company truck. It may stay idle for 5 to 6 days at a stretch.

Something is miswired in your control system or two of the conductors are getting together somewhere keeping 24v to your Y terminal I would pull thermostat off the wall and disconnect then tie R and Y wire together and apart and make sure that contols cooling the R and G and see if that controls fan Here is a link that might be useful: Air Conditioning and Heating Repair Made Easy

dave Is it just the outside condensing unit that continues to run or do both the outside unit and the attic air handler continnue to run? If only the outside unit then yes you probably do have a contactor that is stuck closed and will need to be replaced. You can test your contactor by removing the low voltage (24 volts) control wires from the base of the contactor then power back up and try the unit again, if the contactor is still pulled in (stuck closed) and the compressor and fan still run on the outside unit without the control wiring attached then yes the contactor is bad. Note you will need to cut the power off for the outside unit PLUS the attic air handler to shut off all the power. The attic air handler contains the 24 volt transformer and if you do not shut it down there could still be 24 volts coming to the outside unit thru the low voltage wiring. be careful out there people. I would never ever touch any electrical device unless it is proved to be off using a known good and correctly set voltage meter to check it. If you do not have a volatge tester and do not have some knowledge of how to use one please have someone else check the unit out.