I will give you some more background and maybe it will help. The car is an 02 s-type with the 4.0 and belongs to my brother who lives 100 miles from me. It had around 45000 miles.
He complained it was idling rough and would stumble when accellerating. I plugged in my odbII scanner and no codes were present. Pulled off the coil covers and inspected the area around the coils and it was clean and dry. Pulled the front three plugs on the pass side and they had a white ash on them which I would say is normal. They were NGK plugs gapped at .035". Reassembled everything and replaced air filter which was dirty and the fuel filter. I then drove the car 2 days 15 miles to work on the interstate and back home and it run good. He picked up the car and drove home and said it ran fine. The next day it would stumble at idle in park and on accelleration, and the check engine light was on. He had OReileys read the codes and they told him it was a cyl #4 misfire.
I see coils on ebay #370045219684. Will the DG515 fit the 02. They are around $60 a piece locally with lifetime warranty. Is it ok to replace one coil or should all be replaced. I also assume if a new coil is installed all the plugs should be replaced. What is the correct gap? Should antisieze be used? I assume the heads are aluminum.
Hope this helps, and thanks in advance. The closest jag dealership is 6 hours away and we would like to fix this on our own.