I changed the valve cover gasket about 2 years ago because it was leaking down the motor. I notice the wells were filled with oil again about 3 months ago so I took it to an independent shop to have it done thinking it would be resolved better by them. I asked them to diognose it and they said the valve cover gasket was seeping oil past the spark plugs. Fast forward 6 months and again it misses at idle and when I get on it I feel the hesitation and the pause like it's misfiring. Pulled the cover and coils and noticed oil in 3 of the spark plug wells. I also notice they didn't put on the coil gaskets. Could that cause this? What else could it be? No check engine codes show up and it drops about 100 rpm every time it misses. Only use NGK iridiums.

 

Your cam-cover is likely corroded into all sorts of interesting shapes down at the interface with the spark-plug-wells, rendering the rubber gaskets ineffective. Here's a few examples:
https://www.jaguarforums.com/forum/x...thread-163891/

The misfires may or may not have to do with the oil:

https://www.jaguarforums.com/forum/x...solved-175417/

 

Hey thanks for the info and sorry for the late reply. I just took off the cam cover again and after I snugged down the bolts the last time I found no oil in the spark plugs. But it is still missing at idle and at speeds below about 30. The coils are original and are probably in need of change. Do you recommend a certain brand of replacements?

 

Try the factory-correct Champion copper plugs. If that doesn't work get these coils from SNG Barratt; anything else is vulnerable to screwing up again.

 

+1 on using the correct Champion plugs. Over the years there have been many reports of AJ6 and AJ16 engines not performing optimally on other plugs, no matter how expensive or how reputable the manufacturer (NGK, Bosch, ACDelco, Autolite, Denso, etc.). The original Champion plugs were RC9YCC, but Jaguar subsequently updated them to the hotter RC12YCC (apparently no longer available, so most owners use the identically-performing RC12YC). The gap spec is 0.9mm / 0.035 in., and the torque spec is 22-28 Nm / 17-20 lb.ft.

Personally, I would follow labcoatguy's recommendation to replace the plugs first, carefully gapped, and see if they make the engine happy. If so, you're set and no need for coils.

If you still have some missing, scan for any stored diagnostic fault codes (DTCs) related to the coils (the general code P0300, or the cylinder-specific codes P0301-P0306, the last digit representing the cylinder that is misfiring). Since the engine is 20 years old, you may elect to just replace the coil(s) in the cylinder(s) with misfires, and maybe buy 1 spare to keep in the trunk. But since the correct OEM "Made in Japan" coils were unavailable for many years, it might be worth considering purchasing six new ones from SNG Barratt while they're still available.

Cheers,

Don