I have owned my 99 XJR since January. A couple of months ago I changed the spark plugs and discovered several of the threaded female ends for ignition coil and coil cover bolts were stripped or damn close to it. I put repair/replacement on my to-do list, but my wife and I welcomed twins and the job got pushed down the list. A couple of weeks ago while returning from an out-of-town trip, I started getting a misfire and rough engine while passing an 18-wheeler. Codes were P1316 and P0300. I finally got around to pulling the valve covers this past weekend and found one spark plug drowning in oil on the driver's side bank (US), third cylinder from the timing chain (not sure if that is cylinder #2 or #3). The valve cover had cracked at the moulding/retainer for the spark plug seal. The right side cover had the same crack in the same location, although the seal had not completely blown yet. Looks like it could be either improperly seated seals that were being "crimped" by the moulding edge, which then cracked over time as the seal heated and expanded, or the crack was there first and then as the seal expanded with heat it pushed out on the moulding edge and made the crack worsen over time. Given the stripped cover bolts, I suspect over-torque of the valve cover bolts could have caused the problem if the seal was properly seated when installed, but I found it odd that each valve cover experienced the same type of crack at the same cylinder plug.

Any experiences , insight, advice, tips, words of wisdom, etc. with this issue are greatly appreciated. I assume replacement is the only cure. I have found new covers for about $175 each and a used pair for $90, although I am waiting on pictures to determine the quality. I do not want to buy used if this is a typical issue, but I could sure use the cheaper option if it is not common. Also, any ideas on whether the coil will need to be replaced? The oil was about 1/2" past the rubber boot, but I have no idea if that is enough to kill the coil, nor do I know how to test if off the car absent swapping into another car, which I do not have available.

Thanks in advance.

 

The coil itself should be okay. The only alternative to replacing the valve covers is to carefully use JB Weld to fill in the cracks. Also you can use JB Weld to secure the pressed in nuts that the coil cover bolts screw into. There is a proper bolt tightening sequence and torque settings that should be followed when reattaching the valve covers. The heads themselves have a small seam toward the front calls for gasket sealent to fill in that seam. Obviously the heads are aluminium so be careful when tightening the bolts.

 

Thanks a bunch for the advice. I tried JB Weld for the nuts when I changed spark plugs, but it did not work well, at least not to my satisfaction. Probably user error, but I am not so comfortable with that for the crack near the plug seal. I suspect a previous owner didn't know, or used a shop that didn't know, the bolt tightening pattern or torque specs. Not sure if over-torque would cause the crack at the plug seal, though. I pulled the bolt tightening sequence and torque specs from the JTIS cd and will follow them to the letter.