I know this ha probably been flogged to death, but I have finally resolved this issue that took me well over six month to find and repair.

My car Xtype, 02, v6, petrol,l 2.00.

In brief the misfire feeling suddenly developed one day I was travelling up a very shallow hill (more of a slope) the car kicked. (no obd codes) then for about 5 weeks this did not happen again. finally started to get worse mainly noticeable when the car went under more load, up inclines or accelerating harder from a slow speed, (noticeable for me between 35-50 mph). Should you try to drive through the kicking jolting felling finally the amber light would display with the message (no cruse control available) back of the gas a little the onboard computer would re scan itself turning off both the amber light and the message. AA was called on multiple occasions to transport me home as I felt the severe jolting would /could damage engine mountings. (still no obd codes). I found that I could get the computer to display no cruse by disconnecting the MAF sensor whilst the engine was running.

changed the MAF sensor. still the same
checked the top hard plastic hose for cracks none found ,
replaced all the seals I could find with regard to inlet manifold. sprayed carb cleaner at points of possible issue no engine tone change happy no vacume leaks but still the same . (still no codes) I was invited to a jag specialist for a complete scan with particular attention to live data in an attempt see what could be going on. I did manage to get them to agree that should they not be able to diagnose the issue there would be no charge. (they agreed) still nothing found. (this was scanned using IDS software)

finally I purchased a phone app and a wifi obdII interface totalling about £16.00 so I could watch my own live data. finally I got a misfire on pot 1. manifold off for replacement (still the same) run like this hoping another symptom would show up (none did) finally got a misfire on 5, this time I took a much close look totally removing the inlet manifold, (mind the small hard plastic pipe that is connected to the brake servo unit snaps easily) I found that the spark plugs which should have been changed on a service had not the spark gap on five was far to wide , swapped out all the coil packs under the manifold new plugs new top seals under manifold. replaced and cured. In most of the advice I have read in my opinion has not gone far enough to either describe the symptom or the possible cause hence this rather long post, bottom line when you have your car serviced insist they document what they have done on the final invoice pot five is the hardest plug and coil pack to access and is often ignored, Now I don't know if the coil pack had an issue or the spark plug gap was generating to much heat detected by the coil pack.

I swapped it out with lucas coil packs (c2s42673L) £35.00 for 2 and denso plugs (t20tt/ 4617) £15.00 for 6 all from ebay

hope this post is not deleted for some moderator reason as it may help others. I forgot to mention a test I did and that was to deflate the rear tyres so that it adds more drag and the car reacted more severely.

 

NO - but I have moved it from General Tech Help to the X-Type forum. This is the place for technical posts about your model.

Graham