Hi,
Firstly I have to admit to being a real novice with engines of any type but am loving learning about this fantastic car.
So far I've fitted a new quad "straight through" Adamesh exhaust...new mesh grill, replaced the thermostat and water pump, cleaned all of the wheel sensors, upgraded and fitted 20" BBS Persius Wheels, polished it LOTS and simply put have a stunning looking Pacific Blue with Ivory Leather interior XK8 Convertible....what more could a middle aged man want.....mid life crisis I hear you saying!!
However, over the past 6 months I've been plagued by an intemittent "Restricted Performance" message on start up.
Initially it occurred shortly after a service and on removal of the coil and plug I notice a little oil. As this plug is below the oil filler, I assumed that there had been a little spilage during the service!
Having cleaned the small residue of oil I replaced the plug and coil pack and initially all seemed fine and I thought that had solved the problem.
Alas, the next time I took her out for a spin, the same happened again.
At this point I should mention that she only comes out of the garage on good days and only covers about 3k miles pa.
I've now swapped the coils around and replaced the plugs with NGK IFR5N10, but once again the problem came back at the weekend.
I have just put a bottle of FORTE Advanced Petrol Treatment in the tank and am waiting to see if it makes any difference....could it just be a clogged injector?
If this doesn't work, can anyone give me any other ideas as to what it could be?
Presumably as the message always refers to cylinder 6, it's only a problem in this area and nothing to do with air intake os MAS readings.
Many thanks

Get yourself an OBDII code reader. Or if you have one, what code(s) is it throwing ? It could be a leaky plug seal leaking oil onto the spark plug, if it's always the same plug.

Oil+coil = misfires / damaged coil. (Stop the oil.)

If its always #6, no matter if you've changed coils around or replaced the plug...its either continually getting oil into it (easily discovered) in which you'll be replacing the valve cover gasket...or it could be a bad connector or wire from that offending coil. They do get crispy over time! I can't see any other obvious reason for an isolated cylinder showing a misfire. Find it quick, however!

Thanks for your comments and suggestions.
As luck would have it, I've not been able to reproduce the fault in the last few days and get a fault code on my OBD11.
I'm sure it will be back soon and I'll post the code.
Thanks

Apply some dielectric grease to the inside of the boot for extra protection. It may be firing through the boot.

Oh, I'm really showing myself up.
What's the boot....is it the inside of the coil...sorry😁😁

The boot is the little rubber part that insulates the coil from the head. It would have to be really, really degraded for it to be arcing.

Do the new plugs have oil on them as well? Can you take a photo?
this is a good reference to start troubleshooting your plugs:
Beakys Motorcycles SPARK IT UP!!!!!

I'm guessing your valve cover gasket is leaking slightly and allowing oil to seep into the spark plug well.

The coils look pretty good to me and there is only oil on the spark plug screw threads...
Does it sound like a gasket problem?

I'll post photos shortly

If there is oil on the plug the gasket is leaking.

If only on the threads, clean and see if it comes back (& when). I don't see oil just on the threads causing any codes. On the coils - yes.

Thanks again.
I've done that and will check them again, when the issue comes back!
In the last week, after putting the FORTE Advanced Petrol Treatment in the tank, on "cold" start up, it misfires slightly for 10 seconds or so and then clears up and runs perfectly. This hasn't generated any coded though, however I'm sure that after a bit more useage, the gremlins will return....if not this one it will be another!!

Okay...it's back.
I haven't used the car for a few weeks and when she started up yesterday "Restricted Performance" was back for about 5 minutes, whilst ticking over and then it cleared itself.
I managed to pull the following codes....P0306, P1316, P0356 and P1000.
I've already replaced the coil pack, so running out of ideas.

Ideas:
1. new coil is bad - swap with an existing OE one
2. it's a wiring fault
3. maybe you misunderstood where #6 is?

When the car is not being used, do you leave it connected to a battery conditioner, or is the battery left to slowly go flat?

Its always worth checking the battery on these cars when you start getting seemingly random problems.

Hi JagV8,
I've swapped all of the coils around and still get the problem!
I'm beginning to think it could be the wiring, but have you got any idea why it clears after a few minutes?
I'm working on #6 coin being on the right side and 3 back if looking from the front.
Thanks

Hi dibbit,
Thanks for your reply.....no I do not leave the battery on charge, but do charge it up occasionally!
I've had this problem for a while and sometimes the day after I've driven her.
Thanks

With the codes, it is time to go straight to the proverbial horse, and hear from its mouth: X103 DTC.

A quick read suggests the injector's health need to be brought in the analysis...

Best of luck, keep us posted.

Can P0356 be an injector fault? News to me (and Jaguar I think).