2000 XJR Supercharged, 92K miles. After an overnight cold start engine runs smoothly and is responsive to acceleration. After about 15 miles, "Restricted performance" light comes on and engine idles roughly. I've cleaned the MAF sensor (with the spray application). My service techs tell me all 8 ignition coils are bad, even though we replaced one 3 months (1,500 miles) ago. Restarts after engine cools down and runs for up to about 12-15 miles and the "Restricted performance" light comes on and rough idling resumes.

 

Best thing to do is have the codes read, so we can see what's tripping it.

 

Getting the codes read is always the starting point. But try something.Start if from cold and as soon as the restricted performance kicks in open the boot and try to put your hand on the battery. Is it HOT, not warm but really feels hot. If so you may have an internal short in the battery that opens when hot and kicks off the restricted performance due to low voltage.

 

 

+1 all 8 do not go bad at once!

 

You might also have a bad temperature sensor. Car enriches, it is cold, at start up, but sensor never tells the ECU that it is now warm to flow fuel at a reduced rate. Codes!!!

Might also try a fuel system cleaner (BG44K or equivalent) to see if that doesn't clean up the system. It is also very easy to pull the coils and run a cleaning tool where they touch the plugs. A few years back, an XK8 owner had the same problem, most of us were pretty sure it was bad secondary tensioners (jumped timing -- yours are the third generation, right?), wound up dirty electrodes . . .

But up the street, there is a really good Jag tech . . . Bob Goff.

 

If Bob is near or nearish - arrange to see him!

 

I have replaced 5 of my 8 ignition coils & it was over a period of several years. My OBD-II code reader told me exactly which cylinder was misfiring. It is fairly easy to DIY. There s/b threads here with photos.

 

Thanks, CharlzO, Any idea of which code reader device iI should buy? I have an iPhone and understand there is an app that interacts with the code reader? Do code readers provide descriptions of the different codes?

 

I took the car to where I bought the new battery (in January 2017) to have it checked for shorted cells/plates. It showed at 14+ volts and no shorts.

 

It also never was HOT to the touch.

 

Sounds like the alternator volts, not the battery.

 

I just had a similar situation two months ago throttle body got to be replaced