Misfire

[maxim.nicholls718]

Hi
My ‘94 6 litre xjs has developed a misfire.
When cold it runs perfectly but once up to temperature it loses power and starts backfires. Any ideas ?
It’s had all new plugs and leads and distributors
Max

[mike020150]

If the car is backfiring when warm, it is quite likely that the fuel/air mixture is running weak. Thus, start the diagnosis by methodically checking for air leaks from the inlet manifold/head joints to the throttle bodies paying particular attention to the various vacuum lines and valves. Only if nothing is found, move on to check the rest of the ignition system (coil pack) which hasn't already been renewed. Mike.