Although most people consider a misfire is caused by spark plugs or coils, to the car computer a misfire is detected when a cylinder or cylinders do not produce as much power as the other cylinders. A weak cylinder could be due to poor compression in the cylinder, a weak fuel mixture such as a clogged injector.

Before throwing parts at the X I would investigate the health of the engine first. Don't know how it is on the other side of the pond, but over here one can rent diagnostic tools. A simple compression test can be performed at the same time you are replacing all the spark plugs. A fuel pressure gauge can be usede to determine the fuel pressure at the rail. Apps such as Torque or OBD Fusion can be used to log data such as MAF, (g/s) Fuel trims (long term and short term), etc. Posting these data can be more helpful in determining the problem.

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2006 Infiniti G35
2001 BMW 3.0I E53 X5 Build date 08/2000 SOLD
Lotus Europa 1970 Destroyed by fire
Lotus Europa 1970 S2 Renault Powered
Lotus Type 52 1970 Twincam Webers Powered
PORSCHE 911 Targa 1982 The Garage Queen
Audi Avant donated to Kars for Kids
BMW 525IT Sold
Audi 4000CS Quattro Sold
Jensen Healey Lotus Powered Sold
Opel 1900 Sold
Triumph Spitfire 1971 Sold
Triumph Spitfire 1968 Sold
Plymouth "Cuda" 340 Six pack SOLD