Obviously if you have persistent misfire issues that are not from bad plugs or failed ignition coils then it might be time to try a colder plug. There is no "rule" for when to move to a colder plug. There are several colder N54 spark plugs available, depending on your state of tune. This reduces temps inside the combustion chamber and avoids pre-ignition. We fight this by using a spark plug that does not burn as hot, also known as a "colder" plug. This is called pre-ignition and is very similar to misfires. The higher temps lead to fuel igniting separate from the spark plug. Whenever more fuel is burned the temperature inside the combustion chamber rises. In the tuning world, adding more boost and more fuel calls for changes and upgrades in other areas. It's exactly what you want for a mass-market OEM plug and it's what we recommend for everyday use on a non modified 135i/335i/535i. The Bosch plugs use a Nickel-yttrium and copper electrode which works well under a wide variety of temperature conditions and fuel grades. NGK does not offer a direct replacement stock plug for this engine. There is only one direct replacement OEM N54 spark plug, made by Bosch (part # ZGR6STE2 or BMW #12120037244). This is usually preceded by poor fuel mileage, hard starting, and rough running. If left in the car for too long the plugs wear to nothing and will no longer fire.
The original recommendation for spark plug replacement was 45,000 miles - way less than the 100,000 mile plugs of previous BMWs. Misfires can be caused by several different things but failed ignition coils or worn spark plugs are the main culprits. Knock sensors, exhaust gas temperature sensors, and oxygen sensors tell the engine computer that something is off. Ignition problems are usually misfires - the ignition coil and spark plug are unable to completely burn the air/fuel mixture as expected. The N54 has suffered from a few long term issues, with the ignition system being close to the top of everyone's list of annoyances. Certainly some of the engineers working on the engine weren't even born yet! So a few hiccups and teething problems were to be expected. The N54 engine was BMW's first turbo gasoline engine since the 1974 2002 Turbo. Colder plugs mean less chance of detonation and lower combustion chamber temps but only make sense on highly tuned cars that need that extra protection. Note: while this page is specific to the N54 twin-turbo engine, the discussion also applies to other BMW engines as well.
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