The Importance of BMW Spark Plugs and Ignition Coils

We talk about spark plugs and ignition coils frequently in our performance/tuning guides, but why are these parts so important? Looking at the basics, combustion engines work by mixing air and fuel in the cylinders and igniting that mixture. Spark plugs and ignition coils are directly responsible for igniting the air/fuel mixture in the cylinder. The ignition coils are induction coils that convert your BMW’s low voltage battery into thousands of volts required to create an electric spark through the spark plug. As the spark is created it burns the air and fuel mixture, which in turn drives the engines piston to produce power.

Common Engine Problems Caused by Bad Spark Plugs and Ignition Coils

As ignition coils and spark plugs play an essential role in the ignition process, old or faulty spark plugs or coils that generate a weaker spark may not be able to fully burn the air/fuel mixture. This can potentially cause many various issues on your BMW, including:

• Loss of power, responsiveness, and acceleration
• Rough idling or stuttering
• Cold-starting issues, or long cranks
• Engine misfires
• Poor fuel economy

This is not an exhaustive list of all the potential issues, however, these symptoms are typically the most common. Not all symptoms are noticeable, such as loss of power or sluggish acceleration, as they tend to develop slowly as the spark plugs or ignition coils continue to wear down with time. Further, the above symptoms do not guarantee the issue is related to your BMW’s spark plugs or ignition coils.

Diagnosing BMW Ignition Coil and Spark Plug Failures

If you have not replaced your BMW spark plugs or ignition coils within the recommended mileage, then it may make sense to replace them before spending time or money tracking down the issue. Compared to most of the maintenance and repairs on a BMW, it is relatively cheap to replace spark plugs and ignition coils. The installation is fairly straight forward – even for beginner or novice DIY’ers - with basic tools, and a thin-walled spark plug socket. However, for those with newer spark plugs or ignition coils that are believed to be problematic, here are a few ideas to track down the issue:

• Code scanner
  • Engine may or may not be throwing codes. If there is a code, it is typically for misfires and will help guide you to the problematic cylinder or bank
• Data Logging
  • Log a run of your BMW under WOT acceleration and review logs for clues. A cylinder pulling timing may indicate a bad spark plug or ignition coil.

Once you have tracked down the cylinder(s) you believe are the issue then start by moving the ignition coil to another cylinder. You may start with the spark plug, however, the ignition coil will need to be removed to access the spark plug so it is typically easier to start with the coil. Next you will need to test your car once again by data logging or scanning for codes. If the BMW engine code, or timing drops follows to the new cylinder then you likely have a faulty ignition coil. Otherwise, if it remains on the old cylinder then move on to test the spark plugs. As an example, let’s assume you have a code for a cylinder 1 misfire.

1. Move ignition coil from cylinder #1 to cylinder #2
2. Drive the car, and scan for codes again
3. The engine code still suggests cylinder 1 misfire
1. Ignition coil is now an unlikely culprit
4. Move spark plug from cylinder #1 to cylinder #2
5. Drive the car, and scan for codes once again
6. Misfire moved to cylinder 2

In the above example, the misfire followed the spark plug as it was moved to cylinder 2. At this point, you can likely assume the spark plug is the cause of the issue. Replace the spark plug now located in cylinder 2, and scan the car once again to ensure the problem was resolved.

How Often Should You Replace BMW Plugs and Coils?

Obviously, not all BMW engines are the same so it is tough to put an exact number on the lifespan of the spark plugs and ignition coils. Modifications, driving habits, and the brand of the product may all impact the life of your BMW ignition system components. Typically, heavily modified engines that are driven on the track or pushed near their limits on roads will eat through spark plugs. On the other hand, grandma’s stock BMW might preserve the spark plugs and cause them to exceed the recommended replacement mileage without developing issues. As a general rule, the rough lifespan of these BMW parts is as follows:

Stock BMW, minimal aggressive driving

• Spark Plugs: 40,000 – 50,000 miles
• Ignition Coils: 50,000 – 65,000 miles

Heavily modified BMW, significant aggressive driving

• Spark Plugs: 15,000 miles
• Ignition Coils: 20,000 – 25,000 miles

If you fit into the above two categories, those are the rough mileages you may expect to get out of your spark plugs and ignition coils. For others, the life of your BMW ignition parts likely falls somewhere in the middle, leaning towards a longer or shorter lifespan depending on your particular modifications and driving style. For example, my 2007 BMW 335i N54 engine is modified and I fall on the side of an aggressive driving style (I do not track the car – street use only) and my spark plugs have typically lasted around 22,000 miles, while the ignition coils hang in for just over 30,000.

Replace Prior to Tuning or Modifying!

We highly recommend replacing your BMW spark plugs and ignition coils prior to tuning and/or modifying your car, especially if you know they have not been changed for a while.

Jake and I were both disappointed for days after ordering JB4 tunes for our BMW N54 engines. In both cases, after installing the tunes we realized the new-found power was too much for the old ignition parts. All our anticipation and excitement to get the tune installed, only to get slapped in the face with stuttering and significant under-performance. To further add, there were no recognizable symptoms of old/faulty spark plugs or coils until the JB4 tuner was added and extra power was unleashed.