how to fix the P0301 misfire code in my BMW 3 Series E90/E91

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the negative battery terminal to prevent any electrical shorts.
• Allow the engine to cool completely before beginning work.

2. Replace Spark Plug

• Sub-steps:
  1. Remove the engine cover if applicable.
  2. Disconnect the ignition coil connector on cylinder 1.
  3. Remove the ignition coil from the spark plug.
  4. Use a spark plug socket to remove the old spark plug.
  5. Inspect the spark plug for signs of wear or damage.
  6. Install the new spark plug, ensuring it is torqued to the manufacturer’s specification (usually around 15 lb-ft for BMW).
  7. Reinstall the ignition coil and reconnect the connector.

3. Check and Replace Ignition Coil (if necessary)

• Sub-steps:
  1. If swapping the ignition coil with another cylinder eliminated the misfire, proceed to replace the faulty coil.
  2. Disconnect the ignition coil connector and remove the coil.
  3. Replace with a new ignition coil and reconnect the connector.

4. Fuel Injector Inspection and Cleaning

• Sub-steps:
  1. Locate the fuel injector for cylinder 1 and inspect for leaks.
  2. If clogged, use a fuel injector cleaning kit to clean the injector.
  3. Reinstall the injector after cleaning and recheck for any leaks.

5. Compression Test (if misfire persists)

• Sub-steps:
  1. Remove the spark plug from cylinder 1.
  2. Insert a compression gauge into the spark plug hole.
  3. Crank the engine to obtain a reading and compare to specifications (should be around 140-200 psi).
  4. If compression is low, investigate potential internal engine issues.