how to diagnose a misfire in my BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather tools: OBD-II scanner, socket set, torque wrench, multimeter, compression tester, and vacuum gauge.
• Ensure the engine is cool before starting work.
• Disconnect the battery to prevent electrical shorts.

2. Replace Spark Plugs

1. Remove Engine Cover: Use a socket set to remove any bolts securing the engine cover.
2. Access Spark Plugs: Disconnect ignition coils by unclipping the electrical connectors, then remove the bolts securing the coils.
3. Remove Old Spark Plugs: Use a spark plug socket to carefully unscrew and remove the spark plugs.
4. Install New Spark Plugs: Apply anti-seize compound on the threads of the new plugs, and hand-tighten them before using a torque wrench to tighten to manufacturer specifications (typically around 15-20 lb-ft).
5. Reinstall Ignition Coils: Replace the coils and reconnect electrical connectors.

3. Replace Faulty Ignition Coils (if needed)

1. Remove Ignition Coils: Follow the same steps as above to access and remove the ignition coils.
2. Test Each Coil: Use a multimeter to check coil resistance according to manufacturer specifications.
3. Install New Ignition Coils: If any coil is faulty, replace it with a new unit, ensuring proper seating and connection.

4. Inspect and Replace Fuel Injectors (if needed)

1. Access Fuel Injectors: Remove any components obstructing access to the fuel rail.
2. Test Fuel Injectors: Use a noid light to check for proper electrical signal or listen for clicking sounds.
3. Replace Injectors: If faulty, disconnect the fuel line and electrical connectors, then remove and replace the injector. Reattach the fuel rail and ensure no leaks occur.