how to fix P0300 random misfire code in BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the negative terminal of the battery to avoid electrical shock.
• Allow the engine to cool before starting work.

2. Replace Spark Plugs

1. Remove the engine cover if applicable.
2. Use a socket wrench to remove the spark plug boots and then the spark plugs.
3. Inspect the spark plugs for wear; replace with OEM-spec spark plugs if worn or fouled.
4. Torque new spark plugs to the manufacturer's specification (typically 22 Nm).
5. Reinstall the spark plug boots securely.

3. Inspect and Replace Ignition Coils

1. Remove the ignition coil connectors by pressing down on the tab and pulling them off.
2. Unscrew the ignition coils using a socket wrench and remove them.
3. Test each ignition coil with a multimeter (primary resistance should be between 0.4 - 2.0 ohms).
4. Replace any faulty ignition coils with OEM parts.
5. Reinstall the ignition coils and connectors securely.

4. Check for Vacuum Leaks

1. Inspect all vacuum hoses connected to the intake manifold for cracks or disconnections.
2. Replace any damaged hoses with new ones.
3. If necessary, use a smoke test to identify hard-to-find leaks.

5. Fuel System Inspection

1. Use a fuel pressure gauge to check the fuel pressure at the rail (should be approximately 50-60 psi).
2. If pressure is low, check the fuel pump and fuel filter; replace if needed.
3. Clean or replace the fuel injectors if clogged.

6. Clean MAF Sensor

1. Disconnect the MAF sensor connector.
2. Remove the MAF sensor from its housing.
3. Use MAF cleaner spray to clean the sensor element gently.
4. Reinstall the MAF sensor and reconnect the connector.