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

Solution

1. Preparation

• Tools Required: OBD-II scanner, socket set, torque wrench, compression tester, multimeter, fuel injector cleaner.
• Disconnect the negative battery terminal to prevent accidental electrical shorts.
• Allow the engine to cool before starting work.

2. Inspect and Replace Spark Plugs

• Remove the engine cover (if applicable) using the socket set.
• Remove spark plug wires or ignition coil connectors carefully.
• Use the socket set to remove the spark plugs. Inspect them for wear; replace if they show signs of damage or excessive wear.
• Gap new spark plugs to specifications (usually around 0.028-0.032 inches for BMWs) using a spark plug gap tool.
• Install new spark plugs and torque them to manufacturer specifications (typically 18 ft-lbs).

3. Check and Replace Ignition Coils

• With the spark plugs removed, inspect the ignition coils for cracks or corrosion.
• Test ignition coils using a multimeter (check primary and secondary resistance).
• Replace any faulty ignition coils. To install, simply push the new coils into place and reconnect the connectors.

4. Inspect Fuel Injectors

• Remove the fuel rail if necessary, following the service manual for your specific model.
• Inspect fuel injectors for clogs; clean them using a fuel injector cleaner or replace if damaged.
• Reinstall the fuel rail and ensure all connections are secure.

5. Perform Compression Test

• Remove spark plugs to access cylinder heads.
• Insert the compression tester into the cylinder head opening and crank the engine.
• Record the compression readings; they should be within the specified range (typically 140-200 psi).
• If readings are low, further investigation into the engine's internal components may be necessary.

6. Check for Vacuum Leaks

• Inspect all vacuum lines for cracks or disconnections.
• Use a smoke machine or spray a small amount of carburetor cleaner around the intake manifold and vacuum lines while the engine is running; a change in RPM indicates a leak.
• Repair or replace any damaged vacuum lines.