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

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure vehicle is parked on a level surface with the engine off and cool.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Spark Plugs

• Tools Required: Socket set, spark plug socket, torque wrench
• Remove the ignition coil by unscrewing it and disconnecting the electrical connector.
• Remove spark plugs using a spark plug socket.
• Inspect the spark plugs for wear; replace them if they show signs of damage or excessive wear.
• Install new spark plugs, tightening them to the manufacturer’s torque specifications (usually around 18-25 Nm).
• Reinstall the ignition coils.

3. Check and Replace Ignition Coils

• Tools Required: Socket set, torque wrench
• If misfire persists after spark plug replacement, remove the ignition coils.
• Inspect each coil for cracks or arcing.
• Replace faulty ignition coils with new ones, ensuring they are properly seated.
• Torque the bolts to the manufacturer's specifications.

4. Fuel Injector Testing

• Tools Required: Noid light or multimeter
• Disconnect the fuel injector connectors.
• Connect the noid light to the injector connector and crank the engine to check for pulsing.
• If the injector is not functioning, replace it with a new one.

5. Inspect for Vacuum Leaks

• Tools Required: Smoke machine or carburetor cleaner
• Perform a visual inspection of all vacuum hoses and gaskets for cracks or disconnections.
• Use a smoke machine to detect leaks or spray carburetor cleaner around suspected areas while the engine is running to identify a vacuum leak (engine RPM will change if a leak is present).

6. Compression Test

• Tools Required: Compression tester
• Remove spark plugs and insert the compression tester into the cylinder.
• Crank the engine and record the compression readings.
• Compare results to manufacturer specifications; if low, further investigation into internal engine issues may be required.