what causes my BMW 3 Series E90/E91 (2005-2012) to have a rough idle in gear

Solution

1. Preparation

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

2. Inspect and Clean the Throttle Body

• Remove the air intake duct to access the throttle body.
• Use throttle body cleaner and a soft cloth to clean the throttle plate and surrounding areas.
• Ensure the throttle plate moves freely without obstruction.
• Reinstall the intake duct securely.

3. Replace Spark Plugs and Ignition Coils (if necessary)

• Remove the engine cover to access the ignition coils.
• Disconnect the electrical connectors from the ignition coils.
• Remove the ignition coils using a socket set.
• Replace spark plugs with new ones, ensuring proper gap specifications (typically 0.028-0.032 inches for E90/E91).
• Reinstall ignition coils and reconnect electrical connectors.

4. Check and Repair Any Vacuum Leaks

• Inspect all vacuum hoses for cracks or disconnections.
• Replace any damaged or deteriorated hoses.
• Recheck the intake manifold gasket for leaks and replace if necessary.

5. Reassemble and Reset

• Reconnect the negative battery terminal.
• Start the engine and allow it to idle for a few minutes.
• If the check engine light was illuminated, clear the error codes with the OBD-II scanner.