why does my BMW 3 Series E90/E91 (2005-2012) have a rough idle when cold starting

Solution

1. Preparation

• Gather all necessary tools and parts before starting the repair.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the negative battery terminal.

2. Replace Spark Plugs

1. Remove the engine cover (if applicable) using a socket set.
2. Disconnect each ignition coil by pulling up on the connector.
3. Remove the ignition coils using a socket or wrench.
4. Using a spark plug socket, carefully remove the old spark plugs.
5. Inspect the old spark plugs for wear; replace with OEM or equivalent plugs.
6. Install new spark plugs and torque them to manufacturer specifications (typically around 15-20 ft-lbs).
7. Reinstall the ignition coils and secure them in place.

3. Clean Throttle Body

1. Remove the throttle body intake hose by loosening the clamps with a screwdriver.
2. Disconnect the throttle body connector.
3. Remove the throttle body bolts using a socket set.
4. Carefully detach the throttle body from the intake manifold.
5. Use throttle body cleaner and a soft cloth to clean the throttle body interior.
6. Reinstall the throttle body, ensuring a proper seal and tightening bolts to manufacturer specifications.
7. Reconnect the throttle body connector and intake hose.

4. Inspect Air Filter

1. Locate the air filter housing and remove the clips or screws.
2. Remove the old air filter and inspect for dirt and debris.
3. Install a new air filter, ensuring it fits snugly in the housing.
4. Reattach the air filter housing cover securely.

5. Test Fuel Pressure

1. Connect a fuel pressure gauge to the fuel rail's service port.
2. Turn the ignition to the "On" position without starting the engine to check pressure.
3. Ensure that the pressure matches manufacturer specifications (typically around 50-60 psi).
4. If pressure is low, consider replacing the fuel pump or fuel filter.