how to diagnose a rough idle on my BMW 3 Series F30/F31 (2012-2019)

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Disconnect the battery to ensure safety while working on electrical components.

2. Check and Replace Spark Plugs

• Sub-steps:
  1. Remove the engine cover to access the spark plugs.
  2. Disconnect the ignition coils by unplugging the electrical connectors.
  3. Use a spark plug socket to remove the old spark plugs.
  4. Inspect the spark plugs for wear; if damaged, replace with OEM-equivalent plugs.
  5. Apply anti-seize lubricant to the threads of new spark plugs.
  6. Install the new spark plugs and torque to the manufacturer’s specifications (usually around 23 Nm).

3. Inspect and Clean the Throttle Body

• Sub-steps:
  1. Remove the air intake duct to access the throttle body.
  2. Disconnect the throttle body connector and the intake hose.
  3. Use a throttle body cleaner and a soft cloth to clean the throttle plate and surrounding area.
  4. Reconnect all components and ensure proper sealing of the intake duct.

4. Check Fuel Pressure

• Sub-steps:
  1. Locate the fuel rail and attach the fuel pressure gauge.
  2. Turn the ignition on (do not start the engine) and note the fuel pressure reading.
  3. Compare the reading to the manufacturer's specifications (typically around 50-60 psi).
  4. If pressure is low, inspect the fuel pump and filter for issues, and replace as necessary.

5. Inspect Vacuum Lines

• Sub-steps:
  1. Visually inspect all vacuum lines connected to the intake manifold for cracks or disconnections.
  2. Replace any damaged lines with OEM parts to ensure a proper seal.
  3. Use a smoke machine if available to detect any hard-to-find leaks.