why does my BMW 3 Series E90/E91 (2005-2012) check engine light come on and off?

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Address Error Codes

• Sub-steps:
  • Connect the OBD-II scanner and document any error codes.
  • Clear the codes and perform a test drive to see if they return.
  • Focus on the most common codes for the E90/E91, including P0300 (random misfire) or P0171 (system too lean).

3. Inspect Ignition System

• Sub-steps:
  • Remove the engine cover to access ignition coils and spark plugs.
  • Inspect spark plugs for fouling or wear; replace if necessary.
  • Check ignition coils for cracks or damage; replace any faulty coils.

4. Check for Vacuum Leaks

• Sub-steps:
  • Visually inspect all vacuum lines and intake manifold for cracks.
  • Use carburetor cleaner or a smoke machine to identify leaks.
  • Replace any damaged hoses or gaskets.

5. Test Fuel System

• Sub-steps:
  • Attach a fuel pressure gauge to the fuel rail.
  • Turn the ignition to the "On" position (do not start) and note the pressure.
  • Compare readings to manufacturer specifications; replace the fuel pump or filter if pressure is low.

6. Examine Exhaust System

• Sub-steps:
  • Inspect the exhaust system for any visible damage or blockages.
  • Use a vacuum gauge to check for back pressure in the exhaust.
  • Repair or replace any damaged components such as catalytic converters if necessary.

Tools Required:

• OBD-II scanner
• Socket set
• Torque wrench
• Fuel pressure gauge
• Vacuum gauge
• Carburetor cleaner or smoke machine

Parts Required:

• Spark plugs
• Ignition coils (if faulty)
• Vacuum hoses (if damaged)
• Fuel filter (if necessary)
• Catalytic converter (if damaged)

Estimated Repair Time: 2-4 hours

Safety Warnings:

• Disconnect battery before working on electrical systems.
• Allow engine to cool before opening the engine compartment.