how to diagnose the cause of my BMW 3 Series E90/E91 having low power?

Solution

1. Preparation

• Gather tools and parts needed for repair.
• Disconnect the battery before starting any electrical work.
• Allow the engine to cool down if it has been running recently.

2. Addressing Fault Codes

• If error codes indicate specific issues (e.g., misfire, lean condition):
  1. Note the error codes and research their meanings.
  2. Clear the codes using the OBD-II scanner.
  3. If codes reappear after a test drive, focus on the related components for repair.

3. Repairing Fuel System Issues

• If low fuel pressure is detected:
  1. Replace the fuel pump if it is not delivering adequate pressure.
  2. Check and replace the fuel filter if clogged.
  3. Inspect and replace fuel injectors if they are malfunctioning or clogged.

4. Replacing Ignition Components

• If spark plugs or ignition coils are faulty:
  1. Remove the ignition coils and inspect for damage.
  2. Replace faulty ignition coils and spark plugs with OEM or high-quality aftermarket parts.
  3. Torque spark plugs to manufacturer specifications (typically around 20 Nm).

5. Inspecting and Repairing Intake/Exhaust Issues

• For intake leaks or blockages:

  1. Replace any damaged hoses or air filters.
  2. Ensure the mass air flow (MAF) sensor is clean and functioning.

• For exhaust issues:

  1. Inspect the catalytic converter for blockages and replace if necessary.
  2. Repair any leaks in the exhaust system.