how to diagnose P0301 code in my BMW 3 Series E90/E91 (2005-2012)?

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the negative battery terminal to ensure safety.

2. Inspect and Replace Ignition Components

• Sub-steps:
  1. Remove the engine cover if applicable.
  2. Disconnect the ignition coil connector from the cylinder 1 ignition coil.
  3. Remove the ignition coil using a socket and ratchet.
  4. Inspect the spark plug condition; if worn or fouled, replace it with a new one.
  5. Install a new ignition coil if the old one shows signs of damage or malfunction.
  6. Reconnect the ignition coil connector and engine cover.

3. Test and Replace Fuel Injector

• Sub-steps:
  1. Disconnect the fuel injector connector from cylinder 1.
  2. Use a noid light to check for signal to the injector; if no signal, investigate wiring or ECU issues.
  3. If the injector is faulty, remove it using a fuel injector puller or appropriate tool.
  4. Install a new fuel injector and reconnect the fuel injector connector.

4. Perform Compression Test

• Sub-steps:
  1. Remove the spark plug from cylinder 1.
  2. Insert a compression gauge into the spark plug hole.
  3. Crank the engine for a few seconds and note the compression reading.
  4. If the compression is low, perform further diagnostics on the cylinder (valves, piston rings, etc.).

5. Check for Vacuum Leaks

• Sub-steps:
  1. Inspect all intake and vacuum lines visually for cracks or disconnections.
  2. Use a smoke machine or propane to detect vacuum leaks while the engine is running.
  3. Repair any leaks found.

6. Verify Fuel Pressure

• Sub-steps:
  1. Attach a fuel pressure gauge to the fuel rail.
  2. Turn the ignition to the "on" position (do not start) and check the pressure.
  3. Compare the reading with the manufacturer's specifications; replace the fuel pump if pressure is low.

Tools Required:

• OBD-II scanner
• Socket set
• Compression gauge
• Fuel pressure gauge
• Noid light
• Torque wrench
• Smoke machine (optional)

Parts Required:

• Spark plug(s)
• Ignition coil(s)
• Fuel injector(s)
• Gasket(s) (if needed)

Estimated Repair Time: 2-4 hours

Safety Warnings:

• Disconnect the battery before working on the electrical system.
• Allow the engine to cool before inspecting or removing components.