how to fix P0304 code in my BMW 3 Series E90/E91 (2005-2012)

BMW 3 Series E90/E91 (2005-2012)

Error Codes

2-4 hours

Time

medium

Difficulty

medium

Severity

Error Code: P0304

3 min read

574 words

Problem Statement

The P0304 code indicates a misfire detected in cylinder 4 of a BMW 3 Series E90/E91 (2005-2012). This code typically results from issues related to the ignition system, fuel deliv

Error Code

P0304

Estimated Time

2-4 hours

⚠️ Safety Warnings

Important safety information - read before proceeding

•Disconnect the battery before working on electrical components.
•Relieve fuel pressure before servicing the fuel system.

Symptoms

•Check engine light illuminated
•Rough idle or engine shakes
•Decreased engine power
•Poor fuel economy
•Engine stalling or hesitation during acceleration

Diagnostic Steps

1
Scan the vehicle using an OBD-II scanner to confirm the P0304 code and check for any additional codes.
2
Inspect the ignition coil and spark plug for cylinder 4 for signs of wear or damage.
3
Check the fuel injector for cylinder 4 to ensure it is functioning properly.
4
Perform a compression test on cylinder 4 to assess engine mechanical health.
5
Inspect for vacuum leaks around the intake manifold and associated hoses.
6
Examine the wiring and connectors related to the ignition system and fuel injector for any damage.

Solution

💡 Expert Tip

For complex repairs like this, consider taking photos at each step. This helps if you need to reverse a step or consult with a mechanic. Also, keep all removed parts organized - you may need to reference them during reassembly.

Verification

After completing the repair, verify that the issue has been resolved:

✓Test the affected component to ensure it functions correctly
✓Check for any error codes or warning lights
✓Monitor the vehicle for a few days to ensure the issue does not recur

Prevention Tips

To prevent this issue from occurring again:

•Follow the manufacturer's recommended maintenance schedule
•Address warning signs early before they develop into major issues
•Use quality parts and fluids recommended for your vehicle
•Keep detailed records of all repairs and maintenance

Tools & Equipment Required

•Remove the spark plug using a spark plug socket.
•Inspect the spark plug for wear, carbon deposits, or damage.
•Reinstall the ignition coil and secure it with the retaining bolt.

Parts Required

•Spark plug (if necessary)
•Ignition coil (if necessary)
•Fuel injector (if necessary)

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Diagnostic Steps

1
Scan the vehicle using an OBD-II scanner to confirm the P0304 code and check for any additional codes.
2
Inspect the ignition coil and spark plug for cylinder 4 for signs of wear or damage.
3
Check the fuel injector for cylinder 4 to ensure it is functioning properly.
4
Perform a compression test on cylinder 4 to assess engine mechanical health.
5
Inspect for vacuum leaks around the intake manifold and associated hoses.
6
Examine the wiring and connectors related to the ignition system and fuel injector for any damage.

Solution

1. Preparation

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

2. Inspect and Replace Spark Plug

Tools Required: Socket set, spark plug socket, torque wrench.
Remove the ignition coil from cylinder 4 by unscrewing the retaining bolt and gently pulling it out.
Remove the spark plug using a spark plug socket.
Inspect the spark plug for wear, carbon deposits, or damage.
If necessary, replace the spark plug with a new one, ensuring it is gapped correctly (typically 0.028-0.032 inches).
Reinstall the ignition coil and secure it with the retaining bolt.

3. Inspect and Replace Ignition Coil

If the spark plug looks good but the misfire persists, proceed to inspect the ignition coil.
Remove the ignition coil from cylinder 4 as previously described.
Use a multimeter to check the coil's resistance; if it is out of specification, replace the ignition coil.
Install the new ignition coil and reconnect the electrical connector.

4. Inspect Fuel Injector

If issues remain, inspect the fuel injector for cylinder 4.
Disconnect the fuel injector connector and remove the injector from the rail (ensure fuel pressure is relieved beforehand).
Check for clogs or leaks. Clean or replace the injector if necessary.
Reinstall the injector and reconnect the connector.

5. Perform Compression Test

If the misfire persists, perform a compression test on cylinder 4.
Remove the spark plug, insert the compression tester, and crank the engine.
Compare the compression reading with the manufacturer's specifications. If low, further investigation into engine internals may be required.