Owner's Problem Description
The owner reports that after driving the 2002 Subaru Impreza hard, the check engine light began flashing. A diagnostic scan revealed misfires in cylinders 2 and 3. The owner replaced the spark plugs with OEM parts and switched the ignition coils between good and bad cylinders, also checking the fuel injectors by interchanging them. Despite these efforts, the misfires persist, prompting the owner to seek further advice.
Professional Technical Analysis
The 2002 Subaru Impreza, equipped with a 2.5L flat-four engine and approximately 120,000 miles on the odometer, is displaying misfire codes for cylinders 2 and 3, particularly after hard driving conditions. The flashing check engine light indicates an active misfire condition, which may lead to further engine damage if not addressed. The owner has already replaced the OEM spark plugs but continued misfires suggest potential issues outside of the ignition system. Misfire causes could include loss of compression, vacuum leaks, or fuel delivery problems. Given the cylinder misfires following the owner's interventions, it is essential to analyze the engine's overall condition, particularly focusing on compression tests to assess the integrity of the valves and piston rings in cylinders 2 and 3. The successful compression measurement of 150 psi in both cylinders indicates adequate sealing. However, further investigation into the fuel delivery system, including fuel pressure testing and injector functionality, is warranted. The interchanged coils and injectors should also be re-evaluated to ensure they are functioning properly. The possibility of intake manifold leaks or EGR valve malfunctions must also be considered. Any deviation in fuel pressure from the standard 35-45 PSI could lead to lean conditions, causing misfires. A systematic approach to diagnosing the remaining links in this chain will be critical to resolving the issue.
Possible Causes
Most common causes (ordered by frequency):
- Loss of Compression in Cylinders: A compression test revealed 150 psi in both cylinders, indicating that while the valves and rings are sealing adequately, potential issues like carbon buildup or valve timing could still be affecting performance. Any further deviations in compression could lead to misfire conditions if they deteriorate. It's essential to monitor compression periodically, especially in high-mileage engines. - Compression values indicate adequate sealing, but further degradation can lead to misfires. Attention needed for carbon buildup.
- Fuel Delivery Issues: If the fuel injectors are not delivering the correct amount of fuel to cylinders 2 and 3, misfires could occur. A fuel pressure test should show 35-45 PSI under load. If the pressure is outside this range, consider inspecting the fuel pump and fuel filter for blockages or failure. - Inconsistent fuel delivery can lead to lean misfires; fuel pressure gauge needed for verification.
- Ignition System Fault: While coils and spark plugs are replaced, issues may lie in the wiring or connectors. A weak spark can cause misfires, especially under load. Testing resistance of ignition wires (should be 10,000-15,000 ohms) and inspecting connectors can help identify faults. - Misfires can occur despite new components if connections are poor; check wiring integrity.
- Vacuum Leaks: A vacuum leak can cause a lean mixture in cylinders 2 and 3, leading to misfires. Inspecting the intake manifold gaskets and vacuum hoses for cracks or disconnections is essential, as even small leaks can significantly affect performance. - Lean conditions can result from vacuum leaks; use smoke testing to verify integrity of intake and hoses.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Compression Testing Confirmation: With a compression tester, check the compression of cylinders 2 and 3 again to ensure they maintain the previously recorded value of 150 psi. Any significant drop could indicate further issues with the valves or piston rings. It’s a good practice to compare these values with those of the other cylinders for a complete picture.
- STEP 2 - Fuel Pressure Testing: Use a fuel pressure gauge to measure the fuel pressure at the fuel rail while the engine is running. Check for a consistent reading within the 35-45 PSI range. If pressure is low, investigate the fuel pump and filter. Ensure that the fuel injectors are also functioning correctly by checking for spray pattern and flow.
- STEP 3 - Ignition System Check: Inspect ignition wiring and connectors for signs of wear or corrosion. Test the resistance of the ignition wires to ensure they fall within the 10,000-15,000 ohm range. If any wires show high resistance or physical damage, consider replacement.
- STEP 4 - Visual Inspection for Vacuum Leaks: Perform a visual inspection of the intake manifold and vacuum hoses. Look for cracks, loose connections, or signs of wear. If necessary, use smoke testing to identify any hidden leaks that may not be visible.
