Owner's Problem Description
The owner reports that their 1999 Ford Expedition is experiencing a misfire on cylinder number 6, especially noticeable at low idle. They have replaced several components including spark plugs, ignition wires, ignition coil, EGR valve, and fuel filter, but the issue persists. The vehicle also exhibits a slight kickback when idling. The owner has taken the vehicle to a Ford dealership, but no faults were found by the mechanics.
Professional Technical Analysis
For a 1999 Ford Expedition equipped with a 5.4L V8 engine and approximately 150,000 miles, the misfire on cylinder number 6 is a concern that requires a systematic diagnostic approach. The symptoms indicate a possible ignition issue, fuel delivery problem, or a mechanical fault in the cylinder itself. Given the extensive replacement of ignition components, attention should shift to the fuel injector for cylinder 6, which may not be functioning properly despite its recent replacement. Additionally, the misfire can be influenced by air-fuel mixture discrepancies, potentially linked to the EGR system or MAF (Mass Air Flow) sensor performance. The intermittent kickback during idle suggests that the cylinder may not be firing consistently, indicating an underlying issue affecting combustion stability, possibly related to electrical faults or sensor inaccuracies. The vehicle's condition, with the check engine light possibly illuminated, and potential fault codes stored in the PCM, should guide the diagnostic process. It's critical to evaluate live data from the ignition system during misfire events to pinpoint the exact fault, as the issue appears localized to cylinder number 6. A thorough compression test should confirm cylinder integrity, while vacuum leak checks can rule out external air intrusion affecting the combustion process. Observations of spark plug condition, injector pulse width, and fuel pressure should also be collected to provide a complete picture of the running condition.
Possible Causes
Most common causes (ordered by frequency):
- Ignition System Fault: Despite replacing the ignition coil and spark plugs, the ignition system for cylinder 6 may still have a fault. This could include issues with the wiring harness, an intermittent connection, or a defective ignition control module (ICM). The ICM should be checked for proper voltage output, usually around 12V, while monitoring for any irregularities in signal when a misfire occurs. - Check engine light, stored misfire codes related to cylinder 6, intermittent spark issues observed during live data monitoring.
- Fuel Injector Failure: Even though the fuel injector for cylinder 6 was replaced, it may still be defective or clogged. An injector balance test should be performed to ensure that fuel delivery to this cylinder is adequate. Resistance of the injector should be checked, typically around 12-16 ohms, and a noid light can be used to verify that the injector is receiving the proper signal. - Inconsistent fuel delivery to cylinder 6 during injector testing, resistance measurements out of spec, potential clogs confirmed by visual inspection.
- Mechanical Issue in Cylinder: A mechanical problem such as a burnt valve or piston ring issue could cause a misfire that would not be resolved by replacing ignition components. A compression test should reveal if the cylinder is holding pressure adequately; normal compression should be around 150-180 psi. If compression is low, further investigation into the cylinder head or piston assembly will be necessary. - Compression readings below 150 psi, indicating potential mechanical failure, visual inspection reveals signs of oil consumption or valve damage.
- Vacuum Leak: A vacuum leak near cylinder 6 could disrupt the air-fuel mixture, leading to misfire conditions. Check for any cracks or broken hoses near the intake manifold or throttle body. A smoke test can help identify any leaks that may not be visible during a visual inspection. - Hissing noise detected near the intake manifold, unstable idle conditions, smoke test confirms presence of vacuum leaks.
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 - OBD-II Scan: Begin with a thorough OBD-II scan using GeekOBD APP or a professional scanner to retrieve any stored fault codes and live data. Focus specifically on misfire-related codes for cylinder 6, which may provide insights into the issue. Additionally, check freeze frame data to understand conditions under which the misfire occurs.
- STEP 2 - Live Data Monitoring: Utilize the GeekOBD APP to monitor ignition timing, fuel trims, and injector pulse width specifically for cylinder 6. This data can help identify discrepancies in fuel delivery or ignition performance. Pay attention to the voltage signals at the ignition coil and injector during misfire events, which should typically be around 12V.
- STEP 3 - Compression and Vacuum Testing: Perform a compression test on cylinder 6 to ensure it is within the normal range of 150-180 psi. If compression is low, investigate further for mechanical issues. Also, conduct a vacuum leak test around the intake manifold and associated components to rule out any air intrusion that could affect performance.
- STEP 4 - Fuel Injector Testing: Check the resistance of the fuel injector for cylinder 6 using a multimeter, which should be between 12-16 ohms. Use a noid light to verify that the injector is receiving a proper signal. If the injector is suspected to be clogged, perform a fuel injector cleaning or replacement.
