Owner's Problem Description
The owner reports that the check engine light has illuminated, and the engine is shaking. They have replaced the spark plugs, ignition wires, rotor, cap, O2 sensors, and throttle body tube. After driving 60 miles, all spark plugs except for cylinder #5 were clean, while #5 showed an oily brown residue. Compression tests revealed readings of 1-140, 2-140, 3-133, 4-140, 5-147, and 6-145, with leakdown tests showing no issues.
Professional Technical Analysis
The 1998 Mazda Millenia with a V6 engine (estimated mileage around 150,000 miles) is experiencing a misfire in cylinder #5, evidenced by the check engine light and noticeable engine shake. The owner has performed extensive parts replacements, including spark plugs, ignition wires, rotor, cap, O2 sensors, and throttle body tube, indicating a proactive approach to resolving the issue. Despite these efforts, the #5 spark plug shows oily brown residue, suggesting oil contamination, while the compression readings are relatively consistent across the other cylinders, suggesting no significant internal mechanical failure. Current readings show cylinder #5 at 147 psi, which is within acceptable limits compared to the others, but the oily condition indicates an external oil leak or an internal issue such as valve guide wear. The absence of leakage during the leakdown test further complicates the diagnosis but may suggest ignition system faults localized to cylinder #5. Ignition timing, coil pack performance, and potential fuel delivery issues must be considered. Given the vehicle's history and symptoms, a thorough inspection of the ignition system components, including the coil pack and associated wiring, is necessary. Additionally, checking the crankshaft and camshaft sensors for proper operation is crucial as they play a key role in the engine's timing and performance. The vehicle's current condition raises safety concerns, particularly due to the engine shake, which could affect drivability and engine longevity. Component wear patterns, particularly in high-mileage vehicles like this, warrant careful observation for early signs of failure that could lead to larger issues if not addressed promptly.
Possible Causes
Most common causes (ordered by frequency):
- Oil Contamination in Cylinder #5: The oily brown residue on the spark plug indicates possible oil leakage from the valve cover gasket or worn valve guides. This can lead to misfiring due to improper combustion conditions. The presence of oil may also explain the lack of leakage found in the leakdown test, as oil can prevent airflow while still allowing pressure to build in the cylinder. Investigate the valve cover and related seals for wear or damage. - Oily spark plug in cylinder #5, engine shaking during operation, historical maintenance records showing previous valve cover gasket replacement
- Ignition System Fault: Despite having replaced the rotor and cap, the ignition coil may still be faulty or providing weak spark to cylinder #5. A misfire could result from insufficient spark energy, particularly under load. Testing the coil's output voltage should reveal any irregularities. If the coil is shared among multiple cylinders, it can still cause localized issues. - Check engine light, rough idle, possible stored fault codes indicating ignition issues, weak spark during testing
- Fuel Delivery Issue: While fuel pressure seems normal, intermittent fuel delivery or injector malfunction in cylinder #5 could lead to lean conditions, causing misfire. Verify fuel injector operation using a noid light or oscilloscope to ensure proper pulse width modulation. If the injector is clogged or failing, it may not provide sufficient fuel. - Rough running conditions, potential lean conditions during operation, fuel delivery tests showing injector issues
- Camshaft or Crankshaft Sensor Malfunction: Faulty sensors can lead to incorrect timing signals, which might cause misfiring in specific cylinders. Since the vehicle has a distributor setup, the relationship between timing and firing order is critical and must be verified. Testing the sensor output and ensuring proper alignment can help diagnose this issue. - Misfire under load, check engine light, potential codes indicating sensor malfunction, visual inspection of sensor alignment
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 - Comprehensive OBD-II Scan: Use a professional OBD-II scanner or the GeekOBD APP to retrieve stored codes and analyze live data. Focus on misfire codes and any related fault codes that may indicate issues with the ignition system or fuel delivery. Check freeze frame data for conditions when the fault occurred. This initial scan will provide valuable insight into potential issues with cylinder #5.
- STEP 2 - Visual Inspection of Ignition Components: Conduct a thorough visual inspection of the ignition components, including the distributor cap, rotor, and coil pack. Look for signs of wear, corrosion, or damage that could affect performance. Ensure that all connections are secure and that there are no signs of arcing or excessive wear on the ignition wires. This step can identify any obvious issues that may have been overlooked.
- STEP 3 - Testing Ignition Coil and Spark: Use a multimeter to test the ignition coil's primary and secondary resistances, which should be within manufacturer specifications (e.g., primary resistance typically between 0.5-1.5 ohms, secondary resistance 8,000-15,000 ohms). Check the spark on all cylinders with a spark tester to ensure adequate performance. This can help pinpoint the ignition system's efficiency.
- STEP 4 - Fuel Injector Testing: Use a noid light or oscilloscope to check the fuel injector's pulse width and ensure it is delivering fuel correctly. If the injector is found to be malfunctioning, it may need to be replaced (OEM part number for the injector: 195500-2150) with an estimated cost of $40-$150. This test will confirm whether fuel delivery is contributing to the misfire condition.
