Owner's Problem Description
The owner reports that the 1994 Mazda 626 with a 2.0 V-6 engine has been running rough and missing, especially when the A/C is on. The vehicle shuts off when stopped with the A/C engaged but not when it is off. While driving, it behaves like it is running out of gas, gradually slowing down and finally shutting off. The fuel filter in the gas tank has already been replaced, but the car still won't start; it only turns over but does not catch.
Professional Technical Analysis
The 1994 Mazda 626, equipped with a 2.0 V-6 engine and approximately 186,000 miles on the odometer, is exhibiting multiple symptoms indicative of fuel delivery or ignition system issues. The rough running and stalling when the A/C is engaged suggest potential problems with the engine's ability to maintain proper fuel-air mixture under load. This can often be traced back to issues such as failing ignition components, fuel pump performance, or clogged filters. The condition of the spark plugs and ignition wires is crucial, especially given the reported oil presence in the plug wells, which can cause misfiring and overall poor engine performance. Voltage measurements at the ignition coil should confirm if adequate power is being supplied during operation. Furthermore, if the vehicle fails to start, it is important to verify fuel pressure, which should be within the range of 35-45 PSI, and ensure that the fuel pump is functioning correctly. Additionally, the MAP sensor's readings should be monitored since erratic values could cause incorrect fuel metering, exacerbating the symptoms. A thorough inspection of the valve cover gasket is also recommended, as oil leaks may compromise ignition performance and lead to fouled spark plugs. Current conditions indicate that the vehicle may pose safety risks if not addressed promptly, especially considering the potential for stalling in traffic. Therefore, prioritizing diagnostics on the ignition and fuel systems is critical to restore reliable operation.
Possible Causes
Most common causes (ordered by frequency):
- Oil in Spark Plug Wells: The presence of oil in the plug wells can cause misfires due to poor ignition. When oil leaks from the valve cover gasket, it can foul the spark plugs, leading to rough running conditions and potential stalling, particularly under load conditions like when the A/C is on. A visual inspection of the spark plugs should reveal fouling, and a valve cover gasket replacement (OEM P/N 0000-00-0000) is recommended. It's critical to ensure the plug wells are cleaned and the new gasket is properly installed to prevent recurrence. - Visual confirmation of oil presence in spark plug wells, misfiring symptoms, and rough engine idle.
- Fuel Delivery Issues: If the fuel pump is not delivering the required pressure (35-45 PSI), it can lead to stalling and difficulty starting. This vehicle has 186,000 miles, and the fuel pump may be fatigued or clogged. It is advisable to check the fuel pressure using a gauge at the fuel rail test port and, if necessary, replace the fuel pump (OEM P/N 0000-00-0000) if the pressure is below specification. - Low fuel pressure reading, difficulty starting, and stalling under load.
- Ignition System Failure: The ignition coil or associated components may be failing, particularly under increased load conditions like when the A/C is on. If the ignition coil is overheating or has internal shorts, it may not provide adequate spark to the plugs, causing poor engine performance. Verify the ignition coil's resistance (should be 0.5-1.5 ohms) and check the integrity of the ignition wires. If any component fails to meet specifications, replace it (OEM P/N 0000-00-0000). - Poor ignition performance, visual inspection of ignition components, and resistance readings out of specification.
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 - Initial OBD-II Diagnosis: Start with a comprehensive scan using the GeekOBD APP to retrieve any fault codes related to the engine and A/C system. Check for codes that may indicate issues with the fuel system, ignition system, or A/C compressor. This initial scan provides a foundation for further diagnostics.
- STEP 2 - Visual Inspection: Conduct a thorough visual inspection of the engine bay, focusing on the valve cover gasket, spark plug wells, and ignition components. Look for oil leaks, damaged wires, or signs of wear that could contribute to the reported symptoms. Ensure that the spark plugs are not fouled and are within specification.
- STEP 3 - Fuel Pressure Test: Using a fuel pressure gauge, test the pressure at the fuel rail. The expected pressure should be between 35-45 PSI. If the pressure is low, inspect the fuel pump and filter for clogs or failure. If necessary, replace the fuel pump (OEM P/N 0000-00-0000) and verify proper operation.
- STEP 4 - Ignition System Testing: Test the ignition coil resistance and inspect the ignition wires for damage. The coil should read between 0.5-1.5 ohms. If readings are outside this range, replace the ignition coil (OEM P/N 0000-00-0000) and recheck the system performance.
