Owner's Problem Description
The vehicle owner reports an issue with excess gasoline in the cylinder head of a Mazda 626. Previous repairs included fixing the head, replacing the head gasket, and ensuring accurate timing. Despite these efforts, gas continues to enter the cylinder head, even with the fuel lines disconnected. The owner is seeking help to diagnose the reason behind this persistent issue.
Professional Technical Analysis
The Mazda 626 in question is likely from the late 1990s to early 2000s, with a 2.0L or 2.5L engine and an approximate mileage of 120,000-180,000 miles. The owner's report of excess gas in the cylinder head, particularly after head gasket replacement and head repairs, suggests a few potential issues that need thorough investigation. First, it is important to analyze the fuel injector operation. A malfunctioning injector could be leaking fuel into the intake manifold, leading to excessive fuel entering the combustion chamber, especially when the engine is not running. This could also happen if the engine control module (ECM) is commanding the injectors to open incorrectly due to a sensor fault, such as a malfunctioning coolant temperature sensor that misleads the ECM into thinking the engine is cold, causing a rich fuel mixture. Furthermore, the lack of fuel flow through the injectors when the fuel rail is removed indicates the need to verify the fuel pump operation and fuel pressure, which should be around 35-45 PSI. Additionally, checking the integrity of electrical connections related to the injector circuit and the ground connections to the ECM is crucial, as poor grounding can lead to erratic injector behavior and unintentional fuel delivery. The current condition of the vehicle raises safety concerns, as excessive fuel in the cylinder head can lead to hydrolock, potentially causing severe engine damage. It's advisable to conduct a comprehensive diagnostic check of the fuel system and ECM before considering any major repairs or replacements.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Injector Leakage: If the injectors are leaking, fuel can enter the cylinder head even when the fuel lines are disconnected. This can occur due to a faulty injector seal or an internal failure of the injector itself. The injector should be tested for proper operation and leakage under pressure. The standard operating range for fuel pressure is 35-45 PSI. Injector resistance should be checked as well, typically between 12-16 ohms for standard injectors. - Presence of fuel in the cylinder head, no fuel flow through injectors when checked, possible stored codes related to fuel pressure or injector performance.
- ECM Malfunction: A malfunctioning Engine Control Module can incorrectly command the fuel injectors to remain open, leading to over-fueling. This could be due to a sensor error, such as a bad MAP sensor or coolant temperature sensor, causing the ECM to misinterpret engine conditions. This can result in excessive fuel delivery and flooding of the cylinders. - Check engine light may be on, stored trouble codes related to sensor faults, inconsistent fuel injector operation observed.
- Fuel Pump Issues: A malfunctioning fuel pump can provide inconsistent fuel pressure, leading to flooding in the system. If the pump is delivering pressure outside the recommended range (35-45 PSI), it could cause excess fuel to enter the cylinders. Additionally, a failing pump can cause fuel delivery to be erratic, further complicating the issue. - Check fuel pressure at the rail; should maintain 35-45 PSI, erratic pressure readings may indicate pump failure.
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 - Fuel Injector Inspection: Start by removing the fuel rail to inspect the injectors for any signs of leakage. With the ignition off, manually trigger the injectors to see if fuel leaks out. If injectors are leaking, they should be replaced with OEM parts, typically costing around $40-200 each.
- STEP 2 - Check Fuel Pressure: Use a fuel pressure gauge to measure the pressure at the fuel rail. The reading should be within the specified range of 35-45 PSI. If the pressure is outside this range, further investigate the fuel pump and its electrical connections.
- STEP 3 - ECM Diagnostic Scan: Use a professional OBD-II scanner or GeekOBD APP to check for any fault codes related to the ECM or fuel system. Pay attention to codes that may indicate sensor malfunctions or injector issues. Reset any stored codes after repairs.
- STEP 4 - Electrical System Check: Inspect the electrical connections to the fuel injectors and ECM for corrosion and proper grounding. Ensure that all connectors are secure and free of dirt and moisture.
