DTC P0172/P0175 System Too Rich Diagnostic Aids
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NOTE:
If system is rich at certain conditions, then LONGFT PID would be a negative value at that airflow, indicating that decreased fuel is needed.
System rich concerns are usually caused by fuel system concerns, although MAF sensor, and base engine (for example, engine oil contaminated with fuel) should also be checked. The following are possible causes:
- Air Measurement System
With this condition, engine may actually run rich or lean of stoichiometric (14.7:1 air/fuel ratio), if PCM is not able to compensate enough to correct for condition. One possibility is that the mass of air entering engine is actually less than what MAF sensor is indicating to PCM. For example, with a contaminated MAF sensor, engine would run rich at idle because PCM would deliver fuel for more air than is actually entering engine. Examples of this are; inaccurate MAF sensor measurement can be caused by a corroded connector, contaminated or dirty MAF. A contaminated MAF sensor will typically result in a rich system at low airflow resulting in PCM reducing fuel, and a lean system at high airflow results in PCM increasing fuel. - Fuel System
With this condition, the engine may actually run rich of stoichiometric (14.7:1 air/fuel ratio), if PCM is not able to compensate enough to correct for condition. This situation can be caused by a fuel delivery system that is delivering excessive fuel to engine. Examples of this are:- Fuel pressure regulator causes excessive fuel pressure (system rich at all airflow), fuel pressure can be intermittent, going to pump deadhead pressure, then returning to normal after engine is turned off, then restarted.
- Fuel pressure regulator vacuum hose off (causes excessive fuel pressure at idle, system rich at idle airflow). Fuel pressure regulator diaphragm ruptured (fuel leaking into intake manifold, system rich at lower airflow).
- Fuel return line crimped/damaged (fuel pressure high, system rich at lower airflow).
- Fuel injector leaks (injector delivers extra fuel).
- EVAP canister purge valve leak (if canister is full of vapors, introduces extra fuel).
- Fuel rail pressure sensor (electronic returnless fuel systems) concern causes sensor to indicate lower pressure than actual. PCM commands higher pressure to Fuel Pump Driver Module (FPDM), causing high fuel pressure (system rich at all airflow).
- Base Engine
Engine oil contaminated with fuel can contribute to a rich running engine.