Fuel System Monitor Description
FUEL SYSTEM MONITOR DESCRIPTIONThe Fuel System Monitor is designed to monitor and adjust fuel delivery based on fuel trim tables The fuel control system uses Short Term and Long Term fuel trim tables stored in the PCMs KAM to compensate for normal wear due to aging of fuel system components. During closed loop operation, the fuel trim strategy learns corrections needed to adjust a biased (rich or lean) condition.
^ SHRTFT 1/2: Short Term Fuel Trim 1/2 (bank I and bank 2). Fuel trim is the adjustment made by the PCM to keep the air fuel mixture around stoichiometry, enabling effective catalyst operation. If a front HO2S indicates rich, fuel trim goes lean (negative number) reducing the amount of fuel until the front HO2S shows lean, then increases fuel (positive number) until the front H025 goes rich, etc. It is short term fuel trim changes that cause the H025 rich / lean switching during feedback control. Large positive fuel trim numbers may indicate air leakage into the intake air system (unmetered air). Large negative numbers may indicate leaking injectors or other conditions that would reduce air or increase fuel to the engine.The PCM tries to keep SHRTFT as close to 0% as possible through changes in LNGFT.
^ LONGFT 1/2: Also referred to as "adaptive strategy". Like Short Fuel Trim, LONGFT acts to richen or lean the mixture. However, LONGFT does this over a long period of time, in response to continued changes in the engine mixture. LONGFT accounts for wear to engine and fuel components, and acts to keep Short Fuel Trim as low as possible and as close to 0 as possible.
^ Reliable input is required from ECT or CHT, IAT, MAF and HO2S
^ The MIL will illuminate after two consecutive trips
Fuel System Monitor DTC's
Fuel System Monitor Service Tips
^ P0171, P0172, P0174, P0175: These DTCs are not likely caused by HO2S failures. The majority of HO2S returned under warranty for repairing these DTC's are found to be functioning normally. Additional related DTCs are usually present when there is a concern with an H02S. These DTCs are never the result of a problem with a down-stream HO2S. Always look for a condition related to the rich or lean failure.
^ Lean Conditions & Vacuum Leaks (Un-Metered Air): Suspect a vacuum leak if the Freeze Frame Data indicated the fault at idle or low speed. The MAF sensor signal may drop below normal and the Idle Air control may be operating at its upper limit. A lean condition can be caused by un-metered air leaks (leaks between the MAF and the intake manifold). Examples of possible leaks are: loose, damaged vacuum lines, intake manifold gaskets or 0-rings, throttle body gaskets, brake booster hoses or diaphragm, air inlet tube, stuck or frozen PCV valve, unseated engine oil dip stick, partially blocked air filter or MAF inlet.
^ Lean Conditions & Insufficient Fueling: Suspect a restricted fuel system if the Freeze Frame Data indicated the fault at high engine load or throttle position. Restricted fuel concerns will usually be more apparent at high engine load and may be accompanied by concerns of lack of power or poor acceleration. Checking fuel pressure under load is a good place to start. Examples of possible causes of low fuel pressure or fuel starvation are: weak fuel pump, plugged filter, fuel leaks, restricted fuel supply line, contaminated or plugged fuel injectors.
^ False Lean Conditions / Exhaust Leaks: A leak around the HO2S will cause the PCM to sense a false lean condition. This can be caused by leaks upstream near the HO2S or by secondary air malfunctions.