Adaptive Fuel Diagnostic Trouble Code (DTCS) Diagnostic Techniques

ADAPTIVE FUEL DIAGNOSTIC TROUBLE CODE (DTCs) DIAGNOSTIC TECHNIQUES

Adaptive fuel DTCs diagnostic techniques help isolate the root cause of the adaptive fuel concern. Before proceeding, attempt to verify if any driveability concerns are present. These diagnostic aids are meant as a supplement to the pinpoint test steps in Pinpoint Tests. For a description of fuel trim, refer to Description and Operation, Powertrain Control Software.

Obtain Freeze Frame Data
Freeze frame data can be helpful in duplicating and diagnosing adaptive fuel concerns. This data (a snapshot of certain parameter identification [PID] values, recorded at the time the DTC was stored in continuous memory) is helpful to determine how the vehicle was being driven when the fault occurred, and can be especially useful on intermittent concerns. Freeze frame data, in many cases, can help to isolate possible areas of concern as well as rule out others. Refer to Freeze Frame Data for a more detailed description of this data. Freeze Frame Data

Using the LONGFT1 PID
The LONGFT1 PID can be useful for diagnosing fuel trim concerns. A negative PID value indicates the fuel is being reduced to compensate for a rich condition, while a positive PID value indicates the fuel is being increased to compensate for a lean condition. It is important to know there is a separate LONGFT value used for each RPM/load point of engine operation. When viewing the LONGFT1 PID, the value may change a great deal as the engine is operated at different RPM and load points. This is because the fuel system may have learned corrections for fuel delivery concerns that can change as a function of engine RPM and load. The LONGFT1 PID displays the fuel trim currently being used at that RPM and load point. Observing these changes in LONGFT1 can help when diagnosing fuel system concerns. For example:
- A contaminated MAF sensor results in a LONGFT1 correction value that is negative at idle (reducing fuel), but positive (adding fuel) at higher RPM and loads.
- Vacuum leaks result in large, rich corrections (positive LONGFT1 value) at idle, but little or no correction at higher RPM and loads.
- A plugged fuel filter results in no correction at idle, but large rich corrections (positive LONGFT1 value) at high RPM and load.

Resetting Long Term Fuel Trims
Long term fuel trim corrections can be reset by resetting the powertrain control module (PCM) keep alive memory (KAM). Refer to Resetting The Keep Alive Memory (KAM). After making a fuel system repair, reset the KAM. For example, if dirty/plugged injectors cause the engine to run lean and generate rich long term corrections, replacing the injectors and not resetting KAM, now makes the engine run very rich. The rich correction eventually is learned out during closed loop operation, but the vehicle may have poor driveability and have high CO emissions while it is learning. Resetting The Keep Alive Memory (KAM)

P0171 System Too Lean Diagnostic Aids

NOTE: If the system is lean at certain conditions, then the LONGFT PID would be a positive value at those conditions, indicating that increased fuel is needed.

The ability to identify the type of lean condition causing the concern can be crucial to a correct diagnosis.

Air Measurement System
With this condition, the engine may actually run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. One possibility is the mass of air entering the engine is actually greater than what the mass air flow (MAF) sensor is indicating to the PCM. For example, with a contaminated MAF sensor, the engine runs lean at higher RPM because the PCM delivers fuel for less air than is actually entering the engine.

For example, MAF sensor measurement is inaccurate due to a corroded connector, contamination or dirty connector. A contaminated MAF sensor typically results in a rich system at low airflows (PCM reduces fuel) and a lean system at high airflows (PCM increases fuel).

Vacuum Leaks/Unmetered Air
With this condition, the engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition can be caused by unmetered air entering the engine, or due to a MAF concern. In this situation, the volume of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. Vacuum leaks normally are most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates the fault occurred at idle, a check for vacuum leaks/unmetered air might be the best starting point.

For example, loose, leaking or disconnected vacuum lines, intake manifold gaskets or O-rings, throttle body gaskets, brake booster, air inlet tube, stuck/frozen/aftermarket PCV valve, and unseated engine oil dipstick.

Insufficient Fueling
With this condition, the engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition can be caused by a fuel delivery system concern that restricts or limits the amount of fuel being delivered to the engine. This condition normally is most apparent when the engine is under a heavy load and at high RPM, when a higher volume of fuel is required. If the freeze frame data indicates the fault occurred under a heavy load and at higher RPM, a check of the fuel delivery system (checking fuel pressure with engine under a load) might be the best starting point.

For example, low fuel pressure, fuel pump, fuel filter, fuel leaks, restricted fuel supply lines, and fuel injector concerns.

Exhaust System Leaks
In this type of condition, the engine may actually be running rich of stoichiometry (14.7:1 air/fuel ratio) because the fuel control system is adding fuel to compensate for a perceived (not actual) lean condition. This condition is caused by oxygen (air) entering the exhaust system from an external source. The HO2S reacts to this exhaust leak by increasing fuel delivery. This condition causes the exhaust gas mixture from the cylinder to be rich.

For example, exhaust system leaks upstream or near the HO2S, and poorly welded/leaking HO2S boss.

P0172 System Too Rich Diagnostic Aids

NOTE: If the system is rich at certain conditions, then the LONGFT PID is negative value at that airflow, indicating that decreased fuel is needed.

System rich concerns are usually caused by fuel system concerns, although the MAF sensor, and base engine (for example, engine oil contaminated with fuel) should also be checked.

Air Measurement System
With this condition, the engine may actually run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. One possibility is the mass of air entering the engine is actually less than what the MAF sensor is indicating to the PCM. For example, with a contaminated MAF sensor, the engine runs rich at idle because the PCM delivers fuel for more air than is actually entering the engine.

For example, MAF sensor measurement is inaccurate due to a corroded connector, contamination/dirt. A contaminated MAF sensor typically results in a rich system at low airflows (PCM reduces fuel) and a lean system at high airflows (PCM increases fuel).

Fuel System
With this condition, the engine may actually run rich of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This situation can be caused by a fuel delivery system that is delivering excessive fuel to the engine.

For example,
- EVAP canister purge valve leak (if canister is full of vapors, introduces extra fuel).
- fuel injector leaks (injector delivers extra fuel).
- fuel pressure regulator causes excessive fuel pressure (system rich at all airflows), fuel pressure is intermittent, going to pump deadhead pressure, then returning to normal after the engine is turned off and restarted)

Base Engine
Engine oil contaminated with fuel can contribute to a rich running engine.