Sable V6-183 3.0L DOHC VIN S MFI (1997)
air/fuel ratio during closed loop operation. This range is displayed in percentage (%). A negative percentage means that the HO2S is indicating RICH
and the Powertrain Control Module (PCM) is attempting to lean the mixture. Ideally, Short Term FT should remain near 0% but has the ability to
adjust between -25% to +35%.
LONG TERM FUEL TRIM
Long Term Fuel Trim (displayed as LONGFT1 and LONGFT2 on the NGS tool) is the other parameter that indicates long-term fuel adjustments.
Long Term FT is also referred to as Fuel Trim. Long Term FT is calculated by the PCM using information from the Short Term FT to maintain a
14.7:1 air/fuel ratio during closed loop operation. The Fuel Trim strategy is expressed in percentages. The range of authority for Long Term FT is
from -35% to +35%. The ideal value is near 0% but variations of ±20% are acceptable. Information gathered at different speed load points are stored
in fuel trim cells in the fuel trim tables, which can be used in the fuel calculation.
OPERATION
Short Term FT and Long Term FT work together. If the HO2S indicates the engine is running rich, the PCM will correct the rich condition by moving
Short Term FT in the negative range (less fuel to correct for a rich combustion). If after a certain amount of time Short Term FT is still compensating
for a rich condition, the PCM "learns" this and moves Long Term FT into the negative range to compensate and allows Short Term FT to return to a
value near 0%.
As the fuel control and air metering components age and vary from nominal values, the fuel trim learns corrections while in closed loop fuel control.
The corrections are stored in a table that is a function of engine speed and load. The tables reside in Keep Alive Random Access Memory (RAM) and
are used to correct fuel delivery during open and closed-loop. As changing conditions continue the individual cells are allowed to update for that speed
load point. If, during the adaptive process, both Short Term FT and Long Term FT reach their high or low limit and can no longer compensate, the
Malfunction Indicator Lamp (MIL) is illuminated and a Diagnostic Trouble Code (DTC) is stored.
Whenever a fuel injector or fuel pressure regulator is replaced, Keep Alive RAM should be cleared. This is necessary so the fuel strategy does not use
the previously learned fuel trim values.
To clear Keep Alive RAM, refer to Testing and Inspection / Procedures / Powertrain Control Module (PCM) Reset (Clearing DTCs).
Failure Mode Effects Management (FMEM)
PURPOSE
Failure Mode Effects Management (FMEM) is an alternate system strategy in the PCM designed to maintain vehicle operation if one or more sensor
inputs fail.
OPERATION
When a sensor input is perceived to be out-of-limits by the Powertrain Control Module (PCM), an alternative strategy is initiated. The PCM
substitutes a fixed value and continues to monitor the incorrect sensor input. If the suspect sensor operates within limits, the PCM returns to the
normal engine running strategy.
All FMEM sensors display a sequence error message on the scan tool. The message may or may not be followed by Key-ON Engine-OFF or
Continuous Memory DTCs when attempting Key-ON Engine-Running Self-Test Mode.
Engine RPM/Vehicle Speed Limiter
PURPOSE
To prevent damage to the powertrain caused by an engine overspeed condition.
OPERATION
The Powertrain Control Module (PCM) will disable some or all of the fuel injectors whenever an engine rpm or vehicle overspeed condition is
detected. The vehicle will exhibit a rough running engine condition, and the PCM will store a Continuous Memory DTC P1270. Once the driver
reduces the excessive speed, the vehicle will return to the normal operating mode, No repair is required. However, the technician should clear the
PCM and inform the customer of the reason for the DTC.
Excessive wheel slippage may be caused by sand, gravel, rain, mud, snow, ice, etc. or excessive and sudden increase in rpm while in NEUTRAL or
while driving.
