03-23-2019, 09:17 AM
It’s not only the fuel trims which are lost but also settings related to the IAC, Idle Air Control. I have changed battery’s on cars and they will stall or nearly stall until the idle speed settings are relearned. In the dealer ship we would keep the ECM powered via the DLC, data link connector, to avoid losing settings. Another method is to use alligator clips and an external battery connected to the battery cables when changing out the battery.
The long term fuel trim is accessed via the scan tool and a good indicator as to the health of the engine and engine management systems. The short term fuel trim is designed to switch from lean, low voltage, to rich, high voltage as measured by the O2 sensor so not usually used for diagnostics other than to verify the system is switching constantly from slightly rich to slightly lean as you mentioned for the catalytic converters to eliminate hydrocarbons, carbon monoxide and nitric oxide.
As you mentioned the long term fuel trim percent indicates if the system is adding or subtracting from the hard coded fuel maps to compensate for dirty air cleaners, weak fuel pressure, manifold leak etc. Typically if that percent exceeds 10 percent in either direction the check engine light is set and a trouble code is stored.
Finding and correcting diagnostic codes is not always easy as the code will say if the engine is running lean or rich but the technician has to often eliminate many different possibilities before finding the root cause. That being said I enjoyed driveability work.
The long term fuel trim is accessed via the scan tool and a good indicator as to the health of the engine and engine management systems. The short term fuel trim is designed to switch from lean, low voltage, to rich, high voltage as measured by the O2 sensor so not usually used for diagnostics other than to verify the system is switching constantly from slightly rich to slightly lean as you mentioned for the catalytic converters to eliminate hydrocarbons, carbon monoxide and nitric oxide.
As you mentioned the long term fuel trim percent indicates if the system is adding or subtracting from the hard coded fuel maps to compensate for dirty air cleaners, weak fuel pressure, manifold leak etc. Typically if that percent exceeds 10 percent in either direction the check engine light is set and a trouble code is stored.
Finding and correcting diagnostic codes is not always easy as the code will say if the engine is running lean or rich but the technician has to often eliminate many different possibilities before finding the root cause. That being said I enjoyed driveability work.