Keep Alive Memory (KAM) in tables that are referenced by engine speed and load (and by bank for engines
with two HO2S sensors forward of the catalyst). Learning the corrections in KAM improves both open loop and
closed loop air/fuel ratio control. Advantages include:
z
Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.
z
Long term fuel trim corrections can be used both while in open loop and closed loop modes.
Long term fuel trim is represented as a percentage, just like short term fuel trim, however it is not a single
parameter. There is a separate long term fuel trim value that is used for each RPM/load point of engine
operation. Long term fuel trim corrections may change depending on the operating conditions of the engine
(RPM and load), ambient air temperature and fuel quality (percent alcohol, oxygenates, etc.). When viewing the
LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different RPM and load
points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at
that RPM/load point.
As fuel control and air metering components age and vary from nominal values, fuel trim learns corrections
while in closed loop fuel control. Corrections are stored in a table that is a function of engine speed and load.
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, individual cells are allowed to update for that speed load
point. If both Short Term FT and Long Term FT reach their high or low limit and can no longer compensate
during adaptive process, MIL is illuminated and a DTC is stored. Whenever a fuel injector or fuel pressure
regulator is replaced, RAM should be cleared. This is necessary so PCM does not use previously learned fuel
trim values. To clear RAM, see KEEP ALIVE RANDOM ACCESS MEMORY RESET PROCEDURE
under CLEARING CODES under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - CNG, FLEX-
FUEL & GASOLINE article.
Idle Air Trim
Idle Air Trim is designed to adjust Idle Air Control (IAC) calibration to correct for wear and aging of
components. When engine conditions meet learning requirement, the strategy monitors the engine and
determines the values required for ideal idle calibration. Idle air trim values are stored in a table for reference.
This table is used by PCM as a correction factor when controlling idle speed. The table is stored in Keep Alive
Random Access Memory (RAM) and retains the learned values even after engine is shut off. A DTC is output if
idle air trim has reached its learning limits.
Whenever an IAC component is replaced or cleaned or a service affecting idle is performed, it is recommended
that RAM be cleared. This is necessary so idle strategy does not use previously learned idle air trim values. To
clear RAM, see KEEP ALIVE RANDOM ACCESS MEMORY RESET PROCEDURE under CLEARING
CODES under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - CNG, FLEX-FUEL & GASOLINE
article. It is important to note that erasing DTCs with a scan tool does not reset idle air trim table. Once RAM
has been reset, engine must idle for 15 minutes (actual time varies between strategies) to learn new idle air trim
values. Idle quality will improve as strategy adapts. Adaptation occurs in 4 separate modes. The modes are
shown in IDLE AIR TRIM LEARNING MODES table.
IDLE AIR TRIM LEARNING MODES
Transaxle/Transmission Range
Air Conditioning Mode
Neutral
A/C On
2003 Ford Pickup F150
2003 ENGINE PERFORMANCE Theory & Operation - CNG, Flex-Fuel & Gasoline