Re: Do ECUs learn?[S70/98] posted by abe crombie on
Friday, 26 June 1998, at 6:40 p.m.

There are certain parameters that the control unit learns.

1. idle air control valve opening signal needed to keep correct idle

2. idle fuel trim, i.e. what center range fuel inj. opening time in milliseconds is needed to keep O2 sensor in its best centered range between rich and lean

3. above idle fuel trim, i.e. what computational algorithm keeps the O2 sensor in its best centered range between rich and lean.

The idle one is simple, it corrects for dirty throttle housing, wear, any air leaks, and altitude and actually has several different values depending on whether the a/c is
on or off or whether it is in gear or out of gear.

Idle fuel trim will compensate up to a point for wear, leaks between mass air flow sensor and throttle, leaks in intake manifold and fuel pressure varation.

Off idle fuel trim does the same more or less as idle but it is not a single point value but is a correction to a computation of what injector signal needs to be.

All trims are retained unless battery is disconnected for a period or if a scan toll commands the ECM to clear them.