What it's for
For the first moments after the engine fires, it's common to run idle a bit higher than normal. The engine is still stabilizing, oil is getting where it needs to go, and a slightly higher idle helps avoid a stall. The Post Start Target Offset raises the target RPM for that period.
Raising the target alone isn't enough, though. If the idle valve is only opened to its normal base position, the closed loop controller has to find all the extra air on its own, and the engine can sag or stumble while it catches up. The Post Start Base Offset solves that by adding extra Base Output up front, so the valve opens roughly the right amount straight away.
How it's usually mapped
Treat it as the air-side partner to the target offset. The table is normally set up against time since start, with its largest value just after the engine fires and falling back to zero over the post-start period. It can be up to 3D, so you can also scale it with something like coolant temperature, giving a cold engine more help than a warm one.
- Match its timing to the Post Start Target Offset, so the extra air fades out as the extra RPM target does.
- Size it so the engine lands near the raised target without the closed loop working hard.
- Make sure it reaches zero, or it will quietly affect your normal idle.
Log RPM, target RPM and idle output through several starts. If RPM lags behind the raised target, add offset. If it overshoots, take some away. Get the closed loop base table right first, since this offset adds on top of it. More detail is in idle control.
