CIS theory ?

[Orin Eman]

> 1. open loop occurs when the engine is cold, unless your O2 sensor is 
> heated [3-wire], and when the WOT switch closes. At WOT the ECU puts out a 
> fixed, pre-determined duty cycle, about 85% I think, but it should be in 
> Bentley. The ECU ignores the O2 sensor input, and keeps it at 85%, giving a 

65 to 70% usually, depending on RPM.  Boost over a certain value is
also treated as WOT for this purpose, regardless of the WOT switch.
Higher values of duty cycle (in the 80% range) occur at startup and
during warmup.

> rich mixture for maximum power. I don't know if the ecu is open or closed 
> loop when the idle switch is closed, though.

Closed loop if the engine is warm enough.

Orin.