I agree, that the mixture adjust must be right....(as well)

BEST!

Doyle

"Often, they don't realize that they are not "tuning" the engine in any way, as the air/fuel mixture is taken over by the DME once the idle off switch is activated."

Actually, even at idle (switch closed) the DME ECM monitors the O2 sensor
and 'attempts' to control the mixture, i.e. If the mixture is within its controllable
range.

Loren,
is this range" (mixture range) established by the ICV centering (if you will)? Or, is it (ICV) ONLY an idle (RPM) centering?

I understand these two to be separate issues......sorry for the simplistic....just need to be taught what is "correct" and absolute!

Thanks!

Doyle

The DME controls the mixture by adjusting the fuel pulse length. The engine speed is controlled adjusting the ICV vane. This only happens while the idle microswitch is closed. Once the microswitch opens the ICV moves into a default position.

If you misadjust the idle bypass screw the ICV will not be at its center once it found a stable idle speed. This limits its reaction range one way or the other. That's why you jumper the test plug. It centers the ICV and then allows you to adjust the bypass srew.

When you mess with the bypass screw without the jumper it results in the ICV moving off center. This is the comon trap people fall into. Once the ICV is at close to either end of travel all sort of bad things happen. The response characteristics change of the loop change. Hunting and poor idle are the result.

The idle mixture is adjusted by allowing a certain amount of air to bypass the barn-door AFM. The more air bypasses the barn-door AFM the less the flapper deflects and the lower the AFM signal (resulting in leaner mixture). This of course requires to measure CO before the catalytic converter.

Ingo

"is this range" (mixture range) established by the ICV centering (if you will)?"

The idle control range is fixed by the vane size in the idle valve,
i.e. the amount of air it can bypass the throttle body for maximum
idle is the upper range point. The lower range point occurs when
the valve is fully closed. As mentioned before and in other posts,
the jumper only allows one set the idle to correspond where the
DME ECM will have the idle valve at its middle opening.

The idle jumper does not affect the mixture range controlled by
the O2 sensor. Either a too lean or a too rich a mixture can result
in the O2 sensor not being within its controllable range, i.e. the
mixture has been either fully leaned or fully rich in an attempt
to compensate for the incorrect mixture not resulting in a Lambda
of 1.0 and can no longer affect the mixture.