I recently read a post by Steve Brotherton of Florida who explained in a detail I have not heard before about the operation of the CIS-E Lambda control.

Essentially, in a closed loop stable idle, any Duty Cycle setting within reasonable boundaries of within 100% will have the same fuel air ratio. The EHA will obey the commands of the Lambda control and trim the fuel flow to maintain the correct setting.

This is news to me (slightly painful news).

My 1988 560SEC (US Federal 70%) has a rough idle.
I have replaced, literally, everything. At great expense.
This is my "fun" car and not a daily driver so this is a DIY adventure.

I thought by trimming the Duty Cycle towards richer I made a better idle, but the car wouldn't start well and bogged on acceleration.

Now I think I see that the hard start was due to too much fuel since the mixture screw was too far CW and the Lambda control starts in Open loop which basically follows the mixture screw alone. Hence too much fuel for start.

Bogging on acceleration happened because the system is already "maxed out" trying to lean the mixture to compensate for the incorrect screw setting. When the acceleration enrichment (AFP overswing) kicks-in the system cannot lean out enough since it is already maxed out and the mixture goes too rich for a moment.

If all this is true... I been barking up the wrong tree.

A lean mis-fire at idle then can only be one thing. Vacuum leaks.

My duty cycle swings as much as 5% to 10% either side of the average.
Is that too much?

Tj