1983 380SL.

Background: Car failed smog. I tracked down that the O2 sensor was no good. Replaced it and now the car can go into closed loop. Lambda is very low so...

I am adjusting the lambda by turning the 3mm adjuster CCW = leaner. lambda increases and stays steady up to 36%. If i continue leaning it out the car starts hesitating and finally stalls. The leanest i can get it at idle is 36%. Does this suggest an air leak?

I sprayed carb cleaner at all the vacuum connections i could see as well as the intake manifold rubber rings that connect the upper and lower manifold but could not detect a leak. Would you still be convinced of an air leak or could something else cause this. Car runs well (smooth idle, plenty of power) but i don't want to bring it back to smog untill i understand why I can't go leaner than 36%. At idle the duty cycle is very stable.

O2 sensor is oscillating as expected.

BTW, when I measure Lambda at diagnostic terminal #3 with key on engine off the duty cycle shows up as 0% (0 VDC was also measured). I'll just assume that this year and model does not put out a fixed 70% or 85% with KOEO.

I've spent many hours reading over all the great info on this site but still am stumped.

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1972 450SL
1982 300D Turbo