2005 C230K SS 1.8L S/C L4. I run octane booster in every tank in my W203, and recently I checked the codes with my Star diag. just to see if there were any (CEL is NOT on, car runs fine). I found: P201D - Self Adaptation of mixture formulation, mixture is too rich at part load range. Could this be from having to lean out the AFR from the higher octane fuel? I use 94 octane at the pump and a (verified) 60 point/6 number octane booster, 100 octane rating total. I'm guessing the Octane booster is the issue, since the car is factory tuned for 91 octane fuel. Anyone had this issue before? Air filter is brand new (replaced today), spark plugs replaced 4,000 km ago, full fuel system cleaning and flush done 4,000 km ago, engine oil doesn't smell of fuel.

 

Why use the octane booster? Unless you're chasing a land speed record, you're just pissing away money, and yes, the ecu could be doing cartwheels to keep up with your mixture.

Try to go without for a while and see what happens when the ecu has had a breather to go back to normal levels.

 

Octane boost in a can does sooo little to raise octane that's probably not the problem. Maybe something in the can did something else though.

 

octane won't do that.

check MAF, 02 sensors - those are two other points of self-adpatation.