I recall you are using 89 PON fuel. Have you detected any detonation?

I use 87 in my '88 190E 2.6 five-speed. I get a little transient detonation on upshifts, shifting at about 2000 if the ambient temperature is over about 70F and the coolant is 80C or higher or if I "second gear" a stop sign - brake until I'm nearly stopped and engine revs are down to idle speed or less, then accelerate if the intersection is clear.

Detonation is less likely with an automatic since the torque converter will will allow revs to increase to stall speed if you load it up below that speed.

Duke