Do your auxilary fans work when it gets so hot? They should be on much earlier due to the A/C.
The corresponding device on your car, to the OVP, is the Base Module. It has four fuses in it. It has self-diagnostics. Your tech should look at the lower partial versus upper partial adaptation numbers (on the LH controller). Based upon a bunch of testing I would worry about the differences between the two. In other words, if you get lower partial numbers of .85 and upper partial numbers at .91 then there is a reason why the mixture is richer at low flow rates.
There has been some really good info given by JimF and Willy about adaptation problems in early 119 engine. I would encourage you to search for the thread.
I can offer a story about a 600SEL that I learned a lot about. The car had been a dealer with the check engine light on. Code 19 on the diagnostic module. It was found that the lower partial adaptation values were at .85 on the left side and .91 on the right. Upper partial values were .89 left side and .93 on the right. The first time I reset the values with my scanner to "1" it took two days of intermittant driving to get the car to relearn itself. Upon doing some research I found the criteria for the car to relearn itself. The car must be held to 1600-2200rpms at a load of 40kg/hr to 80kg/hr. The upper partial must be driven for a period at 3500-4500rpms (not sure about the upper rpm) at a load of 150-300kg/hr. I found that I could zero the adaptation values and relearn both sides on my dyno in about five minutes. After doing this repeatedly I got consistant numbers like above that increasingly got lower on the lower partial as the car got hotter.
It was really special to be doing this on a 600 as there are two mirrored systems. I moved the air mass meter from side to side and found that the measurements moved with the device. I got a new unit and the values went to an even .94 on both lower and upper partials. We managed to fix this car without new controllers as the dealer would have done.
The real kick is that the reason the dealer would have changed the controller is that MB has changed the software in the new units to allow adaptations for much greater range at least double the old one. The old controller would set the light when adaptations bottomed at .85 or at the other end 1.15. The new controller may go to as low as .68 or as high as 1.32. This would be a good idea but it seems outrageous that they make people buy a $2000 piece of hardware just to get the software improvement. Both BMW and Volvo have done similar software improvements with either chip installation or straight software downloads. Volvo gets less than 50 bucks for their updates (parts only - the labor is probably more).
Bosch Master, ASE Master, L1
33 years MB technician