Attach your OBDII scanner and read the air mass flow measurements generated by the sensor at WOT in 3rd gear and at 4000 RPM. You'll have to shift the car into 3rd. You have to catch your reading at 4000RPM so you may want a helper to read the scanner just when you hit 4000RPM so that you can remain concentrated on the road while accelerating on the highway. You want to see a MAF reading that is about 550kg/hr or more. Convert 550kg/hr as needed to match your OBDII scanner's mass flow units. For example multiply 550 by 2.2 and divide by 3600 to get it into lbs/sec. That's all the help I can give you.

BTW, don't do this during rush hour! you need an open highway.

Just realized you have a different car so you need to get the spec for your engine. Any MB tech with access to MB specs should be able to give you this. I have a friend at the dealership who gave me this for the car I was working on.

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1972 450SL
1982 300D Turbo