The voltage of Mercedes-Benz W 140S300 air flow sensor is too high or too low.

As you said, if there is black smoke when refueling, it is about the oil return valve, which is the valve on the nozzle holder head.

There may be something wrong with the throttle. You need to see if there is a broken cylinder. If there is, it may be a big bag. See if there is something wrong with the electric wax sleeve and find a guide.

The general situation is the ones shown above. You'd better use HHT computer to check and see the flow value.