Before you replace your mass flow sensor, check the connector which
plugs into it. On my '91 200q, I found that one of the little gold
connector pins would get pushed back into the connector when you
pushed it onto the mass flow sensor connection. I pulled back the
rubber boot on the connector, pulled out the offending pin, and bent
the locking pin a little bit so that it would `click' when re-inserted
into the connector. If the pin slides in the connector, you have a
maybe-yes, maybe-no connection just at the tip.
BTW, I had one of the bad connectors on my '84 4000q which caused
intermittent and finally non-operation of the heated rear window.