No power

My MAF is disconnected at the moment. It is fine is urban traffic, but just went on a motorway run & it's very gutless in 5th gear. Auto box. Doing 80mph, then if you start going up a gradient it goes down to 65mph. If I drop it to fourth, you don't gain much speed & all I can see is a load of black coming out.the back of the car. Will only replacing the MAF cure this?

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