Be careful to drive the car until you have cleared the problem.

I just had exactly the same symptoms (w140 300SEL) with lack of power, and I contiued to drive (carefully though) while searching for the problem.

The problem was indeed the air mass sensor causing too lean mixture. With lean mixture the catalytic converter gets hot and eventually burns down. And when it burns down the internal breaks into peaces and fills up the muffler system with the destroyed catalytic converter.

Total repair cost $4.000.

The air mass sensor is only $500. Expensive, yes, but cheaper than the consequences. Not to talk about if also valves were burned.

Good luck with your repair.

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1991 300SEL (W140)
1987 190E 2.3 (W201)
1987 Honda Accord 2.0