A redish brown smoke is an indication of just a little too much fuel for the air available to burn right. So a bad mass airflow sensor is a good guess, but as you said it wasn't the problem. The computer should limit the amount of fuel the engine burns to amount of air brought in. If this car is turbocharged you might have an air leak. Check the EGR system. Excessive EGR will create smoke. Other than that the only thing I can think of is it would be a problem with the injectors injecting too much fuel.

Is it possible the previous owner put a performance chip in it?

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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