When there's a leak in the piping after the turbo, the car will barely run. Under any amount of boost, it almost dies and tons of black smoke pour out the tailpipe. Why?
The Mass Airflow Sensor (MAS) has already counted the amount of air entering the engine, and adjusts the fuel quantity accordingly. So, ten units of air, the computer squirts enough fuel for that amount of air. But if half of the air is leaking out before it gets to the cylinders, there will be WAY too much fuel in the cylinders. The coughing and black smoke is from a super-rich condition in the cylinders.
Note that if you drive around this way for too long, you can melt your catalytic conveter, foul your spark plugs, and ruin your oxygen sensor.
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