Powered ventilation systems should be turned on for how many minutes before starting the engine to ensure all gasoline vapors have been removed before ignition?

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Minimum of four minutes.

Explanation:

The essence of this is to avoid explosions by removing flammable gases due to reaction with sufficient oxygen. Evaporation of gasoline creates vapors or fumes that are denser than air which would settle to the bottom of a vessel and cause explosion if enclosed areas are not properly ventilated to remove the fumes.