Cold weather starting problems...

Normally my X3 is garaged every night, but when I travel the car gets left outside. Every time I've traveled in the cold and the car is left outside overnight it won't start the next morning. It cranks over fine but just cranks and cranks and cranks until it finally just starts or I floor the gas pedal. Any ideas on what could be? I don't think it's the battery, the starter turns the engine over just fine. Maybe an air/fuel mixture issue, like a sensor or something?

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