wierd problem at start-up

My 93 sierra 1500 runs very rich for a few seconds at start-up. Like a carb. when it loads up. The odd thing is that it only does it when parked outside. It wont do it when it's been parked in the garage. It doesnt matter what the temperature is either. hot or cold it doesnt load up when it's in the garage, but it will do it every time when parked outside. Anyone have a clue?