my STS started overheating, so at one time i found myself pouring water into the coolant reservoir because i didn't have coolant. now everytime i drive i get a gauge tellin me i'm overheating, then i go open my coolant reservoir and steam comes out. as soon as i let it out, the car instantly cools, not quite back to normal but close enough. is the steam from the water i added? have i done something to my engine for it to overheat so quickly? any help or ideas?