When changing oil in a car, after draining the pan and replacing the plug – isn’t there now air in the pan? The space that used to be filled with oil is now filled with air? Is this correct?
So once the plug is replaced, you unscrew the cap on the oil fill hole (top of the engine block) and pour in new oil. Doing so (or at least when I do it) completely occludes the fill hole (at least momentarily). Why doesn’t the air now trapped in the oil pan come “blub blub blub” out through the fill hole?
Pouring oil slowly into the engine doesn’t completely (albeit momentarily) block the fill hole, thus air can flow out as oil flows in. But… with the drain plug in the pan and the fill hole cap removed, that’s essentially the same as, for example, a gallon milk jug with the top removed. If you completely submerged the jug in water, with the (open) top pointing up, thus completely occluding the fill hole, the air would need to bubble out in order for the water to flow in. Why doesn’t the same thing happen with the engine block and oil?