Why doesn't air bubble out when filling an engine block with oil?

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?


Oil sticks together a lot better than water. Whereas Water would take the trapped air under the liquid and make several bubbles out of it, Oil will allow only a few or one. Oil moves neatly around this bubble and the bubble moves up. So with Oil, if you occlude the tube completely, you tend to get just one Blub.

My guess is that it’s actually not completely occluding the hole. Are you positive that there isn’t even a notch or cut-out on the hole that it could be escaping through?

On an old car it might escape through the PCV system - might, depending upon whether the valve was opened or closed.

On the great majority of situations there will not be an airtight seal at the oil filler neck – notches for the filler cap, funnel not making full contact around the circle, oil bottle spout (squarish) not making full contact with the round filler opening, etc.

Then there’s the PCV system, as mentioned, which on the great majority of cars will vent the crankcase when filling.

I have seen a very few cars where the venting is so limited that the oil will back up in a funnel, but I don’t recall seeing a “blub.”

I don’t get this question. If I pour liquid into a milk jug, the air escape without glubbing, unless I submerge it and block the opening. But adding engine oil doesn’t do quite the same thing as submerging an open bottle completely.

Who is adding oil and completely blocking ‘the opening’. Oil caps are quite wide.

The crankcase is vented (often through the intake) so it’s not a sealed system. If there wasn’t some kind of venting system then any pressure that passes the piston rings would build up in the crank case.

I am convinced by your argument.

Thanks everyone!!