Truth regarding crankcase vacuum and oil drain

[JHZR2]

Hello,

Say I have a car that Id like to replace the drain plug on... Say I want to replace it with a magnetic plug, but don't want to drain the oil.

I was under the impression that there is a trick where if you remove the oil drain plug with the engine on, the oil won't drain out because of crankcase vacuum.

Is this the truth? I was under the impression that it was. Don't want to find out the hard way if others can confirm/deny.

Thanks,

JMH

 

[mechtech2]

NO! A running engine will spew oil out faster!
What some do is put a shop vacuum on the oil filler, and it slows down or stops the oil from coming out the bottom.
But if you have the parts ready in hand, you can quickly swap them with only a cup or less loss. Just top it off with fresh oil afterwards.
You unscrew the drain plug by hand the last few threads, and then jam your finger in the hole - quickly switch to the other drain plug, and insert it and tighten it.

 

[Steve S]

When an engine is running there is a positive pressure in the crankcase.

 

[JHZR2]

thanks... my mistake.

I appreciate the info.

Best,

JMH

 

[The_Jeff]

Open a bottle of oil and hold it upside down. The "airspace" above the oil in the bottle will be at or near a perfect vacuum but of course attract air bubble up through the oil beneath it. The oil will still fall out. It'll "glug" for a while.

 

[BarkerMan]

If you are quick you won't loose very much.

 

[Shannow]

Quote:
When an engine is running there is a positive pressure in the crankcase.

Quote:
NO! A running engine will spew oil out faster!


Not in my very well worn E30.

Open the oil fill cap at idle, and the thing stalls from the lean out that occurs when the air gets sucked in.

Won't make much difference over a switched off engine 'though.

 

[johnd]

I have changed drain plugs using a shop vac. in the oil fill hole with the engine not running. You won't spill a drop as long as the vacuum is running. There will be some air sucked in the drain hole but no oil will flow out. It is a great way to get the plug out for a hot oil change, position the drain pan just right and then drain. And _NO_ the vacuum will not suck the oil out the fill hole.

 

[PT1]

Best way to do that is when the car is running at freeway speed...

 

[pajim17057]

Why not just wait until it is time to change the oil? Its not like your engine is going to wear out without a magnetic drain plug. It just seems like alot of trouble to go through with not real benifit.

 

[Triple_Se7en]

You might lose around three ounces using the "thumb over the hole very quickly" method..... that's about it.

 

[Eddie]

Sounds like something I'd do if I had nothing useful to do.

 

[SHOZ]

Don't do this if the oil is hot.......

 

[Titan]

If you want to wait until the oil change is due, just put a magnet from a computer hard-drive on the drain plug...that'll do the trick.