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Air coming out of oil fill cap

I purchased a car a month or two ago and was recently told that if there is air coming out of the oil fill cap it was a sure fire way to tell if your car was about to die. So out of curiosity I checked my car and discovered a light flow of air coming from the oil fill cap. My friends car does it much worse and he is also concerned about it. So my question is, do I need to be making plans to buy a new car or is the current motor fine?

That’s just the positive crankcase ventilation system you’re seeing. Positive meaning positive pressure. Put the oil cap back on, and don’t worry about it.


Its is OK along as that postive pressure that is created in the combustion chamber and blows by the rings (hence its name "blowby)is not excessive. The PCV valve uses engine vacuum to scavange the cranckcase.

You are correct excessive blowby is a indication of poor engine mechanical condition

Many cars have so much NEGATIVE pressure (you won’t feel anything blowing OUT)in the cranckcase that the engine will run rough if the dipstick is not seated.

Engines that do not use a PCV use a fixed orfice system which is really a calibrated vacuum leak.

I would get a wet/dry compression test to check how bad those rings are leaking.

Year, make, model and mileage would be helpful if you want an intelligent answer…One thing for sure, that’s not “air” blowing out. That’s combustion blow-by and it’s not a good sign…

I wouldn’t say that it’s a particularly bad sign if the amount of pressure isn’t significant.

its a 98 civic with a newer motor in it, the motor has less than 50k miles on it

Unless there are leaks, it isn’t a condition which requires repair.

Your friend’s engine is probably a high mileage engine and worn more than yours; so, has more crankcase pressure. The only remedy for it is a rebuild with new piston rings, etc. Maintenance wise, s/he will have to change spark plugs, pvc valve, air filter, and motor oil more often. S/he will need to do ‘intake cleaning’ more often, as well.

I bought a used Corolla at 90KM with this “feature”. It went for another 60KM without much trouble and then I sold it mostly because of Carburetor issues rather than oil consumption.

This is the case of a little bit of knowledge is a dangerous thing.

Trust us, there is no problem here. You and your friend are seeing the very normal by-product of combustion in a gasoline engine. This ‘air’ you feel is blow-by gasses, which happens in new cars as well. Every time a cylinder fires, a lot of cylinder pressure is created, and some slips past the sealing rings of the piston, and gets into the crankcase. This is called ‘blow-by gasses’. It naturally increases with age as the cylinder rings wear.

Placing your hand over the oil fill cap is not a great way to measure blow-by, and determine an engine’s condition. You and your friends cars have a lot of life left in them, most likely.

If your really concerned, you can have a competent mechanic run a compression test. This is a much better indicator of cylinder condition that trying to read the blow-by gasses. A leak-down test is even better.