Q: Air blows out of oil cap when car running

asked by on September 09, 2016

Looking to buy a used car, I was told that if air is blowing out of the oil fill cap when open while the engine is running it's a bad sign but I'm wondering if this could be just how this model works.

My car has 95000 miles.
My car has an automatic transmission.

Hello. From my experience most motors will blow air from the oil filler cap due to the pressure generated in the crankcase from all of the moving components. If the air blowing out seems excessive, smells like fuel, or is accompanied by loud hisses or sounds, then it may be possible that the vehicle has worn piston rings, and is experiencing what is called combustion blow by.

Engine blow by is when the engine’s piston rings have worn out enough to allow combustion gases and pressure to escape past the rings, into the engine crankcase. This is more likely for high mileage engines. I would also check to make sure that the vehicle’s PCV valve and system are not clogged, possibly causing a vent restriction, and an increase in pressure. If you are serious about buying this car, I recommend having a professional mechanic conduct a pre-purchase inspection on the car so that you are fully aware of the vehicle’s current condition before you hand over your money.

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