Q: Air blows out of oil cap when car running

asked by on

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.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Air blows out of oil cap when car running

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Should I buy this 1997 BMW 528I

Hello. BMW makes a fantastic vehicle that, as you know is well known for its performance and luxury. With that comes a little more of a price tag in terms of maintenance and upkeep as with any performance or luxury...

Q: 2010 dodge journey overheating but replaced thermostat and antifreeze

Hello, thank you for writing in. There are various reasons the vehicle would overheat, and the thermostat was a good place to start. You are also worried about the radiator, cooling fans, water pump, and engine block. You will want...

Q: The car has no compression and there is a lot of oil in the cylinders how would I fix this?

Hi and thank you for contacting YourMechanic. If there truly is no compression and signs of oil inside the cylinders, the only way to fix this issue is to have the engine rebuilt. The piston rings are worn or broken...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.