Skip the auto shop - Our mechanics make house calls

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?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

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: How do I know if the timing belt has been replaced?

You are correct in that the car fax may not necessarily always indicate every service performed as not all shops report the services they perform. Unfortunately, there is no definitive way to tell for sure whether or not the timing...

Q: What type of engine oil should I use?

Oil is essential for your engine – it lubricates moving parts and keeps friction to a minimum. While it’s important that you keep your oil level within the specified limits and have it changed regularly, it’s just as important that...

Q: steering wheel turned to the far right and locked in place while the wheel are straight. and I can't turn the key.

Hi there. From the description you have provided, concerning your vehicle's steering wheel is locked far right and key won't turn with the front wheels in a forward default position, is somewhat confusing to me. It could be, undo pressure...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...