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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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


Experienced Mechanic?

Earn up to $70/hr

What others are asking

2007 Dodge caravan won't start

Hello, thank you for writing in. This is a great question. The battery is solely responsible for starting the vehicle. This is assuming your connections and wires are clean, dry, and tight. Once the battery starts the vehicle, the alternator...

Car will not stop beeping even while driving

Hello, thank you for writing in. The vehicle will give you an audible beeping for several reasons. Typically it is to indicate the door is open or that people sitting in the seats are not buckled in. If all doors,...

Idle is running high when driven. I put it in neutral and it idles at 2300 rpms. Just had a cold air intake put in and throttle cleaned

Hi there. Assuming your vehicle has ETC (electronic throttle control), "drive-by-wire" problems can develop when cleaning is performed. Manually opening the throttle plate may have affected the close position. An idle relearn performed with a capable scanner may resolve the...

Malfunction of electrical components - 2006 Cadillac XLR

There is a Recall for a possibly related electrical issue with your vehicle. The Recall issue (NHTSA recall campaign 06v263000) addresses an electrical issue that has damaged various electrical systems with your model vehicle. Discuss this with your Cadillac dealer....

Blower motor works sometimes

Your car has an automatic temperature control system and it sends signals out to the blower controller in the evaporator housing in the engine compartment. The blower controller is the most common failure along with the blower motor. The blower...

Steering not working

Hello. It sounds like your engine drive belt may have broke. If this occurs your engine accessories will no longer spin causing the steering to be hard and the voltage light to come on due to no charging from the...

Car shifts erratically and hesitates when accelerating

Hello. When the Check Engine Light comes on in your car, it will automatically turn off other systems like the traction control. Depending on why the light is on, it may also cause shifting problems. I would typically start with...

Related articles

2012 Ford Edge vs. 2012 Honda Accord: Which One Should I Buy?
Comparing a small SUV or a crossover to a sedan or coupe doesn’t quite seem fair: while the crossover is likely to cost more, it will also have a different skill set than the car as well as a completely...
How to Replace Oil Cooler Lines on Most Cars
Oil cooler lines fail if there are bends in the hose, low oil levels, or oil visibly puddling underneath the car.
Is It Dangerous to Buy a Car with Aftermarket Parts Installed?
It's It's not always possible or practical to buy or lease a new car. Sometimes, you're faced with having to purchase a used automobile. While this process may seem straightforward, searching for the right used car is a lot different...