Why Is Air Coming Out Of Oil Cap [3 Easy Fixes]

Air coming out of oil cap is normal and it’s known as engine blow-by. Every engine has some sort of blow-by. But a lot of air coming out of the oil filler cap is unusual. Worn-out piston rings clogged PCV valve, and loose adapter fitting is some culprits responsible for the abnormal issue. 

air coming out of oil cap

Below, we will break down each of the culprits to solve the air issue. So, let’s get started right here. 

Why Is Air Coming Out Of Oil Cap [3 Easy Fixes]

In this chapter, we will introduce you to those culprits that cause the air to blow out of the oil cap. 

Note: You can also read how to fix air only blows through front vents.

1. Worn Out Piston Rings

Deteriorated piston rings are our first suspicion that causes the air to come out of the oil filler cap. A general sign of damaged piston rings is- the air comes excessively out of the cap and its smell seems like fuel.

Plus, it will make a hissing noise. And the technical name of this problem is combustion blow-by. To get rid of this vehicle issue, you should replace the piston rings.


It requires mechanical knowledge to replace the piston rings. However, many of us are mechanically inclined and this video will help in replacing the worn-out piston rings: 

On the other hand, if you are not mechanically inclined, you can call a professional. And the bad news is- it will cost you at least $1000 to replace the piston rings. But the price may go up to $5000 based on the mechanic and how many of them need to be changed. 

Note: We highly recommend you hire a professional mechanic as it’s very complicated to replace the piston rings. It will take up to 15 hours as you need to disassemble the whole engine. 

2. PCV Valve Gets Clogged

A clogged PCV valve can also be responsible for air to blow out of the oil cap. It will cause a vent restriction, which increases pressure. If you feel pressure instead of a slight suction when you pull out the oil cap, we bet the PCV valve gets restricted.


It’s pretty straightforward to clean the PCV valve. First, locate the PCV valve in your car. So, open the car hood. Then, look for the PCV valve, which is usually located somewhere on the valve cover. 

Once you find it, pull it out. Then, use a brake cleaner and spray the valve with the cleaner to remove every dirt and debris. 

3. Adapter Fitting Gets A Bit Loose

A loosened adapter fitting is another culprit that may create pressure and the air will puff out of the oil cap once you release it. 

This problem basically occurs when there appears a lot of leaking oil residue around the adapter fitting. Turns out that, if the adapter fitting wobbles around instead of sitting in its firm, it will cause the air pressure issue. 


What we recommend to solve this issue is to use gasket sealant to prevent the adapter fitting from wobbling around. Also, you can ditch the filter and get a better seal on the oil filter housing connection. 

Is It Normal For Air To Come Out Of Oil Cap

Yes, the air coming out of the oil cap is normal. Imagine the way the engine is built. The movement of pistons inside the cylinder (basically they slide up and down) makes the bottom of the piston exposed to the oil pan. As a result, air moves by the top and the bottom of the pistons. 

When you remove the valve cover from the motor, you can see a passage right into the oil pan. And when you remove the oil filler cap, you make a path of least resistance for the moving air to come out of the cap. 

So, if you feel air when removing the oil cap, you don’t need to worry about it. It’s just a by-product of combustion in a diesel engine.

This air is also known as blow-by gasses, which occurs in a new car as well. Keep in mind, that it’s not a sure-fire method to measure blow-by by keeping your hand over the oil filler cap. 

If you are really concerned about this, perform a compression test. And the following video will show you how to run this test: 


Should air be blowing out of the oil cap?

Yes, air should be blowing out of the oil cap as it’s pretty normal. Every modern engine comes with positive crankcase ventilation, which under normal conditions, maintains a bit lower pressure inside the crankcase. So, you will feel a slight suction when you pull out the oil cap.

How much Blowby is normal?

Every engine has a minor blow-by, which is normal. But how much blowby is normal basically depends on the engine size. For example, a functioning 12-liter engine will experience 1.5 CFPM of blow-by under normal operating temp. 

How do I lower my crankcase pressure?

You can lower the crankcase pressure by installing a vacuum pump. 

What happens if the PCV valve is stuck open?

If the PCV valve is stuck open, it will cause a vacuum leak, rough idling, and engine surge. 

Can I drive without a PCV valve?

Yes, you can drive for a short distance without a PCV valve. In this case, you must plug the vacuum side of the PCV line. 


As we mentioned earlier, it’s normal for air to come out of the oil cap. But if excessive air comes out of the oil filler cap, there is something wrong with your vehicle. 

In this case, our suspicion goes to worn-out piston rings, clogged PCV valves, and loose adapter fitting, which we already broke down. So, follow the troubleshooting steps we mentioned to fix.

However, we highly recommend you run a compression test to measure the engine blow-by before replacing anything. If needed, bring your car to a professional mechanic. 

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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