Have you ever opened your oil filler cap and felt a sudden rush of air or pressure coming out of the hole? This is completely normal if there is less air because the air is nothing more than a small amount of pressure from moving engine parts in the crankcase.

However, if the amount of air is excessive, this could be due to faulty piston rings or a clogged PCV valve.

If you notice the following characteristics, you should be more concerned about the air coming from the oil cap:

  1. The air that comes out after removing the oil cap smells strongly of fuel.
  2. You hear a hissing or leaking gas sound when you open the oil cap.

Now let us see the causes behind air coming out of the oil cap.

Causes of Air Coming Out of an Oil Cap

Worn out Piston Rings 

Worn-out piston ring seals can allow air to escape from the oil cap. Piston cycles seal the piston against the cylinder wall. These piston ring seals are responsible for keeping oil in the oil sump and allowing oil to splash onto engine components below them to ensure proper lubrication. When the crankshaft powers the piston rings, they store the compressed air. 

If the piston ring seals are tight and working properly, the compressed air will seep back into the oil sump. If the seals are faulty, the compressed air can pass through the piston rings while cycling. This air may then enter the oil fill tube. If the compressed air quantity is excessive, air may escape when the oil fill cap is opened.


The only way to resolve this issue is to replace the piston ring seals. You can determine the correct size by having a mechanic remove the piston ring. Remove any remaining seals from the piston ring, as this will allow the mechanic to properly identify the size of the seal that will fit the compressor piston. The cost of replacing the piston ring can range from $1,000 to $5,000.

Clogged PCV valve

The primary function of the PCV valve is to prevent air from escaping by routing air and fuel from the crankcase to the intake manifold. If the PCV valve is clogged, the pressure in the engine will rise as the pressure-regulating valve fails to function properly.

It is straightforward to determine whether or not your PCV valve is clogged. Remove the oil filler cap(preferably when engine is cool and not hot); if there is slight suction while opening, the PCV valve works properly. However, the PCV valve is most likely clogged if you feel pressure when opening the oil cap and air comes out.

Cleaning the PCV valve can resolve the problem of air coming out of the oil cap. Let’s go over how to clean the PCV valve –

  1. The PCV valve is usually located on the valve cover. To loosen the bolts on the PCV valve, use a screwdriver. Please remove it from the air intake.
  2. Remove the caps from the PCV valve to properly see the parts.
  3. Using brake fluid or carburetor cleaner, clean the parts of the PCV valve.
  4. Blow compressed air into the PCV valve to remove any liquid.
  5. Reinstall the cleaned PCV valve.

FAQ’s on Air Coming Out of Oil Cap

The engine blowby is caused by worn-out piston rings or a clogged oil filter.  If you notice excessive air, a fuel odor, or a hissing sound from the oil filler cap, the blowby in your engine is not normal. It is completely normal for your 12-liter engine to produce 1.5 CFM (cubic feet per minute) of blowby.

It is normal if the air from the oil cap is less. However, if the air is excessive, you may notice an overheating problem in the engine. Because compressed air escapes from the engine, if the piston rings or PCV valves are not functioning properly, the engine parts will not receive proper lubrication.

This can cause increased friction and engine overheating.


To summarize, some air coming out after removing the oil filler cap is completely normal. However, if there is too much air, your piston rings may be damaged, or the engine’s PCV valve may be clogged. Sometimes, you can also spot sludge on the filler cap which is due to some other reasons. You should get that checked if that’s the case.

A clogged PCV valve can be easily repaired by cleaning it with brake fluid. However, the only way to repair the piston rings is to replace them. Make sure to hire a professional to replace the piston rings.

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