Air coming out of the oil fill tube

Is air coming out of the oil fill tube on your air compressor?

That is a fairly common problem for DIY air compressors that have seen a lot of use.

Of course, if your air compressor is factory lubed for life (also known as oil less or oil free) you will not have an oil sump, and have no need of an oil fill tube.

When folks put the cap back on the oil fill tube after topping up their compressor oil sump, they may not realize that the oil fill tube cap is vented.

Why is oil fill tube cap vented?

In the sketch below you can see the typical  flow path of air from the compressor oil sump, up the oil fill tube, through the vent cap, to atmosphere. Your compressor oil fill tube and cap will differ in appearance. Not all caps are threaded. Some are a friction fit.

Compressor air coming out of oil fill tube will vent through the vent cap

The oil fill cap is vented because reciprocating air compressors have cylinders and pistons. Your air compressor may be a single cylinder, or it might be a multi-cylinder design. Each of the cylinders has a piston. It is the reciprocating of these pistons, the cycling up and down, that alternatively pulls air into the compressor, and then drives it down into the tank and the reason why your oil sump has to have a vent.

The oil that lubricates the piston components splashes, or is pumped, up into the cylinder sleeve  from the oil sump.

As the piston cycles the seals on the piston wipe the cylinder wall,  sealing the compressor lubricating oil below the piston seals, and the air to be compressed into the compressor air tank above the piston seals.

When these seals start to wear, and it is inevitable, you will have more air coming out of the oil fill tube since more air will be getting past the seals into the oil sump.  This air is vented harmlessly out the oil fill vented cap continuously as the compressor is running.

At the same time, a little bit of oil is wicking past the piston seals and migrates up into the air side of the piston. The oil sump needs to be vented to allow air into the sump, otherwise, in time, the loss of oil would pull a vacuum.

Air coming out of the oil fill tube – a lot of it!

The venting of air happens all the time the compressor is running. It is when the oil fill cap is venting a lot of air that the compressor owner notices and wonders what is going on.

If you have a lot of air coming out of the oil fill tube that is a strong indication that your air compressor piston seals are badly worn, and you will need to replace them.

It is not recommended that you wait. to replace the piston seals when you have a lot of air coming out of the oil fill tube. As the piston seals wear there will come a time when they have worn far enough that you will have metal-to-metal contact between the cylinder and piston, and once that happens, what might have been a fairly simple repair becomes quite a bit more complex and costly.


  1. Interesting – great article!

    My compressor – 2 cylinders – doesn’t normally have a noticeable amount of air movement into or out of the crankcase during operation. But I just broke the tab off the unloader valve mount when I chasing down some minor annoying leaks, and the unloader is not functional (at the moment). Now, shortly after the pump cuts out, I notice a ‘hiss’ from the crank vent for several seconds. I’ve never noticed a crankcase hiss when the unloader is working normally. Actually, I find it an advantage, acting as a sort of ‘unintentional’ unloader 🙂

    Anyway, I’m thinking a new set of rings may be in the not-too-distant future.

  2. peter dyer says:

    Hello, not just air is coming out of my compressors oil fill vent, but shooting oil too, in copious amounts…

  3. I just replaced the rings on my two-stage compressor …. I’m noticing some air at the crankcase vent. There’s about 10 hours on the rebuild. How long does it take the rings to seat? I hate to think of tearing it down again.

    • Too many variables including not knowing the make or model of compressor to offer a specific guide, Roger. Since the vent is there to allow outflow, and some inflow, when the compressor is running, is it a substantial flow out? If not, it’ll just mean that your compressor will run longer, and you may get some oil into the compressed air tank. If it doesn’t abate in time, then I’m afraid it might mean a tear down again.

  4. Are you talking about piston rings when you say cylinder seals?

  5. Hi.

    I think I have this problem on a new compressor, a 2.5hp 24ltr bought from aldi. Could you please tell me what is considered to be to much air through the oil plug. i can feel a good flow on my hand.

    Thanks for any advice you can offer.

    • The oil fill plug is normally vented to allow flow of air into and out of the sump as the oil level changes. You should not normally feel a flow of air coming out of the vent. If you are, that suggests strongly that your pump cylinder seals are worn to the point where they are allowing substantial air past as the pump cycles. That’s not good. If you are feeling lots of air out of the sump and the compressor is not running, that’s the same problem but you may also have a failed tank check valve too.

  6. Allan Grant says:

    My compressor has started blowing the oil filler plug out and erupting oil under pressure. I have fitted a new 0 ring to the plug (the existing one looked OK but could have shrunk, I guess) and it seems firmer now but I am a bit afraid to turn it on in case it happens again and I get another mess to clean up. It is a few years old but has had only moderate use. Is this related to worn seals or some other cause?

    • You may want to check and be sure that the vent hole in the oil fill tube cap is open. If it has become blocked with crud, air pressure may build up on the sump and blow the cover, and oil, out the oil fill hole.

      While the compressor is running, if you can feel air blowing out the oil fill tube, you likely have badly worn cylinder seals.

      An overfilled oil sump can cause this too.

      • Chuck Parkerson says:

        I have same problem compressor sat for years. I installed new plug same problem. Compressor pumps up fine but are my seals/rings worn? doesn’t have many hours?

        • Sorry, I’m not sure of your question? If air is coming out of the oil fill tube on your compressor while the compressor is running, that air has to be bypassing the piston rings, else how could it get into the sump? That’s regardless of how old the compressor is.

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