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Why does oil come out of the oil fill tube?

Published Categorized as Uncategorized 20 Comments on Why does oil come out of the oil fill tube?

by Don
(Jax Fl)

There are two small holes in the top of the oil stick and when the compressor is on, air leaks out from them. It won’t build up any more than 40 psi. Central pneumatic 2hp twin tanks, 5gal.

Hello Don…

What separates the compressed air from the oil sump is the piston rings or piston seals.

As the compressor reciprocates, the piston moves up and down. The oil splashes from the sump up under the piston, lubricating the walls of the cylinder as the piston cycles.

What keeps the oil in the sump and out of the air stream (mostly) are the piston seals.

If air is entering the oil sump while the compressor is pumping compressed air, that tells me that the piston seals are badly worn or nearly worn out, and air is bypassing them into the sump.

The sump vents out the oil fill plug, and voila, you have air coming out the oil fill cap.

Usually that seal bypass isn’t enough to stop the air buildup in the tank completely. That your air compressor fills to 40 PSI and continues to run and not fill past that point suggests to me that you have potential valve leakage or perhaps a gasket leak in the pump as well.

Air is supposed to vent out the oil fill cap.
Air is supposed to vent out the oil fill cap.

Oil sprays out when I start it, did I overfill it?

by mike
(kapuskasing canada)

oil sprays out when I start it, did I overfill it?
Hard to tell Mike, since I don’t know anything about your air compressor or where the oil is spraying from.

I suspect it’s coming from the oil fill tube vent?

If so, and the compressor is fairly knew, it’s possible you overfilled it and vibration is kicking the oil out or… you’ve overfilled it enough that for the first time you are seeing what happens when the piston seal(s) a worn to the point that compressed air is pressurizing the sump, and the oil fill is high enough to block the air from getting out the vent easily.

LP335SK (040-0387) Pump blowing oil

by: Troy

I have oil blowing out of the oil breather on the pump. It is blowing out while the pump is running. Can you tell me what my problem is?



Troy, if what you mean by breather is the air intake port (maybe with a filter on it) and oil is coming out there, I suspect two problems.

One is that the intake valve is failing, allowing compressed air and oil to blow back out as the compressor cycles. The other is that the piston rings / seals are worn, and you are getting oil blowby into the air pressure chamber as the compressor runs.

Time for some repairs, I fear.

What are holes for?

by: Don

Thanks Bill. Are the holes in the oil stick cap there for a reason? They are on opposite sides at the top of the stick.
The vent holes in the oil fill cap are there to let air in and out.

As the compressor works, small amounts of oil are wicked up into the compression chamber, and sent into the tank with the air. This could pull a vacuum in the sump, and the oil fill vents let small amounts of air into the sump as necessary. Also, when cylinder seals get really worn, there is more oil moving up out of the sump and over the piston and more air needed in the sump to overcome vacuum.

By the same token, some compressed air will wick past the cylinder seals and into the sump. The oil fill vent lets that air out.

If you have lots of air blowing out the vent cap you have an indication of significant wear on the piston seals.

Gianesi edilio air and oil mixed

by: Kamran

Gianesi edilio air compressor. Air and oil mix problem, how to solve, sir.


You say the air and oil are mixing on your Gianesi Edilio air compressor?

From what I’ve seen, these are pressure vessels, not air compressors?

If it is air and oil in the pressure vessel, what’s the compressor that’s pumping the air / oil into the tank? It may need work.

Also, please note that this thread is about air/oil coming out of the oil fill tube on a compressor.

If this isn’t what your question is about, please start a new thread on the ASK page on this site and we’ll see if we can help.

Also, see:


for info about your equipment if this helps.

Additional oil reading:

Do you have a question about why does oil come out of the oil fill tube? Please add your comment here along with photos to help others help you with your compressor and equipment problem!

By Bill Wade

About Air Compressors has been helping folks with their Air Compressor Problems since 2002 online. We're a community of DIY and Compressed Air professionals who are keen to support everyone across the globe with their air compressor issues and troubleshooting. Whether you're trying to identify an old air compressor, or troubleshoot an error code on a sophisticated new industrial air compressor - the community at About-Air-Compressors.com is here to help you

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