Why does oil come out of the oil fill tube?

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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:

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Hello, good info here. Certainly appreciate your time. I’ve got a 1985 Acme 7.6 cfm 20 gal. A small bit of oil comes from the vent tube and a good bit comes from the shaft seals on both sides. I did have the oil slightly over H on the level indicator on the fill tube.

So, is this machine new to you? If you recently overfilled it, and have since removed the excess you may still see some residual oil for awhile. I wouldn’t panic. Yet. ?

Hi Bill, I found this site and cannot believe how much knowledge is on here. It is fantastic so keep up the great work. I just brought a 2nd hand compressor that was a little older but seemed like a great bargain given the condition. you know the saying they don’t make them like they used to. Well this one had a fairly new paint job but the seller was selling on behalf of his dad who was a spray painter by trade. Anyway I used it first time no issues. 2nd time later the same day and there is… Read more »

Could be rings, but usually this is overfill. Can’t tell where the oil is on the dipstick, but probably should not be above the small end.
I’d suggest you drain some oil into a clean container to see if you can see through the site glass (viewer). If you don’t see the oil level at some point before it is completely drained, you might unscrew and clean it.
Still, it’s almost certainly overfilled…

I just recently bought my used compressor. 2006 De Vilbiss EXLC7060V. I changed the oil and put approximately 16 ounces in the motor. The unit seems to be running fine but I am seeing oil spraying out the hole in the top of the cylinder head. It seems to be a fairly significant amount that pools on top of the head.

I was looking through some parts suppliers and discovered there is a felt filter missing on my machine. Would that account for the amount of oil I’m seeing on top of cylinder head?

No, but you should get a filter on there.
More likely, it’s got worn rings, or if you’re lucky, it’s just got too much oil, The pump must be level, and then the oil should be at the bottom thread of the fill port, give or take a thread.

Let us know, please.

I totally filled the unit to the top of the fill plug and then a bit to make sure there was enough oil. Should I try and get some out?


But don’t expect the oil coming out from the intake to stop immediately. There’s probably more sitting on top of the pistons, so it may take awhile for it to clear.

Thank you so much for all your help. I’ll keep you posted.

GREAT NEWS UPDATE: I removed some of the oil from the case so that approx. 5 threads could be seen at the filler cap. The compressor ran great and no oil was coming out the top hole on to the cylinder head. I will try and pick up that felt filter pad.
Thanks once again for the assistance.

You’re welcome. Good you noticed the oil – it could have done some damage over time.
And thanks for the update.

Hi Bill,
Just rebuilt my pump new rings and bearings. At 60 psi I start venting oil. I can pump up to 100 psi. The pressure switch shuts off the compressor and I can hold the pressure for about 30 minutes. Any suggestions would be nice

What pump?
Oil venting from fill cap? or?
Maybe a couple pix?

Sorry for the delay. B5900 and oil is coming from the pin holes in the fill plug

Did you maybe put too much oil in to start? 40oz. 30 weight, depending on revision? Otherwise it sounds like the rings aren’t doing their job – sure you put them in correctly? You honed the cylinders? What about the valves and head gaskets (not much pressure for a two stage)? Or are you running it for CFM at 100psi? Finally, the pressure dropping so quickly suggests you may need to service or replace your tank check valve. Not related to the oil problem. You might also get some ideas from this: https://about-air-compressors.com/b5900-air-compressor-pump-issues-and-repair-steps/ Good luck, and please let us know… Read more »

If oil filter is dirty. Block air to get into the piston chambers . The air compressor oil spilled out.

Aziz of technicalaircompressorservice.com

Aziz, Is this a question or an answer to another question?
Either way, what’s the machine you’re talking about?

Hi Don, Saw this site and i find it helpful for like me. i have a question you can enlighten me about the issue in my homemade compressor. i build a homemade air compressor made out of two 1/2 refrigerator compressor and a 20kg propane tank, it operates well nice, but i notice that oil in may tank and airlines. so when i check everything i discover that the copper tube from the compressor through the tank pump oil out of the compressor. I search about this problem but cannot find any thread about it. hope you can help me… Read more »

Carlo, how ’bout some pix of your setup?

This is normal since refrigeration compressors are designed for a closed loop setup where the system is filled with refrigerant as well as oil. The oil gets pumped around along with the refrigerant and is there to keep the compressor lubricated. I’m not sure how your system is set up, but maybe if your tank is located above the compressors, aka compressors should be at the lowest point in the system, the oil would stay in the compressors and would not be pumped into the tank. It’s important to keep your compressors lubricated if you want them to last so… Read more »