Air coming out of the oil fill tube

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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.

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Donna Philpott
Donna Philpott
September 19, 2021 12:15 pm

Help please? I have a Burisch twin air compressor, I noticed oil spraying out of the oil fill cap. Took off the sump cover and found a thin rubber sump gasket, theres a hole at the bottom that is factory made, but its split all down one side I’m presuming this is whats causing the oil to spray out? Can anyone telk me if its a common wear and tear thing or would the gasket have split because of more expensive repairs are needed? Curious to find out as much as possible before ordering new gasket, dont want to pay out of it suggests its damaged beyond repair 😱

Bob Arbuckle
Bob Arbuckle
September 27, 2019 4:24 pm

I have a new Rollair bulldog twin tank. The oil fill tube fits very loose and blows the dipstick out. My old Emglo compressor fits tight in the bulldog. The groove for bulldog o ring is wider than the emglo. All available o rings are not wide enough to fill gap and the dipstick wiggles back and forth and gets blown out.

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Bob Arbuckle
September 27, 2019 5:57 pm

Are you sure it’s getting “blown out”? As in, there’s enough air in the oil sump to blow it out? If so, it’s not the fit of the dipstick that’s the likely issue, it’s the high volume of air blowing by the piston seals and filling the sump. Or is it just vibration from the pump running that wiggles the oil dipstick out?

Bob Arbuckle
Bob Arbuckle
Reply to  Fix My Compressor Moderator
September 27, 2019 9:57 pm

I tried different things today. Put a new dipstick in and it did not wiggle as much. Used veneer calipers and measured diameters of o ring on dipstick of the Emglo, old bulldog , and new dipstick. Measured width of groove that o ring fits in and every other measure I could think of. Put original dipstick back in Bulldog and opened drain cock slightly and let it run for 15 minutes and it stayed. I could feel a very steady stream of air being blown out of dipstick vent holes. Dipstick stayed in. A new dipstick’s channel width is slightly narrower than the old one and does not wiggle as much. The emglo’s is a tight fit and the other flanges are wider and fits very securely. The new bulldog dipstick did not wiggle as much as the old. I did fill the bulldog up to the max level. No luck finding a slightly wider O ring. I forgot to check the old Emglo for air blowing out vent holes to see how they compare. Emglo pump’s up a lot faster so that might not be accurate. I will stop by Acme Tools and see if their new Emglo dipstick is the same as the old. A new dipstick is only $8.00. I got Rolairs number so I will give them a call. Wish I had spent the extra few bucks and bought another Emglo which has treated me well for decades. I think it is a combination of air pressure and a wiggly fit Have you heard of this problem before?
Thank you

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Bob Arbuckle
September 28, 2019 10:36 am

It’s my opinion that there should be almost NO air blowing our of the vent holes. I really think your compressor has a pump seal problem, and it’s not the tube dipstick at all. Can you think of any reason why air should be coming out of the oil fill tube, except that the piston seal is compromised? I’ve used and owned many compressors over the years and none, that’s absolutely none, had a “steady stream of air” coming out of the dipstick vent hole.

Myles
Myles
June 11, 2019 9:30 pm

I have a Powermate 5hp 80 gallon compressor that has large amounts of air coming out of the crankcase vent/ oil fill. I believe it is a B5900 compressor head. I figured it was bad piston rings, but when I tore it down the piston rings seem in perfect condition. They aren’t broken, have good spring, and don’t seem overly worn/ thin. They also still had a decent seal to the cylinder body as it took what I would guess is an appropriate amount of force to pull it off and the bore was still a mirror finish. The only other thing wrong with it was there was quite a bit of crud around the reed valves as the previous owner had obviously run it for years without an air filter. I was thinking if the build up was preventing the valves from sealing correctly it might lead to a problem. However, my guess is this would much more likely lead to air flowing back out the air intake, not the crankcase vent. Once cleaned the reeds look fine as well. I’m currently cutting new gaskets for it and hoping to reassemble soon. I’m also hoping to keep the price of this compressor in the amazing deal zone and not put $100 dollars of unnecessary piston rings into it, unless truly needed. Are there any measurements/ diagnostics that I could do to confirm the piston rings are good/ bad? Are there any other problems that could cause air in the crank case?

Thanks,
Myles

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Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Myles
June 13, 2019 7:04 am

Regarding the dimensions of the rings, if they appear unworn, measuring them carefully will give you the specs. Unfortunately, I know of no source for the original dimensions. Perhaps another viewer can comment?

If air is coming out of the sump while the pump is cycling, where is that air coming from? The only path to the oil sump from the pump that I know of is if air is bypassing the piston seals, into the sump, and out of the vent. There is no other source. Ergo, it has to be a damaged piston, the cylinder barrel is badly scratched, or the piston seals are allowing air by as the pump cycles.

Brad
Brad
July 19, 2017 12:42 am

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.

peter dyer
peter dyer
April 28, 2017 12:32 am

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

Jory Lopez
Jory Lopez
Reply to  peter dyer
September 18, 2019 7:06 pm

Have you found out what’s wrong with it, mine is doing the same thing

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  peter dyer
April 28, 2017 9:37 pm

I would think your pumps piston rings need replacing, stat!

Roger
Roger
March 27, 2017 12:24 am

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.

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Roger
March 27, 2017 10:03 am

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.

John
John
February 16, 2017 1:35 pm

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

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  John
February 16, 2017 3:38 pm

Yes.

Leigh
Leigh
November 5, 2016 10:28 am

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.

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Leigh
November 6, 2016 4:35 pm

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.

Allan Grant
Allan Grant
January 19, 2016 5:06 am

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?

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Allan Grant
January 19, 2016 11:01 am

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
Chuck Parkerson
Reply to  Fix My Compressor Moderator
March 6, 2017 7:48 pm

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?

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Chuck Parkerson
March 7, 2017 11:58 am

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.