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.
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.