An air compressor that will not build pressure is a very common problem amongst compressed air users. The compressor runs, and runs, and seems to be working fine. There may or may not be air in the compressor tank.
Or, the air pressure in the tank will reach a plateau, say 40 PSI for example, and even though it runs continuously, the compressor will not build pressure past that point. If this is the kind of issue an air compressor is facing, then this is the page to help you resolve it!
An Air Compressor Not Building Pressure
Aside from not delivering enough air for the air tool, this issue can create another problem. An air compressor that will not build pressure runs on long past the point where it normally would reach the cut-out pressure level and stop. The result is the compressor motor will overheat.
Even though the compressor motor might shut down due to it having a thermal cut-off switch, overheating the motor is never good. It also has a negative impact on the compressor start and runs capacitors too. If your air compressor will not build pressure even though it is running, shut it down until you get it fixed.
Reasons Why an Air Compressor Will Not Build Pressure
Let’s start with the various issues that may cause your air compressor not to build pressure, and then address each of these issues in more detail in order.
- compressor intake valve failing
- compressor pressure valve failing
- compressor gasket failure
- compressor piston seal failure
- compromised tank check valve is
Compressor Intake Valve Failing
As the reciprocating air compressor – that’s an air compressor with at least one piston that moves up and down or side to side to pump air – cycles, on the intake cycle, the piston moves away from the intake valve which opens as the piston moves and pulls a bit of a vacuum. Air from outside the pump head rushes into the cylinder through the intake filter. Then the piston moves into the compression cycle, and the air pressure building up inside the cylinder forces the intake valve closed.
In the image below, on this air compressor pump, the intake filter is circled in red. On your air compressor, the intake filter may look different from this one, but it will be located at or near the top of the pump housing. On the smaller air compressors, the intake filter may be under the shroud, if that model of air compressor has one.
If the intake valve has been compromised, the symptom typically is air blowing back out the intake port.
Remove the intake filter, start the compressor, and carefully feel around the intake port. If the intake valve is failing, odds are good you will feel air pumping out this port. That being the case, it’s time to replace the intake valve.
Depending on the model of the air compressor, the intake valve may be part of a “valve plate” which will also contain the pressure valve. If that is the case replacing the plate will also replace both valves, and there is no downside to that.
Depending on the make and model of the compressor, finding replacement flapper valves for an older compressor can be a real challenge. You can, if needs be, make your own using spring steel and the old one as a template.
Compressor Pressure Valve Failing
A compressor pressure valve failing is harder to diagnose. When the compressor piston is moving into the compression stroke, the intake valve gets closed by the increasing air pressure in the cylinder, and, almost simultaneously, the pressure valve is blown open.
The pressure valve allows the compressing air into the airline running from the pump head into the tank. If this valve has failed, air will flow into the line to the tank, but as soon as the piston moves into the intake cycle, air will still flow into the piston from the intake valve, but it will also back up the line from the tank into the piston bypassing the pressure valve or flapper. The air cycling back and forth will not allow pressure to build in the tank past a certain point.
With the compressor off and the tank drained empty, pull the line from the piston head. Start the compressor. If you are able to prevent air from blowing out the pressure port (not using your finger or hand for safety’s sake) then that is a good indication that the pressure valve has failed.
This method of diagnosis is not foolproof, however. You will need to check and eliminate other potential problem sources before deciding that it’s the pressure valve.
If you determine to your satisfaction that it is the pressure valve that has failed on your air compressor, once again, it’s a pump tear down to repair or replace it.
Possible Air Compressor Gasket Failure
Quite often, as noted on the previous page, an air compressor will have a “valve plate” on which will be found both the intake and pressure valves. What separates these two valve areas are gaskets. When a gasket fails in the pump, the motor will run, the pump will pump, but the compressor still will not build pressure.
As the piston cycles, the air is drawn in, and when the piston compresses, it’s supposed to drive the air down into the tank.
With a failed compressor gasket the air simply blows back and forth inside the pump, cycling between intake and pressure, and this limits the air getting down the airline to the tank.
The image above is of one air compressor pump gasket. Yours will almost certainly be different. The image is to give you an idea of what your air compressor gasket will look like.
If you tear down your air compressor pump, two things to note. You may not be able to tell if the gasket has failed as often the gasket leaks when the tank pressure reaches a certain pressure, and until that pressure point, the gasket will appear to be sound.
The other is, if you tear down the pump, you will most likely damage the gasket, so be prepared to replace the gasket even if you are not sure that it’s the problem. This will save you a lot of frustration later when you find that replacing the valve plate, for example, solved one problem, but the compressor still will not build pressure.
Compressor Piston Seal Failure
This issue is pretty easy to diagnose if your air compressor is a lubricated model and has an oil sump. Less so if the air compressor is lubed for life.
If there is an oil sump then there will be an oil fill portal, often a tube with a cap on it. If the piston seals have failed sufficiently – they are a wear item after all – while the compressor is running, air will be felt exiting the compressor oil fill tube vent cap.
While any air getting into the oil sump, and some always do, will vent through the fill cap, a badly leaking piston seal will allow enough air to bypass the seal to create a flow of air into the sump and out the fill tube cap.
Solution? Time to tear down the pump and replace the piston seal(s). When doing so, it might be a good time to replace the valve plate, depending on how old and used the compressor is, and, of course, the gasket kit as well.
Tank Check Valve is Compromised
As air flows from the pump head, through the airline, and into the tank, it passes through the tank check valve. The tank check valve is normally located where the line from the pump enters the compressor air tank.
The photo above shows where the tank check valve is located in typical air compressors.
The tank check valve, like most of the components on the imported air compressors of today, is not a high-quality item, normally containing a simple flapper-type valve resting on a seat.
When air from the pump enters the tank, it blows this valve off the seat, and air enters the tank. While the compressor is off, the flapper valve is pressed onto the seat by air pressure, sealing the air in the tank.
If the check valve fails, it may prevent air from flowing into the air tank properly. Or, if the check valve has failed, compressed air won’t be staying in the compressor tank when the air compressor is stopped, it will vent out the unloader. That being the case, a new check valve is in order.
If you have any questions regarding an air compressor not building pressure, please leave a comment below!