Your air compressor will not build pressure.
It 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.
The creates another problem, aside from not getting enough air to your air tool. 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 negative impact on the compressor start and run 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 by the various issues that may cause this, and then address each of these issues.
- compressor intake valve failing
- compressor pressure valve failing
- compressor gasket failure
- compressor piston seal failure
- tank check valve is compromised
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 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 down side to that.
Depending on the make and model of 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
This is a bit 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 air line 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 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 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.
What else happens when a compressor will not build pressure?