The air compressor bogs down.

By that, I mean that that the air compressor starts normally, it begins to build pressure in the tank, but at a certain point, the compressor motor begins to slow down. The compressor motor begins to labor, the compressor motor slows, the sound changes, and the buildup of air pressure in the tank either stops completely, or slows substantially.

Sometimes, when an air compressor bogs down, that process causes a problem with the power. The motor may pull too many amps, and the circuit breaker lets go. Or, the on-compressor thermal cut out will shut down the compressor due to motor overheating.

An old air compressor

Older pancake style air compressor

What do you do when an air compressor bogs down?

Diagnosis of the problem may not be that easy.

Many things might be the cause. A list of the most common reasons why an air compressor bogs down follows:

  • Use of a too small extension cord to power the compressor. If an extension cord is being used, remove it, and plug the air compressor power cord directly into the wall socket to see if that helps.
  • Use of a power bar to power the compressor. Same advice. Remove the power bar from the power supply and see if that helps.
  • Too many other power-drawing appliances on the same circuit as the feed to the compressor. Ideally, an air compressor should be powered from a socket that leads directly back to a 15 amp (minimum) or a 20 amp breaker or fuse at the panel. If other appliances are in the line that feeds the compressor, shut them all down, and test the compressor again.
  • The compressor unloader valve has failed. To test this, open the tank drain and let all of the air out of the tank. This will emulate the unloader valve function, and at the same time, remove water buildup in the tank. Close the drain and power up the compressor to test. If the compressor starts easily and runs up to cut out pressure, it’s time to service or replace the unloader valve.

More comprehensive checks when an air compressor bogs down on start up.

The previous checks are simple. There is one more that you should do before getting to the more complex checks noted below.

If the compressor is belt driven, remove the belt (power cord out please) and turn the pump sheave by hand. If it turns relatively easily with no binding, that helps eliminate the pump as the cause.

Most frequent cause of an air compressor bogging down!

The reality is that the most frequent cause of an air compressor bogging down on start up is a failing start capacitor. It’s now time to test this component. See the page on this site about how to do that.

If the start capacitor checks out, and your compressor has a run capacitor, check that one next. If either is not up to snuff, they must be replaced.

Now it’s decision time. Do you take your compressor motor to an electric motor shop for a load check, or do you tear down the pump to see what’s what? Your rotating of the pump sheave may help. If there was any hint that the pump may be malfunctioning mechanically, tear down the pump to check the piston rings, intake and pressure valves, and general condition of the pump innards.

If you feel that the compressor pump is OK, and the power supply to the motor and the motor caps are good, it may be time to pay for a motor load test.

If you opt for tearing down the compressor pump, depending on the age of the compressor and level of use (ie; did you forget to put oil in a lubed compressor?) you may want to replace the piston ring, valve plate and ALL gaskets to ensure that when you reassemble the pump, none of these items can be an issue for some time.

Make sure when you install the valve plate that it right side up, else your compressor pump will not work.

One last thing. Know the duty cycle of your air compressor and make sure you run the compressor within this time frame. Over use of the compressor can, too, be a factor in why an air compressor bogs down.

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