SnapOn 6.5HP compressor runs, bogs down and squeaks, then picks up until shutoff

by David Betz
(Grand Rapids, MI)

SnapOn; upright; 6.5HP; 80 Gal; 2 stage compressor; 175 psi; 10+ yrs old

Use it in a metal-fab shop (air for a plasma table). I am not the poster-child for preventative maintenance, but the oil gets changed at least once a year. SAE-30 Non detergent from Auto zone or other. Worked pretty flawlessly for the past decade, but has picked up a problem.

It fires up from no pressure every morning. But when it nears 130psi, the compressor bogs down (almost like it is seizing), but picks up before it stops and resumes pumping until cutoff. Sometimes it will bog down a second time at about 150-160 psi (very infrequent). Sometimes, if I have the plasma drawing air, it will pump until cutoff without bogging down.

I have changed the oil, as it has been a while. No effect.

Shop temps are in the 40's as it is unheated, but these temperatures have never affected it before, and it happens in each pumping cycle, not just at startup.

I was hoping you might have some insight into the problem.


Hi David...

A few things I would check and they are pretty easy to do are...

1) Have you cleaned or replaced the intake air filter?

2) Any change in the power supply to the compressor? By that, I mean are there now appliances drawing power from the same circuit that were not there before? A shortage of power could allow the compressor to bog down when it's working hardest.

3)Compressor off and tankd drained, remove clean and replace the tank check valve.

4)Could also be a run capacitor failing or even the pump valves, particularly on the high pressure side.

See the troubleshooting section for pages about why air compressors won't run properly and build pressure.


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