DeVilbiss compressor slow starting and venting on starting

by Dan Huttunen
(Wasilla, AK)

My air compressor is a large, older deVilbiss 2-stage.

When starting, the motor is lugging as though there is a lot of resistance from the compressor and the valve releases air in short bursts repeatedly.

After 15 to 30 seconds of that, the motor speed increases, the release valve stops venting and the compressor runs normally up to full pressure.

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Dec 02, 2014
Venting issues
by: Bill

Dan, thanks for your kind comments about the site, and the book acquisition.

With tank empty and the compressor off, pull and rinse the check valve is solvent, let it dry, and reinstall. That should deal with any sticking issues.

If the unloader valve is part of the pressure switch, the if it is cycling on and off, that suggests to me that the pressure switch too is cycling on and off, otherwise, there is no other reason why the unloader valve should be venting and not venting. Monitor the switch motor side voltage to see if the power is cycling as the unloader is venting. If so, that suggests a new switch is needed.

As to tripping the unloader valve manually, I suppose that's possible, but then, it would only vent for a second.

Sounds to me like your compressor may have a switch problem, and, quite possibly, if the symptoms persist after cleaning the tank check valve, a pump valve problem.


Dec 02, 2014
More on the DeVillbiss Compressor
by: Dan H

Thanks for your web site and advice. I did buy and download your book which I read completely (skimming over some of the very basic stuff).

I think the check valve is sticking in the closed position on starting (when the pump is beginning to fill the tank).

Symptom 1: The motor (pretty new single phase 220) is working hard (low rpm) and the unloader valve is going on and off sporadically and under pretty high pressure from the sound it is making. After about 15 seconds of that, the unloader valve stops venting and the motor gradually picks up speed to a normal sounding level and fills the tank to the preset level of 200 psi.

If I am using air a lot (i.e. the sandblasting cabinet), once the compressor head is warm, it doesn't seem to be a problem. Mostly just after it has set for a while.

Also, as the compressor stops at full pressure, the unloader valve vents normally.

One more possible symptom is that I manually depressed the unloader valve while the compressor was off and did get a small blast of air venting. Is that normal?

Thanks again,


Nov 30, 2014
Checking unloader valve
by: Bill

That's covered on the unloader valve page, Dan. See that, check it out, and then post your findings if you wish.


Nov 30, 2014
Unloader valve
by: Dan H

Thanks for the quick thought. Q-2: how do I check the unloader valve?

Nov 30, 2014
Valve preventing compressor start
by: Bill

When you say "the valve releases air in short bursts repeatedly" are you referring here to the unloader valve?

If so, it would appear as though that valve is not working properly, and a replacement may be in order.

If the unloader isn't venting on compressor shut down, that would increase the load on the motor on start up, and may cause lugging.

First thing then is to ensure that the unloader valve is working properly.


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