Air Compressor Won't Pressurize Tank

I have a Craftsman, oiler, 1HP, 125psi, 3 gallon tank. The pump motor runs but the pressure will not go beyond 40psi. I have to manually shut it off or it will just keep running.

Is this a regulator problem?

Thanks!

 

The regulator should shut the motor off when it reaches a certain pressure probably around 115-120 psi. I had my portable air compressor [sears 1hp, 11 gallon] do the same thing about 15 yrs ago. I suspected a leak in the pump but it turned out to be a slipping belt so try the easy stuff first

 

Hi marksr - this unit has no belt. It has a motor that turns a shaft - no belt.

 

One or both valves or the valve plate would be my first guess. Pop open the head and take a peek. There should be two reeds in there. If they are bent or damaged in any other way, replace them.

 

Hi markiz37 - ok, I will pop the head off and take a look. Thanks!

rn

 

OK, I pulled the head off and examined the reeds. No damage at all - they look great. Put it all back together and cranked it up - no change. The pressure builds to 40psi and stops, while the motor keeps running.

I've checked all the bolts and screws, etc., and tightened everything. There's no leaks either - when I manually turn the motor off, the tank stays at 40psi.

I'm curious - could this be a check-valve problem? What if the check-valve is shutting prematurely? The reason I ask is because I noticed that the oil fill cap has a very small hole at the top by design. When the motor is running, air streams out of the hole very fast. It seems the only way it could get there is if the check valve shuts and the motor keeps running.

What do you guys think?

 

Hmmm...

Sorry about the wrong diagnosis, happens to the best of us

I doubt it's the check valve because it is strictly spring action and air pressure operated. If it lets air in the tank at 25, why wouldn't it at 40? Unless it's completely busted and just lets air back in the head on every intake stroke. You can check it out, it's the easy thing to do.

But usually, what you are describing is a ring problem. Worn out rings will not hold compression and let air in the crankcase where it escapes out the breather (small hole in dipstick). That'd be a little harder to fix. Are you the original owner of this machine? How old is it? Was it always lubed properly? Was it ever run hard for a very long time?

Keep posting your findings, will figure'er out!

 

Yes, I'm the original owner. The compressor is only 4 years old and has never been run hard. I use it primarily for my small nail gun - I don't use it to drive continuous air tools. I did do a roofing job with it a few years ago, but it really didn't work all that hard, mainly because I'm pretty slow. :~)

I pulled the head and valve plate and the piston looks good. There's no oil or air blow-by. What about a blown head gasket? I haven't pulled the top valve plate off of the head, but that gasket is all that separates the intake chamber from the air output. If it's leaking between the in and out sections then I'll never reach full pressure. Correct?

 

Definitely. Do you get any air pumping OUT of the intake filter?

 

I pulled the intake filter off and fired up the compressor. Definitely air flowing out the intake. I'm thinking the head gasket is blown.

One other question. I looked up the gasket part in my manual and it comes in a gasket kit - so I will change out the head gasket and the value plate gaskets at the same time. I noticed that the manual diagram and kit contains a gasket that fits between the upper and lower valve plates. However, when I took the head off and the value plates there is no existing gasket. On a diiferent site a guy posted a picture of the exact same compressor I have with the cylinder head off with the upper valve plate still attached to the head - it shows the same thing: no gasket between the two valve plates. What's with that? If the diagram shows the gasket, shouldn't it be there?

 

Chinese logistics

I don't know. Be careful if you put the extra gasket in that you don't create enough extra space for the valve to be blown out of its normal seat position in the plate. If that's not a concern, put it in. Good luck.

 

I agree markiz37. I'm concerned that using the gasket will create too much space and the reeds will get damaged. Before the head gasket blew it was working fine, so I'm thinking I will leave it out.

Thanks for all your help!