Thanks for the post!

Good chance you have a worn or damaged Piston Ring. This is a very common problem after some time. Being it's an oil-less pump, the Teflon-coated Piston Ring wears down over time and of course when this happens, it will create little compression or no compression, which will in turn cause the unit to run constantly until it overheats.

I would also check the Reed Valves in the Valve Plate to confirm these are not damaged, but most likely you'll see the Piston Ring has a tear around the outside perimeter; most likely the cylinder's walls are worn down pretty badly as well.

I would take the plastic shroud off of the motor/pump assembly, then remove the pump head and valve plate which will allow you to see the piston ring.

Lastly, make sure the drive Belt isn't torn.

You can see a breakdown of this unit by clicking on the below link:

www.mastertoolrepair.com/portable-oil-fr...1900aj-p-746922.html

- MTR

Glad to help!

Good question, not sure if Campbell Hausfeld (who manufactures this unit) improved the motor/pump assembly necessarily, it could simply be a part# change.

You can ask a technical rep with Campbell by calling them at 800-543-6400.

- MTR