Air impact wrench always spins - fixable?

I have a trusty old Kobalt 1/2" drive 3 forward speed impact gun.  When I connect it to air it always spins at low rpm.  Slow enough that I can still seat it on lug nuts without drama, but I can't keep it connected to an air line.  Is this something to try to repair or just take a trip to Harbor Freight?  I really like this one for the three forward speeds and being able to use it to seat lug nuts to around 30 lb-ft.

Have you oiled it lately? 

Try oiling it first, but if you do go the HF route, I've been loving my Earthquake XT. 

Not sure what it actually torques to, but it has 3 selectable speeds as well. 

Have you tried the tool on another airline? Could there be a problem with the fitting?

Oil it.  The O-ring on the trigger mechanism is leaking.

If that doesn't fix it, then it needs a new O-ring.

Alternatively, if it's one of the 2131/2135 style guns, there's a funky pin-in-grommet arrangement that the trigger pushes on, that can crack and fail.  Usually those don't "idle", they just run full bore when air is attached and the trigger feels dead.

It's possible something is stuck under the valve too. I used to sometimes get dessicant from my cheap air dryer stuck under a valve. I've since added a second filter. 

The trigger pushes a pin which upsets the inlet valve, either pushing a ball off it's seat or cocking a poppet valve over depending on the trigger design.

The pin can rust in to the bore and stick if the gun isn't oiled regularly.

OIl it and hope it loosens up or remove the fittings from the bottom of the gun, dismantle the pin assembly, clean and re-install.

They're pretty simple devices when you get them apart.

Having run a number of 3/4" and 1" impact wrenches for a number of years the key to longevity and reliability was to fit an inline oiler.  Cheap enough and available in sizes that won't hamper usability very much.  I know this won't address your current problem,,,,just sayin.

Shoot, i thought this would address the problem I have with my impact gun.  The socket connection will easily spin, making it difficult (and two hand job) to put a socket on it.

I'd like the socket nipple to stay in place.  Is this just the HF unit?  I don't remember this being a problem on my other one, it seemed the socket connection stub did not spin freely.

Thanks everyone, a little oil, some nice words, rinse and repeat, and it's back to working well.  Added to the shop list board is the reminder to oil the tools.