AC works, then it doesn't. Then 5 minutes later it does. WTH?

Driving home in the WRX, 94F-ish degrees so I've got the AC going pretty well.  10-15 minutes into the drive, I can barely feel the fan but hear it running.  I even crank it up to #4 and hear it but barely feel anything.  Then I notice fog coming from the vents (not smoke, it's fog).  I'm on the highway so I say berkeley it and turn off the AC, turn off the fan, roll the window.  I'm almost home anyway.

I get to the bottom of the offramp maybe 5 minutes later, decide to try again.  Everything is working fine...  Is something just freezing up?  ('02 WRX for reference)

Evaporator is freezing over.

froze it up.  usually it means low freon, but I've had it freeze up before just because it was super humid out and I had been driving with the windows down for a bit. 

I've seen them freeze up due to inadequate airflow -- mouse nests or clogged cabin filters.

Hmm.  I'll check the filter, after some dusty rallycrosses what are the odds?  Otherwise summer is almost over for now anyway, maybe deal with it next spring.  (Google-fu mentioned water leaking into the system somewhere?)

We had that happen, twice, in the One Lap Civic. A can of R134 fixed it both times. Now would be a good time to check your pressures and if they're low, add a can with dye in it. 

Pretty much all systems have low-pressure cutoff switches or evaporator temp switches which are supposed to cut off the compressor to prevent this from happening, so there's a good chance something in the system has failed. Maybe more than one, as usually the operating pressures in the system won't get the evaporator cold enough to freeze. You probably have low refrigerant AND a failed failsafe of some kind. 

Vigo said:

You probably have low refrigerant AND a failed failsafe of some kind. 

Winning!

I have the same thing going on moreor less, although with mine a bump will turn it back on. Its on my winter list of things to do

You say you can hear it, but barely feel it. What happens when you shut off the AC and just run the fan? Do you notice it slowly losing strength, or is it just like a switch is flipped? 

This almost sounds like there is a relay issue too, otherwise wouldn't the fan continue to run?

In reply to mtn :

Have to try again, I'm not sure.  Finally cooling off for a few days, it might be awhile before I have to use it again.

For what it's worth, when we had the core freezing up we'd manually cycle the compressor every 90 seconds or so, keeping the fan on, and we could get fairly consistent cool air that way. Fun way to drive across the country. Way better than no AC at all. 

My subaru does this too, because there is no low pressure switch in the system.   It was left out when the engine swap was done.  I just run the a/c for 5 minutes or so, switch off for a minute or two, repeat.  Works well enough.