Problem w/ intermittent defrost a/c


Have a '11 Crown Vic with 290k miles and I've been having a problem with the windows fogging up all the way around. The defrost seems to be working about half the time. On humid rainy days it'll work for say a half hour keeping the windows clear, then the windows will fog hard for 10-15 mins. And then it might decide to start working again, back and forth it goes. When I started it just now the compressor clutch turns when the a/c is turned on, although I haven't had a chance to observe it when the windows are fogging.

A secondary problem it has is that it only blows air through the vents on the front of the dash, straight out into the cabin. The air flows only very weak out the defrost vent at the windshield or the floor. It does this on all settings. Although this might contribute somewhat to the problem, it seems like the a/c does completely cut out as the windows will still fog hard with the a/c set to on and on full fan. When it cuts off the air in the cabin feels immediately humid. One that that helps somewhat is to turn the temp toward cooler and it seems to not fog as much and gives it a chance to maybe clear the fog some.

Any idea what's causing this or what I should do to start testing?
Fogged up glass can be a sign of a leaking heater core. Do you smell antifreeze or is the passenger floor carpet wet?
The heater core angle is a good idea. You'd probably smell that, and/or see little "whisps" of glycol coming out the vents.. Worth checking, but this could be a few things.

With regard to it not switching between "modes," I'd be highly suspicious of a vacuum leak somewhere. Although if memory serves, these things will usually default to "defrost" mode as a fail-safe, so you could be looking at something more like a stuck blend valve.

Certainly the A/C compressor plays into this, so getting that resolved is pretty important. Additionally, if the car is kept outside, I'd check to see if rainwater is gathering somewhere you don't want it to. (ie: running into the air intake at the base of the windshield) if you're collecting water in the system, that will exacerbate these problems. :)
Thanks for the help and sorry it's been so long since I've been back. Life got complicated all of the sudden.

Actually I had to replace the heater core a couple months back so I'm fairly sure that's not the cause.

The a/c is having the same problem it did last summer. The air cycles back and forth from being medium cold to only slightly cold about once every minute or two. Not sure if this problem is related to my intermittent defrost.

Does this help with any more ideas?
Just when it rains.

I'm gonna assume my problem is being caused by low refrigerant and take a shot at fixing it as cheaply as I can. But at the same time I'd like to take steps at identifying where the leak might be.

So what will this look like? Renting some gauges from the local car store and checking my refrigerant lines? How would you go about this and what readings should I be looking for?

And then if things read low I would get a can of refrigerant with leak revealing dye and put it in the system?

Sorry I'm so remedial here, first time working on an a/c system.
Hooked the gauges up and it seems like I've got a low refrigerant condition. Before turning the engine on both gauges read about 75. After starting the engine and setting the AC on max you could really tell the compressor was cycling fast. The needles cycled every 5-6 going up and the other down, then reversed. The blue gauge cycled between 22 and 38 PSI while the red went between 90-140. The outside temp was about 60 F.

From what I've read, especially with the compressor cycling so fast, this indicates low refrigerant. Hopefully my compressor isn't hurt by running it for so long like this.

How do I go about using this 2 gauge setup to add refrigerant? Should I add it through the middle hose somehow or get a can of 134 with a gauge on it and just add it directly from the can?

And how do you know how much to put in?
I'd like to offer some sage advice but in my experience, I've learned to leave the AC system diagnostics to the pros.
Much like Driller, I also treat the A/C as something of a "black box."

I have done the occasional "top off" however, and have once replaced the condenser coil. This taught me the hard way that it's important to pull a vacuum on the system prior to recharging it. My first attempt failed pretty miserably.. :)
Thanks for the advice guys. I def know I'm on the edge of my expereince here and if my next step doesn't get the problem fixed I'm going straight to a pro.

After I took the gauges off I noticed one of the schrader valves was hissing, and now the system is down to 0 psi. So I believe a faulty valve has been the culprit all along.

Now that the system is down I'm going to replace both valves, but my regular schrader valve removal tool won't fit down on the valve. The stem of the valve is too long and the tool won't go down around it to twist it out. I need a tool with prongs longer to fit down into the recessed sides of the valve. Does this '11 Crown Vic have schrader valves or a different kind of valve?

Also, when I go to fill the system with refrigerant (after vacuuming out) , do I need to add any PEG oil or just add refrigerant up to the specified capacity?
I believe the schrader valves are different sizes versus a standard tire valve. As for oil in the refrigerant, most shops will add oil before filling the system and most refill kits have oil mixed with the refrigerant.