First I gotta say that It seems as though that right after I had my headgasket replaced, this started happening, and I took it to the dealer shortly afterwords, and they found that this wasn't caused by any of their work, but I have my doubts.

Anywho, the problem is that when it's below approximately 15* outside, the HVAC system will default all the air through my defrost vents when I give it any throttle, until the car thoroughly reaches operating temperature, then it works fine, even at WOT. But for those first 10-15 minutes, it's wierd. And if I let it sit for a while, and get back in it, it works fine, but it seems it'll only act up when I first start it up in the morning...so basically it only happens if it sits for like 6+ hours.

The first thing that came to mind was that I was getting ice in a vacuum line or a check valve, and that's why it defaults to Defrost only when it's very cold, but after it reaches operating temp (and thaws out) it works fine, so I told them that when I went into the dealer a few weeks ago, and they looked it over, checked for any errors on their part, and they didn't find any thing that they could have caused, so they're thinking that it was a problem with that "vacuum box" (for lack of a better word, it's in the dashboard, and it takes the vacuum line that runs from the engine, and diverts it to operate all of the blend doors). And obviously, it wasn't caused by the HG repair...it's just a tremendous coincidence.

So now I've gotta go back there, if I decide to look into this any further, pay for the one hour of diagnosis, figure out what it exactly is, and if I'm lucky, the warranty company will cover it. Otherwise, I'll be out the $115 for the diag, plus the repair costs if I wanna pay out of pocket for it, or I could just live with it and wait until I've got more money to deal with it.

 

You're gonna need a mity-vac to diagnose this problem. If you don't have one, they're sold at most auto-part stores, and Sears. <$20.

Scratch that - the testing process is too complicated for me to describe here.

The players in this game are:
Engine - primary source of vaccuum for HVAC
PSE pump - backup source of vaccuum for HVAC
Vaccuum tank inside passenger fenderwell, behind tire - storage of vaccuum produced by PSE pump.

All automatic HVAC systems I've ever encountered default to full defrost when in failure mode. This is a safety issue.

It sounds like the system functions normally, unless you step on the gas, until things get warm. I will assume this means you get air through the defroster vents when accelerating - does the air return to the footwells when you reach cruising speed - I doubt it. Does the air return to the footwells if you turn the system off and back on again - I'll bet it does.

It may be a combination of things:
1) Moisture in the vaccuum line from the engine is freezing and clogging that line.
I wouldn't think it would take very long for underhood temps to get above freezing, but I'm from TX, not MN, so I have no experience with arctic cold like you get up there. I can't figure out how moisture would get into this line, although the dealer would have had it disconnected while doing the head-job.

2) PSE system not able to "charge" up the vaccuum tank due to a faulty/leaky hose, manifold (under passenger carpet, behind metal plate - has about 10 hoses attached.) or canister Also can be the PSE pump is kaput, but you'd see other issues, like no door locks.
The PSE system provides "backup" vaccuum for the HVAC system for when the HVAC is in REST mode, or when engine vaccuum is very low, like when accelerating. If this circuit is depleated, then the HVAC will fail over to defrost when the car accelerates. I'm not sure it will return to normal vent settings without a power cycle - I don't think the system has enough "brains" to know where the various valves are actually set, it just calls for vaccuum to the right locations at the right time and "assumes" everything happens correctly.

If you're up to the task of diagnosing this problem yourself, AllDataDIY has an excellent section on the MVA circuit (Manifold Vaccuum Assist). Sign up through v12uberalles.com to give Pete some well-deserved $.

Hope this helps.