Hey everyone,
I've picked up a 2001 LX Manual Sedan for the low low price of free. It had some electrical issues among other things, but I took care of the electrical issues: on to the other things.

I've read the cooling issues sticky,
-edit:posted too early on accident.-
Anyway, I read the sticky, have some other concerns as well.
The car runs great, has not overheated or even tried in the slightest. No black film inside of reservoir, no pancake syrup smell, but it does push coolant from the reservoir. No smoking, misfires, milky oil, or any of the other super obvious symptoms.

I pressure tested the cooling system as recommended to 16 PSI, walked off for about an hour, no change.

It will push coolant into the reservoir but it will suck it back down, no stream of bubbles coming from the filler neck or out of the reservoir, and the coolant hasn't ever boiled really.
The one symptom that confuses me is this: I can run the heat while driving, for maybe 5 minutes, before it goes cold. While it goes cold, the coolant temp gauge will actually drop to COMPLETELY cold while it's running and driving. The heat will stay cold, and it will show no other issues.

I assumed it would have a head gasket issue when I picked it up. It doesn't seem to be bad, if it is blown, but that very last symptom make me want to believe I've got another cooling issue that's faking a head gasket.
For the record: if it is a head gasket, that's fine, I'll fix it. I just want to explore other troubleshooting options and methods considering I'm in the car for $0. Thanks

 

It's normal for the reservoir level to go up a little when the coolant is hot. Is it rising a lot? What is the ambient outside temperature where you're located? A thermostat stuck open could result in no heat if the outside temperature is very cold.

Also, there are tests that can be used to diagnose head gasket issues. There's a sniffer test that can detect hydrocarbons in the coolant. A leak down test might also detect head gasket issues in certain circumstances.