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First off, I'd like to thank sdaidoji and BlueEM2 for their incredible work putting together one of the most comprehensive troubleshooting threads I've seen to date. The quoted tutorial by gearbox proved to be very helpful in my trobleshooting.

About three weeks ago I noticed a lack of heat first thing in the morning in the Civic. At first I assumed low coolant so I checked the reservoir bottle and found it just above the minimum line as it had been when I've checked it periodically. No indications of running hot back and forth to work but progressively staying colder for longer periods on my morning commute. Then this morning after driving to the bottom of our driveway to drop my son off at the bus, the temp gage rose above the thermometer symbol for the first time. By this point the engine had been running about 20 minutes. I quickly shut the engine off and waited for it to cool down a little before driving back up to the house. Once at the top I lifted the hood to look for what I thought would be indications of a coolant leak. I looked at the reservoir bottle and it remained at just above the minimum as it had been. I started it back up and started checking hoses for heat. The two thermostat hoses were hot so I looked for the heater core lines at the firewall. They were warm but not hot. That's when I noticed the top of the heater control valve. There appeared to be water around the top lever. I first thought it was runoff from the hood as we've had a bit of snow which is in the process of melting. I took a break to let everything cool down before checking any further. Once cooled, I took off the radiator cap to find the level right at the bottom of the hole 90 degrees from the filler neck. That didn't seem exceptionally low so I came back in to do some research on this website. I have to admit I was a little worried after reading all the threads on head gasket failures. I went back out and drove the front end up onto a snow bank before removing the radiator cap again. Sure enough, the level dropped out of sight into the radiator. I started it up and proceeded to fill the radiator. At first a lot of foam developed but kept receding into the filler neck. After repeatedly topping off until solid coolant remained I replaced the cap and pulled it onto level ground. The coolant was green and after reading a number of posts on the Honda all weather type 2 coolant, my next order of business is to get this thing flushed and filled with the right stuff. Anyway, with the level full, I checked for heat. Sure enough it was hot once again. I then took it for a spin down the highway looking to see if the needle started climbing at all. Thankfully it remained rock solid right at the base of the thermometer symbol. As I went down the highway I set the temp dial to cold to see if the temp needle changed at all. No movement whatsoever. Once home I lifted the hood once again to find the reservoir right where it had been. I thought this was odd as I'd just come off the highway and the level should have risen slightly i would think. That's when I decided to check the temp control valve. Sure enough, the top was wet once again. At this point I'm thinking the system was venting though the control valve which explains why the level never changed in the reservoir bottle. Upon cooling it must be sucking air back in through the same point. The heater core was getting more air than coolant. Either way, it's coming out. Hopefully I dodged the bullet by catching it in time. We'll see tomorrow. Thanks again guys for all the great info on this site. Sorry for the long winded first post. Hopefully the update will be far shorter.

 
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thanks! hope all goes well with yours

 
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Well, it's been two days with the temp gage rock solid at the base of the thermometer symbol. The valve is $30 but will be a b**** to get to behind everything. Still better than a HG though. Thanks again.