Ok, so here's the problem.

When I start my car, it warms up fine - right to where it should go. However, once I start moving, the temperature gauge goes right back down to a VERY low temperature (like back down to the bottom line)

I tried putting the heat on today and it's coming out cold, so this is going to be a problem if its not fixed for winter.

I'm almost positive it has something to do with the coolant sensor, but I have no idea WHERE the sensor is, or how I would even check it.

Can anybody either describe where it is, show a picture, and/or say how to check/fix it?

Thanks in advance!

thermostat, it's stuck open, it's in the same housing as the coolant fill cap right off the front of the head

how do I close it? Is there like just kind of little valve I can switch to close it or something?

It's not a "valve" you can control that way. It opens and closes depending on water/coolant temp. Just replace it. I think it's under 10 bucks at autozone, and the new gasket is a buck or two.

This will make it clear for you.

http://auto.howstuffworks.com/cooling-system8.htm

ha.

If the thermostat doesn't help, the coolant temperature sensor is the bigger of the 2 sensors on the thermostat housing below the radiator hose. I believe it has the 2 wire harness plug that connects to it. The one below that is the dash water temp gauge sensor.

Or you have no water in it. :shrug

awesome, thanks everybody.

I'll check it out ASAP