I was opening my hood to change my spark plugs all by myself for the first time, and I noticed that the engine was still REALLY hot, after it had been sitting for an hour. I took note of the coolant level (as I always do when I open the hood for anything), and it was normal. Three hours later, I go back and my coolant level has gone down at least an inch, and my engine is STILL really hot. It was driving very oddly today too, but I don't know enough to be able to describe it better than... it sounded very cloggy, and was very unresponsive. My first thought when I saw the coolant loss was OH NOOOOO!!! intake failure!!! But I looked under the car, and it'* all on the ground. I was under the impression that when the intake failed, it went into the engine, and I wouldn't see any on the ground. I'm really worried :(

Oh yeah, and it hasn't leaked coolant for over 2 months, that leak was caused by a bad radiator cap, so basically nothing like this til now. I also have very little history on the car, so I don't know if the UIM/LIM was ever done before.

Oh yeah, and as for possible leak sources, I just did the upper radiator hose, but not the lower. If that helps.

You could check for the date on the UIM to see if it was changed. But if you have coolant on the ground it sounds like an external leak, which could be good – better – news. Where was the coolant leaking from – on which side of the engine was the pool of coolant?

Left side, near the coolant container thingy.

Water pump? Coolant elbow? :? I'd look carefully in those areas.

Eeep.. where are those? :?

Coolant elbow is as depicted here, the center of the pic:
http://www.clanmob.net/hosted/bonnev...k/DSC00518.jpg

Water pump is in the middle of where your pulleys are (number 5 in pic). I know your car isnt supercharged but the location should be the same, hence here is a pic to show you. If the water pump is bad, it will leak through the weep hole. My money is on the elbow however:

It could be a leaky connection where the rubber hose fits at the bottom of the overflow tank...pull out the battery and see if there'* a puddle on/near/around the battery tray.

(Doesn't explain your drivability symptoms, but an easy thing to check...)

Was it driving cloggy before the spark plug change, or after?

Well, I still haven't changed the plugs because the engine was so hot for so long. I checked the coolant elbow and water pump this morning, and they look okay to me, but no coolant has leaked since last night, so it'* hard to tell. I'll pull out the battery after work. Thank you!!