Coolant issues

If it were the heater core it'd be dripping onto the passenger side floor. That being said, its either going into the engine or leaking out of it.

When I was losing coolant, I had a tiny hole in the heater hoses leading into the firewall. Couldnt see it and took me like a week to find it, but damn, that thing could leak some coolant. Also check under your thermostat, hard to see if its leaking there too, though it would pool under water pump.

Is your rad. cap sealing well enough? If it's not sealing perfectly, you lose pressure, which lowers boiling point and it could boil and evaporate. That being said, if the cap weren't sealing, you'd prob. have at least a little coolant on your battery, etc(maybe not if you drive easy). Just something to keep in mind.

Are you running a water/anti-freeze mix? If you're losing coolant, are you overheating as well? Check your oil to see if you have any coolant in there...I know you said it still has power, but I wouldn't rule out an internal leak just yet.

I've read that you don't necessarily see a puddle in your floorboard below the heater core. I think one guy had one leak down the firewall only. As Andres noted, those hoses in the back for the heater core can sometimes leak. Have you done any work on the car recently? Maybe you didn't tighten the clamps enough or maybe you have a defective hose.

Have you changed your coolant recently? Maybe it's still sucking it into the block...I had to drive mine, add some, and I kept doing that a numerous amount of times and it eventually stayed full.

Lots of stuff, hope this helps. Been there, done that, it sucks.

Yeah man, as having just reworked my cooling system (still working my own kinks out) I can tell you that what "TheUser" says is all true. My heater core blew and the only time it leaked was when I really nailed the gas. Otherwise I don't know what it did. However, once I installed a high flow water pump, it pulled like a mother. Best advice is to check those hoses leading into the firewall and the ones from your radiator and water pump. If you still can't find it, get some 5/8 tubing and bypass your heater core to see if that was it. Good luck!

Sounds like a slow head gasket leak.

I always say this, but it might be that damn egr spacer if you have the coolant lines hooked to it. It'll leak just enough to notice but not enough to show obvious signs.

I had a similar problem with my car a while ago, water was coming out of the "weep" hole (i think that's what it's called) on the underside of the water pump. Like yours it wouldn't happen unless I got on it, it finally gave way and started peeing like crazy, I had to replace the pump.

I took me 3 weeks to find where it was coming from, the problem was in the beginning it was a small leak that evaporated quickly against the engine...

just a thought