My daily driver (01 Impala) has a coolant leak (about 1gal every 2-3 months in warm weather) And now that it has gotten cold the leak is worse (About 1gal a week) the only difference is the weather and the fact that i have the heater on.

Smoke billowes out of the driver's side of the engine compartment and you can see coolant dripping off of the frame (unibody/ whatever).

Can you give me any pointers? (I have already changed the gasket on the thermostat housing because it had a small leak.)

And there is brown crud on the radiator cap.

 

Your leak has the same symptoms as my 2005, but I've failed to pinpoint it. I initially thought it was a shaft seal on the water pump or the pump gasket, but that doesn't seem to leak though coolant is all around the area.

I let it go too long and experienced some mild overheating. Upon refilling it, I found my heater doesn't work. I was told there's an air relief plug somewhere, but I can't find it. Anyone know where it is?

 

Do you mean the plug to let air out of the system? On my 3.4 it is on the housing for the thermostat (i think, haven't looked for it in a while.)

 

Pretty sure it's under the plastic cover on top of the engine, unscrew the oil filler neck and the cover will come off. It tells how to bleed the system in the owners manual.