Check your oil, Is it Milky or Green?
Pull the plugs...Crank it over, does it "Spit" water out the plug holes?
Does the Starter spin fast when trying to start, a few revolutions, and then kinda lock?
All signs of a blown gasket or damaged casting material. Do a compression test..are a few low next to each other? Bad gasket..
ODD, though, That is is , as you say, not heating up and still using that much coolant.