Using up a lot of water

If it's not a noticeable leak it might be going out the exhaust because of a bad head gasket or cracked head. Pull your dipstick and see if you notice any moisture. Look and see if you see any leak tracks behind the water pump pulley area. If you can't find the problem you can get a smoke test done on the cooling system. It'll let you know where it's leaking.

 

put a rag over the stack for a few minutes, if it smells like antifreeze. probably a head gasket

 

Check for external leaks. If its going out the exhaust you can generally smell it or see the smoke. Check your trans level if you have a oil to water cooler.

 

I'm with the others its going up the stacks more than likely, I had the same problem with a C-15......It was a cracked head..