You are sucking coolant!

For sure If your engine "drink" that much it not because it's hot
outside!!!

Leak down test followed by a coolant pressure test by Bentley or read my
page on the subject at:
http://www3.sympatico.ca/huotx/pressure_test.htm

This will tell you if you have an internal problem.

#1 problem: one of the internal round head gasket (botom or upper)
http://pages.infinit.net/engine/heads/head_sleeve1.jpg
is leaking.

Second problem: crack in the head (you should have some white smoke in the
back

Do the test!

Ben
http://www3.sympatico.ca/huotx/engine1.htm