no leaks but the can was a gallon and a half low, i been hearing air going through the heater core behind the dash board too.
checked the stick and cap no mix there either.

what is going on ?

 

maybe you got a set of those leaky castech heads

 

if no external leaks, then it must be internal.

 

I had a 5.3L that consumed a quart of coolant about every 12 to 14 days.
After 2 months, it had built up a half gallon of very thick, black TAR like sludge, bottom of oil pan. Very nasty stuff.
Replaced the heads, and camshaft. After 3 oil changes, the bottom of each oil drain still presents some residual sludge, but the OIL on top was always clean, translucent, and looked just fresh from the bottle.

 

pull a valve cover and see if you have the castech heads

 

BOTH of my coolant sucking 5.3 LS engines had head(s) NOT sporting a Castech Logo inside.
One engine was from 2002, with 194,000 miles on.
The other was a 2004 model, with only 84,000 miles.

 

I had the same issue and thought that it was the weaping head issue. Turned out it was a bad water pump that was leaking so slowly that I never caught it until I noticed that runout.