Coolant Disappearing

So I replaced the plastic elbows that run to the
tensioner about 3-4 months ago. I'm still going
through coolant. I've gone through 3-4 gallons since
then. No visible leaks. We originally thought the
heads were cracked, as white smoke was coming
from my exhaust. And just the other day the engine
temp was up to almost over heating, and my car
was blowing cold air (set on 90 degrees.) I took it
into the dealership where I bought it, as it is still
under warranty. However, they pressure tested it,
checked the spark plugs, and drove it. Told me it
ran fine, didn't over heat, and that I can come pick
it up. Any ideas??