99 Envoy heater blowing cold air

I know you posted your message a while ago, but I know that most no heat concerns on vehicles like yours are caused by restricted flow through the heater core. You will most likely have to perform a chemical acid flush on your vehicles cooling system and replace the tstat and radiator cap and refill with a 50/50 dexcool mix. You will most likely have to backflush the heater core two times by removing the hoses that go to the core and running air, then water in reverse direction of normal flow, and then driving the vehicle for 100 miles or so, and repeating. This is necessary because the chunks of debris in the engine block will begin to break off, and will plug the core again. I believe the heater core is just acting like a filter in cases like this, and I know some people prefer to replace the core, but that is a huge job. I also would recommend having the Intake manifold gaskets replaced with the latest design because the old design plastic gaskets were prone to leaking, and Dexcool can become crusty and rust-like if there is a leak in your sytem.