You are correct. The heater core hoses run to the firewall (rear of dashboard engine side). If neither is hot, there is no coolant flowing. Either you have a plugged heater core, air pocket, low on coolant, or you could have bad head gasket allowing compressed air to get into the system and blocking it.
I would start simple. First, with the engine cold, remove the radiator cap and make sure coolant is full. If it is, replace the cap and loosten the heater core return hose and start the vehicle. See if air comes out. If it does, make sure the coolant overflow stays full. If that still doesn't help, replace the thermostat and flush the radiator and heater core. It that still doesn't help, you need to check engine compression to make sure there isn't a bad head gasket.
This sounds like a lot of work, but really it isn't. If you want to do it, let me know and I will walk you through it. It isn't as bad as it sounds.
Let me know. I have to admit, the hissing sound you are hearing has me a bit concerned. Are you sure none of the hoses are collapsing causing a restriction and a hissing sound?
Wednesday, December 29th, 2010 AT 5:59 PM