|Originally posted by what would a josh do:|
When adding coolant do you have to drain the original coolant in the engine already?
I wouldn't worry about that. You just got to get the air bubbles out of the cooling system or the coolant won't flow properly. I would worry about bubbles first and foremost.
You have to "burp" the system. I have had luck with a "gravity/burp" fill. I jacked the rear end way up, pulled the thermostat out and slowly filled it. Then started it for a second, filled more, started for a second, filled more etc. Then lowered and used the "burp" system to double check it.
You really gotta get all the air out of there. Even the heater core. Or you might get air lock. With any luck, hopefully this is all your problem is!