You just got a new engine put in your 1997 Saturn sl and now the heater blows only cold air your mechanic said that the heater core is blocked How do you fix it?

Things to check. Is the radiator full? Are the hoses connected to the heator core at the firewall? All vacumn lines connected. When engine warm, feel both heater hoses to see if water is circulating thru the heater core. If you think the heater core is pluged, remove both hoses from the core and try blowing air or water thru it. This will show if it is plugged or not and may point you to the cause. Fill the radiator with the car running as this is the only way to insure the cooling system is full and circulating.