i have no heat in my 2000 chevy suburban...?

[gunter_22]

had shop install new motor into truck last week....(new used motor). now have no heat coming out of vents. will blow slightly warm after running car for while. worked with old motor. how do i see if thermostat is bad? can hear door slide for warm to cold air and car is not overheating. (has been -15 degrees in michigan last week). please help thanks

 

[lrgboy06]

It may be the heater core after putting the new used engine in it