Heater Issue: Blend Door or Heater Core?

heating

#1

1999 Jeep GC has strong AC but no heat from vents. Any thoughts on how to easily determine whether a blend door isn’t opening (easier/cheaper) or whether the heater core is shot (tear out whole dash to fix)? Anyone know how to flush to core without removing?


#2

Get the engine up to operating temp and turn on the heat with the fan on high speed. Feel the two heater hoses where they come out of the firewall. They should be almost no difference in temperature. If the core is clogged, the outlet temp will be much lower.

I backflush my heater core this way.

drain the coolant
disconnect the outlet hose from the heater core where it connects to the water pump.
shove a garden hose into the heater hose and run water through it for a minute.