I have install a new heater core and thermostat,and flushed system. I stiil have no heat in 1978 old 98?

Answer 1

Either the plenum door is stuck in the wrong position or the blower fan isn't blowing or drawing air through the heater core.

Some have a valve connected inline with the heater hose and temperature sensor will open or close the valve with the connected vacuum line.

You need to run the engine for a few minutes and grab the heater hose and it should get rather hot to the touch on both the hose coming in and going out of the heater core. If not then there is no coolant or water running though the heater hose/core. This would either be the result of the heater hose valve not opening (if it has one) or the water pump not working right.

If the hose is hot going in and coming out then the flap inside the plenum is in the wrong position or closed. This can be because of a broken cable (older models) or a vacuum motor not working to open the flap inside the plenum. Newer models use an electric motor to open and close the flap(s) inside the heater or A/C plenum. You most likely have a vacuum motor on this model that opens and closes the plenum flap to adjust the air flow and temperature. Sometimes things (like an ink pen) can fall down inside these passages and block the flap and make it stick open or close.

There's a small electric coil inside the plenum as well (most are next to the blower motor or fan) and this senses temperature changes and adjusts the flaps automatically depending on the position of the "hot - cold" lever or knob on the dash or console. Rarely these will fail and then the flaps will not move since no signal is sent to make this happen.

Good Luck!

Answer 2

Check to see if both heater hoses at the heater core are actually hot. If they are ,you know you have good flow. That means you either have a blend door or actuator problem. Does the sound of the air change as you move the control from hot to cold? If you loose the vacuum supply the system will default to hot, so you might remove the vacuum supply hose from the intake to see if your heat is restored. If your heater hoses are still not hot, you might have a bad water pump. Had a FORD on Wed. that I replaced the heater core on and the hoses were still staying cold. The water pump impeller was so rusted away that it couldn't flow to the core, but yet the car didn't overheat. Amazing!

Answer 3

try replacing the tempeture sending unit. If it is not overheating it is probably not the water pump