No heat. Had it in to a mechanic, replaced heat door motor. Still does not work. Hoses are working, but still is blowing cold air. Mechanic stated that heater core is working and the thermostate. The mechamic still didn't know what was wrong could tou help.

the car electronic stuff just get a messy little bit...contact the mechanic

Is your air locked slacken off top hose and make sure your getting a good flow of water no air

The blend door motors have to be clocked correctly.

Explain how you had a mechanic work on the problem
& not resolve

If the hose from the water pump is hot & the return from
the heater core is not ,then there is a control valve somewhere

Some mechanic that is

We don't have a data base to look up that ,we are at home like
you,social web site

May have to 'flush' out the system, especially if the heater core has 2 hoses at the 'firewall' that are not getting hot when the engine is hot.

If the hoses are getting hot but it's cold inside the cabin, then it's a bad blend door actuator problem.

Good luck on this repair.

first feel the heater core hoses see if both are hot if so then your problem is in the blend air door must be stuck on cold. is your blower motor blowing cold air if not your blower motor might be the problem. the blend air door is down by the blower motor.

the vent door on your heating and cooling system probably has a broken vent door which is preventing the cooold air from entering your vehicle its either the control switch for the windows or the window relay for the window motors

blend motors

Its more than likely the blend door not moving to the heat position. I'm not familiar with the location of the blend door in the 1999 GMC jimmy, since I'm not a mechanic and don't have a manual on that vehicle. If you can get to the blend door housing you might be able to move it physically.

I know this problem well, it is caused by a failed air temperature control blend door actuator motor or a stuck bleed door in the heating and ac duct, to replace this part requires you remove the dash assembly, there is no other access provided, this not for the home mechanic, it is very time consuming nd requires a good knowledge of repair work to complete the job, a factory repair manual is a must, not Haynes or Chilton, they will not do the job, not detailed enough.