Blend door 1999 jimmy

This is likely caused by a failed or jammed electric air temerature control blend door actuator and or a warped and jammed blend door.

vent door not closing inside or control panel

Because the Main Gear in the Blend Motor has cracked or teeth have chipped and is now not keeping the heat/cold door to the Hot position. 3 fixes: Buy a new Blend motor, takes about 5mins to replace. Buy a new Blend Motor Gear, will take about 30mins to fix. Repair current gear with Epoxy, takes about 30mins and 12hrs for the glue to set up strong enough to use it.

Your blend door may be stuck,or the motor for it may be bad.

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.

This sounds like possibly a broken blend door on the heater box assembly or possibly a vacuum problem if the blend door is controlled by vacuum.

the system has what is called an electric temperature control blend door motor, this is the problem and must be replaced.

I have the Heater/blend door to replace the old one in my 98 gmc jimmy. I cant get the part to slide on the shaft. Any ideas?

No, you just need to fill freon gas.