Heat then no heat

[Rblaney21]

My 99 expy has a new front heater core,new tstat, and has been flushed several times but the heat comes and goes. Has good heat then after driving 20 minuits the heat goes away. I need help bad. This thing has drive me nuts. The blind door is new also. The heater hoses are both hot to begin with but when the heat goes away the outlet doesn't seem near as warm as it should be. Could it be a air pocket?

 

[stamp11127]

Welcome to the site, the problem may be caused by air in the system or the core is full of trash. Even though it has been flushed the passages are small and clog easily.
To bleed the system I usually park on a steep incline so that the surge tank is the highest part of the system. Fill in the normal manner, run engine up to temp. Squeeze the upper hose as necessary to help move any trapped air.

 

[Rblaney21]

I will try the air bleed first. Is it anyway it could be the water pump? Or the rear heat causing it. The rear doesn't work at all even after flushing. It needs a core

 

[Rblaney21]

Didn't work. I think it maybe the water pump. It doesn't have sysymptons of head gasket. I have had so much trouble out of this thing but have too much money invested to sale

 

[David Kimble]

sounds like blend door. try making a hard right or left and see if you notice it changing from hot to cold. Noticed this before I knew about the blend door issue. If the temp gauge is up where it belongs that is. I have same year truck.

 

[Rblaney21]

It been fixed. I reversed the lines going into heater core and it works great. It has a new blend door anyway they replaced it when replacing heater coil. Thanks