I have a 98 Grand Cherokee with a heater problem I have been chasing. No heat comes out of the heater. I have flushed the heater core, so I know that it is not blocked, also the radiator has been flushed and the radiator cap replaced. This is not a dual climate control system and its blend door is working. The problem seems to be a air bubble in the heater lines. The top radiator hose feels like it has very little water in it. Yesterday I installed a modified radiator cap and kept squeezing the upper radiator hose and filling the flush tee with water until I was sure that the lines were reasonably full. I started the engine and when it got up to temp, the heater was blowing 110 deg air. I drove the car and in less that 3 miles the heater temp dropped to 80 deg. I checked the hoses and the top radiator hose felt empty again and I noticed that the overflow tank level had increased. Thinking that I might have a bad head gasket forcing coolant out, I did a compression test on the engine ( 6 cyl ), the high reading was 170, the low was 150. This would seem to indicate that the head gasket is ok. The thermostat and the water pump were replaced about 10,000 miles ago and it does not overheat. I am running out of ideas.

Joe

 

The only thing I've got is this, make sure the hose from the thermostat housing goes to the lower inlet of the heater core. This will allow any air to escape out the top hose.

 

Thanks for your input. On this model I am not sure that it matters as the inlet and outlet are horizontally side by side.