Coolant Flow direction thru the heater core DOES NOT matter.

 

In my experience it does. If you look at the design of the heater core and the factory layout, the flow is designed to go in the bottom of the core and out at the top portion (sorta middle outlet - crap design, I know). The direction of the flow enables the air to rise up and be flushed out of the heater core, if the hoses are reversed the air gets trapped because you are trying to push it down in the heater core and the air gets trapped in the upper end tank opposite the outlet pipes. It physically cannot go down due to buoyancy and gravity. If this is not clear let me know and I can provide more detail.

 

Look at older RRC’s vs a D2. It’s straight in/out vs the flip flop D2 setup. Coolant flows thru the core. I’ve had em mounted either way & zero difference. If you constantly keep getting air in the heater core you have other issues.

P.S. I went out & verified a D1 = 5/8 coolant lines to the heater core are straight in/out from the factory. D2 = 5/8 coolant lines to the heater core are flip flopped. Both D1/D2 share the same basic setup = coolant in/out makes absolutely zero difference. Once the system is bled air should not keep traveling to the core. RRC had the best setup with a tall stack to remove air pockets, D2 has a bleeder on the T while a D1 has nothing except for a bleeder on the radiator itself. Like I said if air keeps getting into the core you have air getting into the system somewhere.

 

No one has mentioned the OP probably has a plugged up heater core?