I’ll restate my question. Does the expansion tank drain hose have a check valve on it so that coolant flows in one direction from the rad to the tank then to the engine?
I bypassed all the plastic connections on the heater core fittings since they were cracked. Now I overheat.

 

Hi all
2006 pathfinder. 4L v6
I have a question regarding the flow of coolant into the expansion tank.
The rad has a pressure cap that I assume opens under pressure and Copland flows into the expansion tank. It has its own pressure cap as well.
the question is about the hose at the bottom of the tank that flows back into the motor at the connectors against the firewall where all the heater connections are. If that hose is open to the motor, how does coolant only flow out of the tank. I would think that expansion happens more easily via that hose, with an hose fittings, than it does out of the top of the radiator. Since that hose is open, I would expect coolant to flow up it in to the tank.
Hope this makes sense.

The bottom hose of the expansion tank allows coolant to return to the engine as it cools. Coolant flows into the tank when pressure rises and back to the engine when pressure drops, so the hose being open doesn’t cause backflow. It just lets coolant move based on pressure differences.

 

Dorman 626-688 is the part number to stop this problem from ever happening again. That part number is with the HVAC there's also one without depending on your vehicle.

 

Are you referring to the hose highlighted below? I don't think the radiator cap works as a pressure release, just the reservoir cap. You may have air trapped in the system that you can't get out if you've blocked parts of the circuit to the reservoir. The procedure I've found for getting air out of the system is to fill the reservoir above the high mark, and getting the front end higher can help. However, if you've modified the system then that may not work.