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Ok I'm back 1 last time. I was wrong on something's I said. After looking over the w8 motor and 1 of my cars again I realized I have been mistaken. These cars feed the engine cool coolant via upper radiator hose and return hot via lower. This being the case the water pump is actually sucking coolant thru the heater via the left line(looking from front). I was looking at this all wrong. I was believing this was pushing not pulling. It's pulling. Now with that being said this is actually simple. It has 1 valve on the outlet side(look at pictures). There is absolutely nothing on the inlet side(right) that can restrict flow or alter flow. So after thinking about this more I have come up with 2 possibilities. 1 is a blockage. Which could be the thermostatic valve(26 I'm point at it in pic which i believe it adjusts to vary in cab temp of shuts heater core down when AC on) on the outlet is closed hence not allowing coolant to pass or heater core clogged preventing it. Those are the only blockages possible that will effect the heater. The aux pump and all that other stuff means nothing once you see the system and think about it. That valve IS the only valve that matters in terms of heat. Its simply using the wp to suck coolant thru the core from what is essentially the lower radiator hose But then it hit me with another idea..... You were talking about air boiling up in the system. Cap on and drive it holds temp fine. This leads me to another question.... Does your cooling system pressurized once everything is warmed up? Because this is were my #2 other possible cause will come in. Let's say you had a crack or pin hole somewhere in the core, hose, pipe, line ECT. Being the wp is sucking coolant not pushing it it would pull air in via any tiny hole or crack(with res cap off and just keep bubbling) before pulling coolant thru the core. Once cap is on the air would have no where to vent so you would most likely then have a leak as the system tried to pressurized. Anyhoooo after look after thinking this thru more I'm thinking 26. I'd almost bet my rent on it. It's in the left hose between firewalls next to battery. Remove it and see if it's closed. Bet it is:) I'm serious no other valves, pumps or anything matter in terms of heater function. Save your time and money. 26 26 26
 
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