I have a 2000 150 with the 330 v8, my question is that i seem to be getting a air bubble in the heater core, which causes my truck to leak coolant from the reservoir as it essentally has more coolant that it needs ones. I can some times get coolant into the heater core by opening the fill cap to remove the extra air in the system. My question is about the blend door. what exactly does this do. I imagine that if it is not the control valve I will just need to replace the heater core under the assumption that there is some sort of blockage. I have searched the message boards and know about the heater hose test to determine if your core is blocked or not, but that does not explain how i have have heat some times and not others. For instance I had heat halfway home for lunch then it got cold. After lunch i tired the heater hose test and found one warm, one cold hose. I wanted to compare this to the main coolant temp hose and squeezed that, and after a few squeezes i heard coolant bubbling flowing in to the heater core. I now have heat again (for a while)
Any insight would be appreicated
Thanks

 

Um, bubbling into the reservoir? Any water in the oil? These motors in pickups and SUVs usually aren't real bad about getting air pockets like they are in a car, but if you're getting bubbles into the reservoir, you may have combustion gases leaking into the cooling system. I might be jumping to conclusions, but I've seen this before on 4.6 liters in Town Cars/Crown Vics/Grand Marquis. Usually the combustion gases blow the heater core up though, weakest link in the cooling system.

That said, search in this forum, I put together a post with some pictures and stuff of the heater core and blend door.

 

I should clarify that when I squeezed the radatior hose the "bubbling" was just ment to describe the fact that I could hear the coolant flowing into the heater core.