Originally Posted by sbowen574
Yes both the hoses get hot. Is there anyway to get to the core without removing the dash? I changed the heater core to one that was new to me and I don't want to spend another 7 hours doing it especially since its now 20° out
If the heater core is flowing well, and everything else is good, it should be working. However since it is not... Perhaps there is some other reason coolant is not flowing TO THE CORE.
If that were the case the heater core would slowly heat up just from being in proximity to hot fluid, so you'd get a bit of heat for 20 - 30 seconds as you said, then it would cool off.
I had this same sort of issue with a car before. Even replaced the thermostat. There was a slow leak in the coolant system. So there would usually be enough coolant to cool the engine, but not enough to reach the core. The engineers usually design the coolant system to put priority to cooling the engine, then if there's only enough coolant for the engine you won't get heat.
Once I topped it off, the heat started working again. I know you said that the fluid was fine, but I would double check. :D