After an engine swap, I'm no longer getting coolant to the heater core of my '80 pickup. The new engine was a lower miles 20R from a '79, though not necessarily the original engine on the '79. Otherwise the cooling system works fine and I've verified the following:

1. The heater inlet valve opens.

2. The heater core is not blocked.

3. The thermostat opens when it's supposed to and coolant starts going to the radiator.

4. The heater outlet hose empties to the bottom of the radiator with no obstruction.

5. The inlet hose goes from a pipe from the water pump through the timing chain cover to the inlet of the heater core. Blow on it and there's a clear path through the water pump to the radiator But here's the funny thing: this is true even with the engine cranking. Whether the engine's cranking or running, comes out of the hose to the heater core, explaining why there's not heat.

So the question is, how can the water pump be working and coolant through the engine and radiator but not pumping any water to the heater core? As I say, there are no obstructions to be found anywhere. Is it possible that the temperature differences can create enough circulation of their own to make up for a poorly functioning water pump?

Any help would be much appreciated.