[chengny]

The engine is generating the same amount of heat it always did. Those BTU's are transferred through the cylinder walls/cylinder head passages to the coolant.

There are only two heat exchangers in the system that can remove BTU's from the coolant. One is the heater core and the other is the radiator. The original complaint was a lack of heat from the heater core so it would be safe to assume that the BTU's are not being removed by that heat exchanger.

That leaves the radiator - as the only place where the heat generated by the engine can be removed from the coolant. The radiator will only remove heat from the coolant system if there is hot coolant flowing through it.

The rate of coolant flow through the radiator is dependent on the degree of opening of the thermostat (or what leaks by the thermostat).

The proper procedure to confirm proper t-stat sealing is to pull the upper radiator hose from the radiator inlet tank and visually confirm a no-flow condition upon an initial cold start.

With the hose off the radiator inlet neck and pointing down towards the ground, there should be virtually no coolant coming out of it.

If there is any more flow than what can pass through the little vent hole in the t-stat disc, something is allowing it to exit the engine. Either the stat is not closed or there is leakage between the joint formed by the intake manifold, the t-stat flange and the water outlet.

If the idea of running the engine with the radiator inlet hose removed is an issue, try blocking the hose. Use a clamp and two blocks of wood to squeeze the hose shut and let the engine run. Observe whether the coolant temperature rises faster than it does with the hose not clamped closed.

It should not be necessary to block off air flow through the radiator to maintain proper operating temperature of the engine coolant. It will only cool what is available to it.