Engine running due to stall converter???

The water pump doesn't "Pull" water. The vanes on the water pump cause a drop in pressure which causes the water to rush to the reduce atmospheric pressure. But it can only do this if there is a void for the water to go to. So until the thermostat opens, there is no void or drop in pressure to create flow. It is a misconception that water can be sucked by a water pump, water has no surface tension, you can't pick up water.