Ask expert's help for my 94 Toyota Previa's coolant system problem.
The coolant can not flow and engine temperature is very high. However, when I drove up to hill, the coolant can work and the temperature is down to normal. I thought it is the thermostat problem. I drained the coolant out and only got about one gallon coolant, 2/3 gallon form radiator and 1/3 gallon from engine. The coolant tank is still full. Could an expert explain what this problem is?

Thanks