Maybe you need a freon boost.
water pump
because the water pump is not working properly....
Assuming it did not overheat before you replaced thermostat and water pump you may have installed the thermostat backwards. It is also possible the cooling system wasn't refilled properly and was air bound.
Would need to know if car was overheating before waterpump and thermostat was replaced.
I had the same probl;em on my 1996 SL1. I found the bypass tube from the head to the reservoir was clogged and would not let water return to the reservoir. When I unclogged the line the overheating went away. I also had replaced the water pump and thermostat before without any luck. The clogged line was the culprit.
Year, make and model would help us help you.
Water pump not pumping or a pluged radiater
Perhaps plugged radiator - have it boiled out or replaced
If the thermostat is broken in the closed position the engine would overheat rapidly. If it was broken in the open position the engine cooling system would not reach normal operating temperature and the heater would produce only warm air. Take the thermostat out and put it in a pan of water on the stove. When the water gets to around 200 deg F, the thermostat should open. If it doesn't it needs replaced. Observe carefully that you have not installed the thermostat upsidedown.
I had the same problem with my 98 jimmy I repalaced the water pump have had heat since.
Of course a water heater thermostat would be defective as water heaters / hydronic boilers would use an Aqua Stat
If you have an old ISUZU Trooper, the radiator thermostat may be not working. Therefore, it will not let cool water from the radiator into the hot engine, even if the radiator is full. Have the thermostat checked and replaced, if necessary. Good Luck