Last year my 1993 F-150 5.8L V8 Engine truck would not start. It was losing coolant like crazy because the water pump seal was bad and so was that little seal at the weephole. So, I replaced the water pump. It still drank a ton of coolant but not as much as it did. Now, I can fill the radiator and the reservoir with coolant and two days later the reservoir is bone dry. The radiator has plenty of antifreeze. So, I fill it again. Two days later, empty. No issues with the gauge on the truck. The truck is at a good temp. Not too hot.

Is it the head gasket? The thermostat somehow?

I just did a leak treatment to see if that helped. No seen radiator leaks and not much antifreeze on the ground. The oil is not milky. It is just a little dirty.

 

Could be any one of many things. Just has to divide-and-conquer through the list of possibilities until found, I likes to start with the easiest stuff to check first.

One indicator of a head gasket is excessive white vapor out the exhaust. Another, in more extreme cases, is seeing bubbles in the coolant with the radiator cap off.

Perhaps inspect the intake manifold area for any coolant puddling suggesting a hose or loose clamp problem somewhere or maybe thermostat housing gasket.

May drop the glovebox, open up the heater core access panel, and inspect the core and surrounding internal area for any leakage.

 

I would pressure test the cooling system to see if it holds pressure and for leaks. If the head gasket were bad, you would either see coolant in the oil, a lot of white smelly smoke out the exhaust or coolant would be pushed out of the system and into the coolant bottle causing it to fill or overflow.

Make sure all hose connections are tight and there is not coolant coming out of your new water pump's weep hole. Also make sure you do not have a freeze plug leak.