[2000izusu]

water pumps can leak when off but not when running, and they are cheap to replace! i just replaced one on a 97 xj for 25 dollars and 2 hours of my time.

i would replace the thermostat just because you are already in that area working.

but in reality my next step would be a pressure test. you can rent the pressure tester at autozone or other big box part stores! pump it up to 16 pounds and see if it looses any pressure, and when it does then go find the leak! most likely it will be your water pump.

also i would consider a full flush and maybe even a new radiator!

also the closed system does not apply to you. you have the better open system which means your radiator cap is on the radiator and not on a plastic bottle on the passenger side fire wall! mike