I have a 91 S10 2.5 motor I have flushed it
completely and replaced the thermostat why is it
Did you purge air out of the system?
Many things can cause an overheating problem. The most common is loosing antifreeze while in operation. A weak radiator cap not holding correct pressure or reservoir cap. bad clutch fan or electric fan not working properly or at all. Other leaks in the cooling system such as bypass hose, heater core hose or unit, oil cooler hose and so on. Replace the radiator cap and check for coolant leaking out at operating temp as well as fan operation. check exhaust for smell of coolant when started cold, check oil fill cap for milky residue or excessive moisture. Pray it isn't a blown head gasket from a past overheating situation.
Yes F_O_R is correct on this one this time. You must permit the cooling system to purge as you fill the radiator don't just fill it and cap it. Give the antifreeze time to recede in the radiator as you fill it. Wait a few minutes and fill as it recedes. When you think it is no longer receding wait about 20 minutes to be sure and install the cap . Run to operating temperature, let it cool a few hours and check it is full.