What make a car run hot?

Answer 1

The radiator is not radiating.

For whatever reason, the heat is not being removed from the engine. In a normally functioning water-cooled engine the water pump circulates cool water from the radiator to the engine where it travels through the water jacket picking up heat as it goes. The water exits the engine past a thermostatic valve that remains closed until it gets hot enough to open and the water goes to the radiator. Air blowing through the radiator cools the water and it cycles through again.

What can go wrong with that?

• You are out of water-more properly called "coolant" because it is mixed with ethylene glycol (antifreeze) to prevent it from freezing.
• The water pump fails. Rare, but the impeller can come off the shaft without any external symptoms. Usually, when the water pump fails the seal becomes compromised and it bleeds coolant all over the ground.
• The hoses are kinked or blocked. Blockage bad. Think "Stroke" or "Heart Attack".
• The engine's water jacket is lined with lime from running really hard water through the system for years. Not terribly likely.
• The engine is really, really low on oil. Oil transfers heat to the water jacket. You'll probably notice worse problems than overheating if you are really low on oil, though.
• The thermostat is stuck closed. Many new thermostats are designed to "fail safe", not all are. The thermostat may have been installed upside down. The thermostat will have a small caged ball valve that allows air to move through the system and eventually be ejected out the overflow hose. Worse, some engines don't have thermostats that use this type of valve. It is up to the grease monkey that changed your coolant to bleed the air out of the system. Sometimes they don't.
• Back to the radiator...I have pulled radiators that weighed 30 pounds when they should have only weighed 10. The difference was from lime scale and stop-leak. When you pull a radiator like that, you just walk away shaking your head. Buy a new one. There's no fixing it.
• The radiator may have no way for air to pass through it. That comes from plastic shopping bags or 'possum guts getting sucked against it and blocking the air passage. That's an easy fix. If its 'possum guts, that's a bonus--now you have supper. If you are from Wisconsin, look and see if you took out the cardboard that you had in front of the radiator so you'd get heat in the cabin last winter.
• There is a fan that pulls air througn the radiator when the car is not moving. The fan may be switched on by a thermostatic switch. If the switch is bad, the fan won't turn on and the car will overheat while you are waiting for your hamburger at the drive-through. The fan motor may also be bad.
• The radiator cap holds in pressure to prevent the coolant from boiling. If the radiator cap is bad, it will allow the coolant to boil out.