Below are some common causes a vehicle may immediately overheat after a radiator replacement:
1.) Air / improper fill of cooling system
If you have air bubbles in your system, they will become trapped inside the radiator and create spike or hot spots which will cause your engine to overheat. It is important to "burp" your system as well as fill the unit with the proper amount of coolant to prevent this type of overheating condition.
2.) Defective Thermostat
The thermostat is an important component in your cooling system. It's main function is to open and allow hot engine coolant to flow through your radiator and cool down before returning to the engine. If the thermostat does not open or close correctly, the engine may run at maximum temperatures regardless of how hot or cold it may be outside. it is suggested to always either replace or check your thermostat when replacing a radiator. Some at home methods of checking would be to remove the thermostat, place in warm water and check if it is properly opening.
3.) Failing Water Pump
The water pump is another very important component in your cooling system that regulates the flow of coolant into the radiator. if failing, this also will cause the vehicle to run hot regardless of outside environment conditions.
4.) Blown Head Gasket
A blown head causes gas to get into the cooling system and will contribute to overheating as well as repeat radiator failures. To test for a blown head gasket, remove all air from your cooling system and monitor if it returns.
5.) Low Coolant
Not filling the system with enough cooling will contribute to immediate overheat. The radiator will only cool what coolant is available. Again burp your system and always add some extra fluid when the engine is on to make sure the vehicle has enough coolant. To see how much coolant your cooling system takes, check your vehicles owner manual.
6.) Blocked Passage Ways
Often older vehicles may encounter rust and debris in the coolant from aging engine components. If you find any debris when replacing a radiator, be certain to treat and flush your system. If debris passes into the radiator, it will often become stuck and block the passage of coolant through the radiator. This will often cause an overheat as well as may cause the radiator to rust out and fail quickly.
5.) Failed Radiator Hoses
Yes, even bad radiator hoses can cause your vehicle to run hot. if you find visible cracks or the radiator hoses are soft and mushy, it is imperative that you replace them when replacing the radiator. We also often suggest if re-using hoses to flush them out and clean prior to use to prevent debris from entering the radiator and new coolant.
6.) AIR FLOW !!!
The radiator can only do so much. It requires ample air to pass through the radiator core to properly assist in the cooling process. Check all existing fans to be certain they are properly functioning. If utilizing a vehicle for parade or idle, additional air flow sources may be required or suggested (such as a push fan in front the radiator to assist air into the vehicle on idle).
7.) Failing belts
Check all belts and replace any loose or worn belts. A worn or torn belt will affect a water pump from driving fluid into the radiator at proper speeds and cause overheating. Other worn belts may cause the vehicle to work harder to maintain speeds and function. Thus, causing an overheating situation.