Car Running Hot?...Burp It !
Any time a cooling system is opened up for service, air gets in. Depending on how much air enters, it can cause problems for your cooling system. To cool efficiently, the coolant liquid must come in direct contact with hot engine surfaces to absorb heat as it moves through the engine, and then it has to come in direct contact with the radiator surfaces as it passes through the radiator to dissipate that heat. This process repeated over and over keeps everything at proper temperature.
If air is introduced to the cooling system, it can get trapped and form a bubble in the liquid. This prevents the continual flow of liquid, and can create hot spots in the cooling system, depending on where the air is trapped.
Symptoms are an engine that runs hotter than normal, a temperature gauge that indicates hotter temperatures during high speed (highway) operation, inconsistent heat coming from your heater, and overflowing when the engine reaches normal operating temperature or after a hot shut down. The reason for all these symptoms is simple: the air can't remove or dissipate heat the way liquid can, and it prevents the coolant from flowing properly through the system. Coolant also expands as it heats up, which is the reason it overflows. The trapped air takes up space in the cooling system normally occupied by coolant, so there's no place else for it to go.
Fortunately, this is an easy problem to remedy. Most mechanics will do this after working on your cooling system, but sometimes it takes more than one attempt to remove all the air, especially after a major service operation like radiator or heater core replacement, although simple tasks like installing a new thermostat or hose can also trap air in the system.
In essence, you want to "burp" your cooling system to remove the trapped air. Start with a cold engine. Remove the radiator cap and fill it to the recommended level with a 50/50 mixture of antifreeze and water. Make sure any auxiliary tanks are also at the proper cold fill level. With the radiator cap off, start the engine. Make sure your heater is set to maximum defrost. This is important because most cars, especially those with automatic temperature control heating and air conditioning; allow coolant to flow through the heater core while warming up, to help clear the windshield. If controls are set for heat, the system may keep the heater core closed off until the engine warms up. You don't want that to happen in this case. You want 100% circulation of the coolant. Let the engine operate long enough to warm up enough to reduce the idle speed to a normal idle.
Let it idle until it's at normal operating temperature, keeping an eye on the temperature gauge or light. Do not let your car overheat! Overheating can cause additional problems, such as blowing your head gasket, even warping your carburetor. Once the thermostat begins to open, it's also very important to not rev the engine, even slightly, as this will force coolant out of the open radiator.
You might notice some bubbling of coolant out of the open radiator during warm up. This bubbling is normal if you have air trapped in your system, and is caused by the air escaping as it reaches the area of the open radiator cap. Unless the bubbling is excessive, continue to let the engine run at idle for a few minutes once it reaches normal operating temperature, then shut it down. Allow the engine and cooling system to cool off, preferably overnight. Then check your coolant level again. If it dropped, that means you've displaced some of the air in your system. Refill to proper levels, and repeat the process. Do this until the level doesn't drop any longer, then replace your radiator cap and check your coolant levels at least monthly, or weekly during hot weather.
If your older car still has its original radiator, you might consider replacing it. Radiators don't cool as well when they're older; even in well cared for cars. Scale and rust build up inside the radiator, and can prohibit proper circulation, or even clog it up completely.
Be good to your old car, and it will be good to you.