Q: How does air get trapped in coolant system

It is the normal operation of the coolant in your cooling system to expand when it gets hot. The purpose of the overflow tank is simply that. The radiator cap is designed to allow coolant to pass into the overflow as pressure increases in your cooling system. It then allows coolant to be drawn back into the radiator from the expansion tank (overflow) when the system cools. In addition, the pressure increase in the cooling system is what keeps the coolant from boiling. All of this is the normal operation of a cooling system.

First make sure the cooling system is completely full. This should be done with the car cold and by filling through the radiator. If it is low and it has a leak, this needs to be fixed first.

If the expansion tank is overflowing, then yes there is most likely a problem. Overfilling can create this as well, but it sounds as though your car is actually overheating. When a motor overheats, the place to begin is to determine if the thermostat is opening properly. This is easily done by feeling the upper radiator hose. This is the hose that comes from the thermostat housing. If it is hot to the touch, the stat is open and in most cases, the motor will not overheat because of the stat. The stat could be opening too late, in which case you will need an expensive infrared temperature gun to determine at what temperature it is opening.

After it is determined the thermostat is working properly, you will want to check to see if the radiator is clogged externally or internally. If it is clogged externally there will be debris lodged in the fins. Testing for an internal clog is a little more involved. You first need the thermostat to be open, go for a quick drive around the block, come to a stop, turn the motor off, and feel the radiator with your hand. It should be hot at the top and gradually getting cooler toward the bottom. You are looking for cold spots. If there are cold spots, you have a clogged radiator that is best replaced.

If your radiator is good, you will need to check to see if the water pump is pumping. This is best done by removing a coolant hose, usually a heater hose and starting the car with the hose disconnected. It should pump coolant out of it quickly. It is important to choose the hose that will flow plenty of coolant. If the first hose you choose doesn’t move much coolant, try another, but do not use either radiator hose.

The last possibility is a blown head gasket. If this is the case, air bubbles will continually be created in the cooling system. With a head gasket leak, air is forced into the cooling system from the pressure in the combustion cylinder. Air bubbles can also be created from leaks and if a recently worked on system wasn’t properly filled. Determining a head gasket failure is best done by an experienced technician as there are many nuances that only experience will make apparent to you. For this reason I recommend booking an appointment with one of our technicians to help you figure things out. Car is overheating inspection

Good luck!