Also known as the overflow bottle or reservoir, the radiator overflow tank of the Jeep CJ7 has the dual purpose of storing high-temperature coolant and to allow you to refill the radiator without disrupting its internal pressure or getting yourself burned in the process. So when it starts to malfunction, you can expect to have a serious cooling system problem in your hands. The following are some common Jeep CJ7 radiator overflow tank problems and how to troubleshoot them.
If you notice that the coolant inside the radiator overflow tank is boiling, it is an indication that one of the radiator's cooling fans or the devices connected to it are malfunctioning and that the radiator is beginning to overheat. First, check if the fuses and relays of the cooling fans are in good condition. If they are, run the engine for around 15 minutes or until it reaches the normal operating temperature and check if the fans have turned on. If the fans are running, the problem may be with a bad thermostat, a clogged radiator, or a bad water pump. However, if the cooling fan stays still, the fan or its temperature sending sensor may be malfunctioning and needs replacing.
There have also been cases of the overflow tank getting filled up with hot coolant and stays there even when the engine cools down. If this happens in your CJ7, check the radiator cap and the hoses for punctures and other types of damage. Damaged caps and hoses can allow air to get sucked in to the radiator and prevent the coolant from the tank from flowing back.
If you find that the radiator overflow tank of your CJ7 is often dry and you're filling it up more often than usual, there might be a puncture in the tank, coolant hoses and the radiator and the coolant is leaking out of it. Check the tank for any signs of damage; if the tank appears to be in good condition, inspect the hoses and radiator for leaks. We also recommend getting the radiator flushed as well, as there may be a clog in the radiator or in one of the hoses that prevent the coolant from the radiator to circulate back into the tank.