Core plugs may go by many names, from freeze plugs to expansion plugs and casting plugs, but they are not a commonly-discussed part of an engine. Since they are susceptible to rust and wear, they should be examined for signs of damage on occasion. These plugs are located on the engine block or cylinder heads of most car engines, and their job is to both fill core holes and prevent coolant leakage so the engine does not overheat and break down.
It might seem like the holes are unnecessary, since they're just plugged up, but the method of manufacturing engine blocks requires them to have entry holes. Most blocks are formed by sandblasting the basic shapes and passageways within the block. Once this is done, the core holes allow the sand to be removed, but then the holes need to be sealed.
Years ago, it was common practice to replace coolant with regular water for the warmer months, but many drivers would forget to switch back to the chemical liquid when temperatures drop. Once it got cold enough, the water would freeze and expand. This often caused the core plugs to pop out, thus earning them the name of freeze plugs.
These parts are not designed to break down, and may never need to be replaced or repaired, but the metal used to manufacture them may be more likely to rust than the material used for the engine block itself. Chemical compounds in antifreeze may weaken the plugs and cause them to rust, which just gives you another reason to keep an eye on the coolant. The car's cooling system should be flushed periodically to protect against unnecessary wear on the plugs, which will help prevent problems with the cooling system as well.
Leaking coolant, whether from the core plugs, hoses or other parts, will cause a decline in engine performance and may lead to an overheated engine. If you suspect there is an antifreeze leak, you should first rule out the possibility of the issue being in areas of the cooling system that are more likely to breakdown or be damaged. Inspect the hoses, antifreeze reservoir, radiator and radiator cap for signs of leakage. If none of these are to blame, the issue might lay in the core plugs.
Since there are multiple core plugs, it is best to replace all of them if one shows signs of rusting or wear. Chances are that if one is failing the others could begin to break down as well, so switching all of them at once will prevent future issues. Factory-installed plugs are typically made of steel, but replacement parts made of different materials such as brass or rubber may be available. Brass core plugs tend to be the most common replacement parts since the metal can withstand the harsh chemical compounds of antifreeze and is less susceptible to rust.
Visually inspecting the core plugs may help you determine whether they are damaged or wearing out. Their location can differ from vehicle to vehicle, but the owner's manual will point you in the right direction. In most cases, the freeze plugs will be located on the lower side of the engine block, so you may need a jack to hoist the vehicle up and get underneath the car to inspect them. Check the plugs for signs of rust or leaking coolant around the edges. If you find any evidence of these issues, the plugs will need to be replaced.