Freeze plugs play an important role in automotive engines. Typically consisting of brass or aluminum plug-like pieces -- some of them are made of rubber as well -- they are designed to fill the otherwise open sand-casted holes in automotive engines. Most automotive engines are manufactured via sand-casting. Holes must be created in sand-casted engines so that the excess sand can be easily removed. Freeze plugs are simply placed into these holes to fill them after the casting process has been completed and the excess sand has been removed.
Why They are Known as 'Freeze Plugs'
The term "freeze plug" is a reference to the parts' ability to protect automotive engines from freezing. In the past, many automotive engines were cooled via water. They used a water-cooling system to transfer heat from inside of the engine to the radiator. During the winter, however, it wasn't uncommon for this water to freeze. As the water inside of an automotive engine began to freeze, it would expand. Freeze plugs allows for the safe expansion of this frozen water by popping out.
While most automotive engines are no longer cooled via water, freeze plugs are still necessary. They are designed to fill the holes in automotive engines that were created during the manufacturing process. Automakers still use sand casting to manufacture engines. And sand-casted engines still have holes that require filling. Without freeze plugs, these holes would remain open, in which case coolant or oil may leak out of them.
How to Diagnose Bad Freeze Plugs
How do you know if your vehicle's freeze plugs are bad exactly? Coolant leaks are common with bad freeze plugs. As previously mentioned, modern-day vehicles aren't cooled via water. Instead, they require coolant to remove heat and transfer it to the radiator. Coolant will flow through the internal passages of your vehicle's engine where it picks up heat before transferring it to the radiator. If your vehicle has one or more bad freeze plugs, some of the coolant may leak out of your vehicle's engine.
Even if you don't see a puddle of coolant underneath your vehicle, you may be able to smell it. Coolant leaks caused by bad freeze plugs will create a distinct odor. Coolant has a sweet odor. If you notice this sweet odor when inspecting the engine bay, you could have a coolant leak. And coolant leaks can be caused by bad freeze plugs.