The freeze plug is leaking coolant or water.
Coolant expansion plugs can leak in two different ways, they either corrode through or the surface between the plug and the block/head surface develops a leak. In either case, however, the plugs cannot be re-used. The coolant expansion plugs must be replaced and installed with a non-hardening sealer. Even though they cost slightly more, brass replacement expansion plugs are recommended as they resist corrosion. You may want to enlist the help of a mechanic, such as one from YourMechanic, who has the tools and the training to install new coolant expansion plugs
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