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Recurring Coolant Leakage

I have a 1992 Buick Century with 195K. Last November, I noticed that coolant was leaking from the radiator drain plug. I replaced the plug and its rubber gasket. I also replaced the radiator cap in case the old cap was allowing too much pressure build-up. The old cap was rated at 15 psi, but the new cap is rated at 16 psi, which is typical. Replacing these parts corrected the problem immediately. Three months and 2200 miles later, the same thing is happening. It only leaks after the car gets warm with pressure build-up in the cooling system. Would a leak stopper additive stop the problem? Should I try to re-tighten the plug? Is the pressure rating of the cap critical?

Do not use “stop leak” products.

Check the tightness of the drain plug and the condition of the gasket. Consider a new drain plug.

Radiator cap pressure is critical. Use ONLY the correct cap for your car.

You might want to have the cooling system pressure tested, and also tested for signs of combustion gasses, which would indicate a leaking head gasket.