GM 5.7L Engine Coolant Leak

If you suspect your Mercruiser 5.7-liter engine, based on the GM 350 block, sprung a cooling system leak, a pressure test, some visual inspection and sharp hearing may lead you right to it. If no leakage is found, the engine is leaking internally -- possibly from loose bolts on the cylinder heads or the intake manifold. It may come from a damaged head or manifold gasket, or even a cracked or porous engine assembly.

Things You'll Need

  • Pressure cap gasket, or replacement pressure cap
  • Cooling system pressure tester
  • Shop rag
  • Coolant
  • Hoses
  • Gaskets
  • Circulating pump seal
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Instructions

    • 1

      Remove the pressure cap from the heat exchanger. Wash the cap with clean, plain water to remove debris or deposits from sealing surfaces. Inspect the cap’s gasket and rubber seal on the cap for tears, cuts, cracks or other damage or deterioration. Replace the gasket if it is damaged. Replace the entire cap if the rubber seal is damaged. Inspect the locking tabs on the cap. Replace the cap if any are bent or cracked.

    • 2

      Test the pressure cap using a cooling system pressure tester, to ensure it holds its rated pressure for 30 seconds without dropping below 11 psi, and it relieves pressure at 16 psi.

    • 3

      Clean inside of the heat exchanger’s filler neck to remove any deposits or debris, using clean water and a shop rag. Examine the lower inside sealing surface of the filler neck for damage. Inspect the locking cams on the filler neck to ensure they are not bent or damaged.

    • 4

      Add coolant to the fresh water section of the coolant reservoir so the coolant level is 1 inch below the top of the filler neck. Attach an automotive-type cooling system pressure tester to the filler neck and pressurize the closed cooling section to 20 psi.

    • 5

      Observe the pressure gauge reading for two minutes. If the pressure drops, inspect the hoses, gaskets drain plugs, petcocks, core plugs and the circulating pump seal for leakage, while maintaining a pressure of 20 psi on the closed cooling system section. While doing so, listen for bubbling or hissing.

    • 6

      Start the engine. Pressurize the cooling system to 20 psi and observe the pressure gauge on the tester. If the needle in the gauge vibrates, compression or combustion is leaking into the cooling section from a leak in the combustion chamber.

    • 7

      Remove spark plug wires one at a time while watching the pressure gauge; the vibration decreases or stops when the plug wire is removed from a cylinder with a leak.

    • 8

      Remove the spark plugs one at a time from the cylinders for further confirmation of a leak. Examine the spark plug tip for the presence of coolant. A spark plug that’s perfectly clean or has a milky appearance on its surface is a sign of a leak in that cylinder. Stop the engine.

    • 9

      Replace damaged or worn hoses and gaskets. Close the cooling drain petcocks and replace missing or damaged core plugs. If necessary, replace the seal on the circulating pump.