More of this Feature • Part 1: Cooling System • Part 2: Gasket Sealants, Battery Chargers
Related Resources • Maintenance Q&A Index

The freshwater section of the closed cooling system on my 1997 3.0L stern drive requires frequent topping off. What could be wrong, and is there a troubleshooting method you can recommend? A.L. via e-mail
Frequent topping off usually indicates a leak. While large leaks are fairly easy to locate, small ones are not because hot coolant evaporates as fast as it leaks out, preventing formation of telltale rusty or grayish-white stains. A pressure-test of the freshwater section can pinpoint the source. This procedure is similar to that used on automobile cooling systems and in fact requires the same type of pressure tester. Here's how to do it.

Remove the pressure (fill) cap from the heat exchanger or expansion tank and wash it with clean water to remove debris and deposits from its sealing surfaces. Check its gasket (if so equipped) and rubber seal for cuts, cracks, tears, or deterioration. Also ensure the cap's locking tabs are not damaged or bent. If any of these conditions exists, replace the cap, and keep a spare onboard.

If the cap appears fine, immerse in a bucket of water and attach a cooling-system pressure tester using the proper adapter supplied with the tool. (You can purchase a tester at any auto parts store.) Pump up the pressure to 14 psi. If the cap fails to hold the pressure at 11 psi or higher for 30 seconds, replace it.

Next inspect the filler neck seat and sealing surface for nicks, dents, or distortion. Wipe the sealing surface with a clean cloth. If you notice corrosion or embedded grit, remove it with an emery cloth. Also make sure the locking tabs are not damaged.

Now check the coolant level; it should be within an inch of the top of the filler neck. Connect the pressure tester to the filler neck and pump it to 17 psi. If the pressure does not hold constant for at least two minutes, you have a leak in the system. Check all hoses, gaskets, drain plugs, drain valves, core plugs, and other potential leak points for signs of weeping, and listen for hissing or bubbling while the system is under pressure.

If you don't find leaks, disconnect the raw-water outlet hose from the heat exchanger and again pressurize the system to 17 psi while paying attention to the outlet connection on the heat exchanger. If you see water flowing from the connection or bubbles in the water or hear bubbling or hissing, there is probably a leak between the freshwater and seawater sections within the heat exchanger.

If you still don't find leaks and the coolant level continues to require frequent topping off, you probably have an internal leak. Possible causes: a faulty gasket in the cylinder head, intake manifold, or exhaust elbow; leakage around the distributor bolts; or a cracked or porous head, block, or manifold.

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