I  More of this Feature • Part 1: Leaky Oil Pans, impeller pitting, and more • Part 2: Total Base Number, and more
Related Resources • Maintenance Q&A Index

I have a 3.0L, four-cylinder in-line gasoline stern drive and do most of my own maintenance. Lately I’ve noticed some oil leakage around the oil pan. What could be causing this? A.V., via e-mail
Oil leakage around the oil pan can be caused by several factors.

One is excessive torquing of the oil pan screws when a new gasket is installed, causing the gasket to be deformed and squeezed out. While the typical one-piece silicone rubber gasket includes metal spacers around each of the screws to prevent this from happening, over-tightening is still possible. You’ll have to replace the gasket. Refer to your engine manual’s specs on the correct torque for this application.

Two other possibilities are a defective gasket and one with insufficient contact or "crush" in certain areas to make a good seal. This is commonly caused by residue from an old gasket that was not removed during installation. Yet another cause could be deformation of the pan’s flange. If this is the case, you can usually straighten out the flange by tapping it lightly with a hammer. If not, a new pan will be needed.

Finally, the leak may be coming not from the pan but from either the front or rear main-bearing seals. The seals could be worn or defective, or if you’ve had any work done on the crankshaft, the problem could be mismatched surfaces. If this is the case, you’ll either need a new crankshaft or have to have yours machined.

During a routine maintenance inspection of my cooling system’s freshwater pump, I found the impeller blades to be pitted. What could have caused this? T.L., via e-mail
Besides the obvious culprits–sediment, rust, and similar contaminants causing abrasion–air in the cooling system can cause a condition known as coolant foaming that will promote pitting of water pump impellers. The situation is worse if the air is caused by exhaust gases.

Leaks are the usual cause, so regular inspection to ensure hose and clamp integrity, proper seating of all seals, and proper pressure cap and thermostat operation is important. Flush your system regularly, following the engine manufacturer’s directions, and always refill with approved coolant. You may also want to check your engine owner’s manual for recommended coolant additives, many of which contain defoaming agents.

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