Chocolate Milkshakes: Contaminants In Your Boat’s Oil

If you do your own engine maintenance, you might have seen oil or lubricant that just didn’t look right. Ever wonder what that meant?

Contaminants in engine and drive oil are big factors contributing to failures. Engine oil is the lifeblood of your engine. It lubricates the cranktrain, pistons and cylinders, and valve train. When it breaks down or becomes contaminated, it loses its viscosity (ability to flow easily) as well as its ability to protect against metal-to-metal contact.


Fuel gets into the oil when the engine is running rich. It’s easier to smell than see; the fuel smell can be powerful, but color-wise, it just turns the oil a darker. The fix is to tune the engine properly—correct timing, carburetor tuning, new distributor cap, plug wires and spark plugs. Of course, drain and refill the oil as part of the tune up.


Dirt gets in via the flame arrestor (air cleaner) atop the carburetor. It usually comes from poorly adjusted alternator belts, which cause dust to permeate the engine compartment where it gets sucked through the flame arrestor. Running without the flame arrestor (a big no-no with marine engines) makes this worse. Keeping the belt pulleys aligned and proper belt tension will reduce dust in the engine intake. Engines that have been submerged usually have rust on the pulleys, which also cuts into belt life and causes belt dust.

Sound-deadening foam can also deteriorate over time and find its way into the engine via the intake. A visual inspection will keep this from becoming an issue.


This is an easy one to spot—the oil will appear coffee-colored like a milkshake, and it will be almost frothy in an engine that’s just been run hard. This can come from a number of causes, all common. A sticky exhaust valve will suck water back through the exhaust manifold. This leads right back to improper tune and maintenance, as a sticky valve is usually the result of the engine running too rich.

Light water contamination can be caused by small leaks, from loose hoseclamps on water hoses and fittings. Check carefully for water leaks with the engine running. Another cause is “sweating.” In humid climates, the engine will sweat inside the engine compartment, creating moisture to find its way inside the engine.

A cracked block, exhaust or intake manifold will also allow water to seep into the oil. This usually comes from improper winterization in cold climates. To check, run the engine with clean oil, then check the level and color. If the engine appears to be “making oil” (the oil level rises) and the oil appears milky, then water is getting in. The block and manifolds can be visually inspected and pressure checked for cracks, but sometimes these can be very difficult to pinpoint.

Overheating the engine (usually from a bad water pump or blocked cooling passages) can also cause these water leaks, although it usually takes severe high temperatures to cause cracks in heads and blocks.

A submerged engine is not uncommon; water enters the engine and contaminates the oil. An oil/mineral spirits mixture is used to break down the water; if the engine takes 5 quarts, the mix is 4 quarts oil to 1 quart mineral spirits. Sometimes this flushing must be done a few times in order to completely clean the engine.


Marine oils with rust inhibitors are a better choice for a boat engine than automotive oils. These inhibitors protect the internals from rust due to water ingestion and sweating.


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