Four-Stroke Outboard Oil Level Rising or Smelling Like Gas: What It Means and What to Do

A wrong reading causes a lot of false alarms. Yamaha's procedure, in short: make sure the outboard is level side to side and front to back, warm it to normal operating temperature (about 5 to 10 minutes in the water, or on flush muffs or in a test tank; not on the built-in flush port unless your manual allows running on it), shut it off and wait about 5 minutes. Pull the dipstick, wipe it, reinsert it fully, pull it again and read it. Your own manual's routine may differ a little, so follow it. Yamaha warns that both overfilling and underfilling can cause severe internal damage.

Wipe the dipstick on a clean white towel. Then judge it:

• Smells clearly of gasoline, or feels thin and watery: fuel dilution is likely. Keep going with this guide.
• Milky, light brown or foamy: water or coolant in the oil. That is a different, more serious problem: stop using the engine and call a dealer.
• Normal smell, level a little high: maybe it was overfilled at the last change, or the reading was taken with the engine tilted. Go to step 3.

Look at your log: how much oil went in at the last change, and did it match the manual's capacity with filter? Re-read the level using step 1 on another day. If the level was high right after an oil change and has not gone up since, it's an overfill. This is common when someone pours in the full capacity printed in the manual: Mercury explains that the listed capacity is approximate and covers the whole lubrication system, and some oil always stays in the engine after draining, so the full amount often won't fit. Fill to the dipstick, not to the number on the bottle. Remove the extra oil (most shops use a small pump through the dipstick tube) until it's within the marks.

Write down the engine hours and where the oil sits on the dipstick (for example "1/4 above the lower mark"). Check it the same way after every trip for a week or two. A level that keeps climbing with a fuel smell confirms dilution. Note how you used the boat each time: hours of trolling, water temperature, number of starts.

If the oil smells of fuel or is above the full mark from dilution, change the oil and filter now, using the oil specified in your manual (see the outboard oil change guide). Don't just drain off the excess: the oil that's left is still thinned by fuel. Note the hours.

Most dilution cases on healthy engines are fixed by how the engine is used:

• After a long idle or trolling session, run the engine at cruising speed for a good stretch, when it's safe and legal to do so, so it gets fully hot and can drive off the fuel.
• Avoid lots of short trips where the engine never warms up.
• On a kicker, run it at higher RPM once in a while instead of only at trolling speed, or run it warm before you put it away.
• On a carbureted engine, use only as much choke or priming as it needs to start, and close the choke as soon as it runs smoothly.

Many outboards use a fairly cool thermostat so they don't build up salt in the cooling passages. A Suzuki Marine USA service manager, quoted by Virginia's Department of Wildlife Resources, says Suzuki offers a higher-temperature thermostat for owners who run at low speed a lot (trolling or working crab pots) because it helps fight carbon buildup and oil dilution. Ask your dealer whether your engine has such an option before you change anything; a non-approved thermostat can cause overheating or warranty problems.

Also make sure the cooling system you already have is working as designed. Yamaha notes that salt deposits or small bits of debris can make the thermostat or pressure-relief (pop-off) valve stick open, so the engine runs too cool and never reaches its normal operating temperature. If your temperature gauge always reads low, or the engine takes ages to warm up, have them checked and cleaned before anything else.

Use the oil grade your manual lists for the temperatures you run in; some manuals give a lower-viscosity option for cold weather. Ask your dealer whether a shorter oil change interval makes sense if you troll a lot in cold water.

With the engine off and cold, remove the cowling and look at the fuel hoses, the fuel filter, the fuel pump and, on fuel-injected engines, the vapor separator tank (a small fuel reservoir on the side of the engine). Look for wet spots, fuel stains or a strong raw-fuel smell. Check the primer bulb and hose connections too. Any leak or strong smell: stop and have it fixed before running the engine.

If the level keeps rising even though you run the engine warm, a part may be letting fuel into the cylinders or crankcase. Depending on the engine, a technician will check things like:

• A leaking or stuck-open fuel injector.
• A float or needle valve in the vapor separator tank (or a carburetor) that sticks and floods the engine.
• A leaking diaphragm in a mechanical fuel pump that sends fuel into the crankcase.
• A fuel pressure regulator fault.
• Worn or stuck piston rings, checked with a compression or leak-down test (Mercury notes that dealers use a compression check to look for carbon buildup or ring problems).

These are dealer jobs: they involve opening the fuel system and, on EFI engines, diagnostic software.

On a brand-new or rebuilt engine, follow the break-in procedure in your owner's manual to the letter. Long periods at idle or a single steady speed during break-in are often discouraged. Check the oil level frequently during break-in, and report a rising level to your dealer while the engine is under warranty. BoatUS notes that the first oil and filter change, which flushes out the normal wear particles from break-in, usually comes somewhere between 20 and 50 hours; your manual gives the exact interval.

After the oil change and any repair, keep logging the dipstick for the next few trips. A stable level with no fuel smell means you've solved it. Yamaha's general schedule calls for an oil change every 100 hours or once a year on its four-strokes (confirm in your manual); if you troll a lot, plan to check the oil before every trip.