What "making oil" actually is A four-stroke outboard keeps its engine oil in a sump at the bottom of the powerhead. A pump circulates it through the crankshaft, rods and cam. That oil is supposed to stay put and stay clean. When a motor "makes oil," fuel is working its way...
Quick answer: "Making oil" means raw fuel is getting past the piston rings into the crankcase, so the oil level climbs even though you haven't added any. It isn't a Mercury defect. It's what four-stroke outboards do when they run cold and slow for hours, which is exactly what a day of trolling is. The fix: change the oil as soon as the level rises, run the motor under load for about 20 minutes after long trolling days, and check the dipstick weekly in trolling season. If the level keeps climbing after a fresh change and proper hot runs, get it looked at.
What "making oil" actually is
A four-stroke outboard keeps its engine oil in a sump at the bottom of the powerhead. A pump circulates it through the crankshaft, rods and cam. That oil is supposed to stay put and stay clean.
When a motor "makes oil," fuel is working its way into that sump. The level rises on the dipstick, the oil thins out, and the dipstick smells like a gas can. Thinned oil protects bearings and cylinder walls less well, and those are the last places you want to cut corners.
The catch: the motor usually runs fine while it's happening. There's no alarm for fuel dilution. The dipstick is the alarm.
Why four-strokes do it
It comes down to temperature.
An outboard is cooled by the water it sits in. On Rice Lake in October that water can be around 10°C, and it keeps the intake manifold and cylinder walls cool too. Fuel hitting a cool intake doesn't fully vaporize. Some condenses on the manifold walls as liquid instead of arriving in the cylinder as a burnable mist.
Liquid fuel doesn't burn cleanly. It films the cylinder walls and gets carried past the rings into the crankcase, a little at a time. Multiply that by six hours at trolling speed and the dipstick tells the story.
| Factor |
Effect on fuel dilution |
| Water temperature |
Colder water, cooler engine, more condensation |
| Engine speed |
Hours at low RPM never make enough heat to cook the fuel off |
| Fuel delivery |
Modern EFI is less prone to it than older carbureted four-strokes |
Kickers are the worst offenders. They spend their whole working life at one low speed and rarely get hot enough to boil fuel out of the oil. A kicker that idles along all day and then gets tilted up at the dock is running the perfect fuel-dilution experiment.
EFI helps, but it isn't immunity. Modern Mercury FourStrokes meter their fuel far more precisely than the carbureted four-strokes of 20 years ago, so they're less susceptible. Hours of cold, low-RPM running still show up on the dipstick.
Mercury says it plainly in the owner's manual: change the engine oil more often when the engine is operated under adverse conditions, such as extended trolling. That one sentence is the whole story.
The thermostat connection
Your FourStroke has a thermostat that controls cooling-water flow so the engine reaches and holds operating temperature. A thermostat stuck open lets full water flow all the time, and the motor never properly warms up. That's the same cold-running condition that causes fuel dilution.
If your motor always seems to run cool, or it makes oil faster than your trolling hours explain, the thermostat is one of the first things a technician will check.
Getting overheat alarms instead of rising oil? That's a different problem. Our Mercury beeping codes guide and overheat alarm decoder cover that side.
How to check the oil properly
Most "my oil level looks weird" readings come from checking it wrong. Mercury's manual procedure:
- Tilt the motor to vertical so the powerhead is level.
- Check it cold, or at least an hour after shutdown, so the oil has drained back to the sump.
- Pull, wipe, re-insert, and read.
- Keep it in the operating range, not over the top mark. Overfilling a four-stroke is its own problem. Add oil in small amounts (roughly half a litre at a time) and re-check.
Do this weekly during trolling season. A rising trend between checks is your early warning. As a reference point, a Mercury 150 FourStroke holds about 6 litres with the filter. Capacities vary a lot by model, so check our Mercury outboard oil capacity chart for yours.
One caution: high and fuel-smelling is dilution. Milky oil means water is getting in, which is a different and more serious problem. That one needs a technician, not an oil change.
What actually fixes it
Do these in order:
- Change the oil and filter as soon as the level rises. Draining a little off to get back to the mark solves nothing. What's left is still diluted.
- Run it under load at higher RPM for about 20 minutes after long trolling days. Under load means actually pushing the boat, not revving it at the dock. That brings the engine up to real operating temperature and drives the fuel out of the oil.
- Change the oil again before winter storage. Old, diluted oil sitting against bearings all winter is how a small fall problem becomes a spring repair bill. It's standard in our winterization routine.
Change it, cook it, store it clean. That handles fuel dilution on the vast majority of motors.
How to prevent it next season
- Give the motor a hard run after trolling. If your Mercury has SmartCraft Troll Control, it holds a set trolling speed somewhere between about 550 and 1,000 RPM depending on the engine. Handy, but it's still hours at low speed. The run afterwards matters more than the RPM you picked.
- Use the main motor when you move spots. The motor that never works hard is the one that makes oil.
- Shorten your oil change interval if you troll a lot. Mercury doesn't give a number for "more often." Your dipstick will. Our 20/100/300 maintenance guide covers the standard schedule.
- Keep the 100-hour service honest. Oil, filter, gearcase lube, and a look at the cooling system on schedule.
When to bring it in
Book a service visit if:
- The level keeps rising after a fresh oil change and proper hot runs
- The oil smells strongly of fuel or looks milky
- The motor consistently runs cool
- You're due for the 100-hour service anyway, which is the natural time to deal with diluted oil and check the thermostat in one visit
Request service at hbwservice.ca. We're a Mercury Premier dealer on Rice Lake, and a kicker that makes oil after a fall of walleye trolling is a routine diagnosis here, not a mystery.
Sources
About the author
Jay Harris runs Harris Boat Works in Gores Landing, Ontario, a third-generation family marina on Rice Lake since 1947 and a Mercury dealer since 1965.
