The F25 on the back of a 14-foot aluminum skiff starts on the second pull, idles rough for a minute, then settles. Or it doesn't settle, and Saturday morning goes by in the parking lot with the cowling off. Small EFI four-strokes like the F20G, F25G, F30B, F40F, the F50 and F60 versions, F70A, F75, F80, F90 and F100 rarely get plugged into anything. That's a shame, because their ECUs keep a short but useful record.
Less data, but the right data
In this family each ECU offers 18 to 30 live values, between 9 and 13 fault messages and 4 to 8 actuator tests. That's modest next to a V6. The values still cover what goes wrong on a small engine: engine speed, intake air pressure, TPS voltage, ISC valve opening angle, fuel injection duration, cooling water temperature, battery voltage, plus the shift position and shut-off switch states. Some versions add the oil pressure switch, fuel flow and trim level. All four ECU variants in the family support idle learning.
The fault list is short and easy to read: thermo sensor, TPS, knock sensor, pulser coil, air temperature and air pressure sensors, water in fuel filter, shift position switch, engine shut-off switch, intake air leak, low battery voltage, overcooling, and an immobilizer communication message on engines that have one.
Three things small engines do, and what the scan says
Rough idle after sitting
Classic on a tender that spends weeks between uses. Watch ISC valve opening angle at a warm idle. If the ECU holds the valve well open just to keep the engine running, the idle air path is gummed up or there's an air leak somewhere. The ECU can log an intake air passage leak on its own. The ISC valve actuator test tells you whether the valve responds at all. After cleaning, run idle learning so the ECU starts from a fresh baseline instead of the correction it built up for a dirty throttle body.
Runs cold and never quite warms up
The overcooling message, engine warm-up not sufficient, sends you to the thermostat. On engines used in cold water for short hops, a thermostat stuck open is common. Cooling water temperature confirms it: the value climbs slowly and levels off well below where it sits on a healthy engine. Left alone, a cold-running engine fouls plugs and never burns off moisture in the oil.
Won't crank, or cranks and won't fire
Check the shift position switch and engine shut-off switch values first. The lanyard clip on a tiller handle gets knocked loose more often than anyone admits. If the ECU logged a pulser coil irregular signal, the check steps go to wiring, coil resistance and output peak voltage, in that order.
Where the scan runs out
A lot of small-engine trouble is plainly mechanical. A weak water pump impeller shows up as a thin telltale stream long before any sensor reacts. Old fuel, a cracked primer bulb or a clogged filter at the tank produce no fault message unless water reaches the detection switch. And a tiller engine that's gone for a swim needs to be pickled and opened up, which no app will do for you.
Where a scan earns its keep is in cutting out the electrical guesswork. A diagnostic dongle and phone app that lives in the boat bag reads all of the above without a laptop, depending on what the ECU of your engine offers. The covered models and the live values each one provides are on the F20 to F100 page. On a skiff I'd check the shut-off switch value and the telltale first, because between them they explain a surprising share of bad Saturdays.
