Most impeller failures on a Yamaha four-stroke don't start with an alarm. They start with a tell-tale that's a little weaker than last year, a temperature that climbs a few degrees faster on a hot afternoon, and an owner who doesn't notice because nothing is wrong yet. By the time the overheat horn goes off, the vanes are cracked or set in a curl and the engine has already been running hot for a while.
What the ECU can and can't see
No sensor sits on the impeller. What many Yamaha ECUs do have is a cooling water pressure value, in kPa or psi, from a sensor in the water jacket. It shows up in the live data on engines across the range, from some F20 to F100 models through the F115, F200 and big V8 families, depending on the ECU. The HPDI Z series doesn't list it.
Pressure is the result of everything in the water path: the intake screen, the impeller, the pump housing and plate, the passages, the thermostat and the pressure control valve. A tired impeller pushes less water, so pressure at a given rpm falls. So does a partly blocked intake. The number tells you flow got worse. It doesn't say where.
Log a baseline when the impeller is new
The values that matter are your own engine's, not a number from someone else's boat. After an impeller change, with the engine fully warm, do this on the water:
- Note cooling water pressure at idle in neutral. Expect it to be low and to wander with the thermostat.
- Note it at 2000 rpm in gear, steady for half a minute.
- Note it at 3000 and at 4000 rpm, same way.
- Write down the water temperature that day, roughly. Cold water and warm water give different readings.
- Note engine temperature at each step too.
The M-JET DIAGNOSTIC app puts these next to each other on one screen, which makes it easy to take a screenshot at each rpm. Keep those screenshots with your maintenance log.
Reading the trend
Repeat the same steps a couple of times a season. A gradual drop at 3000 and 4000 rpm compared with the baseline, at similar water temperature, is the pattern of a wearing impeller. A sudden drop from one trip to the next is more likely debris on the intake or a vane that broke off. Look at the screen before you pull the lower unit.
Pressure that's normal at mid-range but low at idle tells you less. At idle the thermostat is still controlling flow, and on some engines idle readings are naturally low and jumpy.
If the value drops to nothing or reads something impossible with a good tell-tale stream, suspect the sensor. On the larger platforms the fault memory can hold a water pressure sensor entry, out of specification, which points at the sensor, its wiring or its output voltage rather than at the cooling system.
The thermostat side
A thermostat stuck closed shows as temperature climbing fast with pressure looking fine or high. A thermostat stuck open shows the other way. The engine never gets properly warm, and on engines that watch warm-up, the memory can hold an overcooling entry, engine warm-up not sufficient. The check step for that one is the thermostat and the cooling system, not the ECU.
What still needs a wrench
Impeller condition is confirmed only by looking at it. The data tells you when to look. Change it on the schedule in your owner's manual or sooner if the trend says so, and check the pump plate and housing for scoring at the same time. A new impeller in a grooved housing will show a lower baseline than you had before. For more on the cooling side, read the Yamaha outboard overheating guide, and the live engine data page lists the other values that sit next to pressure on most engines.
