Depending on the model, the app shows up to 34 live values, and the first time you open the list it looks like a wall of numbers. Most of them only matter when a specific question comes up. The values below cover the Yamaha four-strokes from 2002 to today, and the trick is that almost none of them mean much alone. They are useful next to each other.
Pressure: intake pressure against air pressure
The app shows intake manifold pressure and barometric (air) pressure. With the key on and the engine off, the two should be about the same, because nothing is drawing air through the engine. If they disagree before the engine has even started, the intake pressure sensor is suspect. This is one of the few comparisons where you do not need a reference table. On the supercharged models the same value also tells you something at speed: in forum reports a failing supercharger clutch leaves the intake pressure near ambient while the ski hangs at 5000 to 5500 rpm on the water.
Temperatures: three sources, not one
Engine temperature, exhaust manifold temperature and intake air temperature are separate values, and they should not all move together. A manifold temperature that climbs while the engine temperature stays calm is a reason to check the water flow before blaming a sensor. An intake air temperature that keeps rising on a supercharged ski can explain why power fades on a hot day. I would not quote a number as normal for all models, since the right figure depends on the engine and the conditions.
Throttle and accelerator: pairs that must agree
On models with an electronic throttle (the VX1100 from 2005 and everything newer), the app lists TPS1 and TPS2 for the throttle valve and APS1 and APS2 for the lever. The two signals in each pair should track each other. When one of them jumps or drops out while the lever is held still, that is almost always a contact problem. On the older GP1300R, the single throttle sensor has to read closed at start, and a poor contact there is a common reason for rough starting and idling.
Battery voltage and switch states
Battery voltage is the quickest check on the whole electrical side. Watch it at rest, then while cranking, then with the engine running. It should rise above the resting value once the engine is running, and if it does not, the charging side needs looking at. Switch and relay states cover the kill switch, slant switch, oil pressure switch and, where fitted, the reverse and no-wake switches. Operate each one and watch the value change. A switch that reacts late or flickers is worn or corroded.
Other values and what they are good for
- Engine speed. The obvious one, and the one to put on the log next to everything else.
- Ignition timing and injection duration. Not something you judge by feel, but a sudden jump when the engine stumbles is worth a look.
- O2 sensors, on a few models only.
What a number cannot tell you
Live values report what the sensors see. They do not see cavitation, a tired impeller, a blocked passage or low compression, so a ski can have perfect numbers and still run badly. A rough idle or a dying engine with normal live data is an honest result, and it moves the search to the mechanical side.
The Yamaha version of the M-JET Diagnostic shows only the values your model actually has, and the app handles the model detection itself. The page on reading live data describes the screen, and the overview of models and typical problems helps you see which values apply to yours.
