A 2021 or newer four-stroke SuperJet starts beeping after a session over a sandbar, or it simply feels flat and there is no warning at all. On a stand-up hull that spends its life in sand and weed, the water reaching the engine is the first suspect and a temperature sensor comes well after that.
This concerns the four-stroke SuperJet from 2021 with the 1049 cc three-cylinder TR-1 engine. The same engine sits in the VX, EX and JetBlaster, so most of what follows carries over, and the M-JET Diagnostic supports all four. The SuperJet has no RiDE system and no reverse, which removes a few of the usual suspects. The model page is SuperJet diagnostics.
Flow first, sensor second
Owners of several Yamaha models describe the same order of causes for overheat alarms. A restricted water path comes first: debris on the intake grate, a nicked impeller, cavitation at the pump, a plugged small cooling-water passage, or a manifold that silted up after careless flushing. Only then does a bad sensor come into play. Shallow water adds a simple extra way to get there, because the pump pulls in whatever is on the bottom. A wad of weed or a handful of sand on the grate can be enough to starve the pump under load while idle still looks fine, which would match an alarm that shows up after a hard run and eases off when you slow down.
What the temperatures tell you
Read the fault memory first and write down what is stored. In the code list used by the newer control units, 15 is the engine temperature sensor and 65 the exhaust manifold water temperature. Numbers can differ between control unit generations and between sources, so go by the description the app shows. The general background is on the page about overheating and temperature sensors.
Then watch the live temperatures during a short run on the water. A real flow problem looks like values that climb with load and come back down once you slow. A sensor or connector problem tends to look different: a reading that jumps around or sits at an impossible value while the engine is obviously not hot. If you don't know what is normal for your ski, a recorded ride from a day when nothing was wrong is worth more than any number from a forum.
What the scan will not find
- Sand, weed or a plastic bag on the intake grate, because nothing monitors that.
- A damaged impeller or a worn wear ring.
- Cavitation.
- Blocked cooling passages inside the engine or manifold.
Some owners call the small tell-tale tube up front the pisser. If water leaves the stern outlet but nothing comes out of that tube, the path is closed somewhere between. If your hull has one, it tells you more than any sensor reading will.
The learning mode catch
The four-stroke SuperJet has a learning mode (L-mode) that limits power. A ski that feels weak, shows normal temperatures and has no stored codes may simply be sitting in that mode, and it is an easy thing to rule out before pulling anything apart.
An order that saves time
- Note the stored codes, then clear them so you can see what comes back.
- Check that the ski isn't in learning mode.
- With the engine off, clear the intake grate and look for damage at the impeller.
- Ride in deeper water and log engine and manifold temperatures against rpm.
- If temperatures climb with load and the grate was clean, suspect the water path further in, not the sensors.
Never run the engine on land without a water supply, not even for a quick check. If the temperatures look fine, the warning is intermittent and the grate was clean, a second logged ride in the same shallow spot is the cheapest next step.
