The overheat alarm on a supercharged 1.8 litre Yamaha, meaning the FX SVHO and Cruiser SVHO, the FZR and FZS, and the GP1800R SVHO, is reported more often than almost anything else on these boats. The usual first move is a new temperature sensor. Owners who have chased it down say the water path was the problem more often than the sensor.
Three devices that can raise the alarm
These engines carry three temperature devices. The thermo sensor at the exhaust manifold reports through code 65. The engine temperature sensor reports through code 15. The third, the thermo switch, works differently: it doesn't deliver a value, it just closes at a threshold.
That matters for diagnosis. If the alarm sounds and neither 15 nor 65 is stored, and the live temperatures look sane, the thermo switch moves up the suspect list. The logic comes up repeatedly in owner discussions, though it is a way of narrowing things down and not a proof.
One more detail from the EX manual: the overtemperature and oil pressure warnings share the warning tone, and both reduce engine speed automatically. For the SVHO models, check what your own manual says before you assume the tone means heat.
Flow comes before electronics
Owners keep coming back to the same short list of flow problems:
- cavitation at the pump, where the impeller draws air instead of water
- a foreign object in the intake grate
- a chewed or damaged impeller
- a plugged small cooling-water passage
- exhaust manifold passages closed up after poor flushing
A popular check is to run the engine with water supplied and see whether water leaves at the back and also at the test tube at the front. Water at the rear but nothing at the front means the path is closed somewhere between them. A sensor swap won't change that.
What a logged ride can show
This is where the M-JET Diagnostic helps. It supports the FX SVHO and Cruiser SVHO, the FZR and FZS and the GP1800R SVHO, and it can record engine temperature and the other temperature values during a ride. A few patterns are worth reading for:
Temperature that creeps up under sustained load and comes down slowly after you back off points toward a restricted water path. Two temperature readings that disagree with each other, or one that jumps in steps, points toward a sensor or its connector. An alarm with every reading calm brings you back to the switch.
Boosted engines also suffer from heat soak, meaning heat building up after a hard run or at a stop. Owners report this as real, so a high reading right after shutting down isn't automatically a fault.
What the app cannot see
Cavitation, a worn or damaged impeller, debris in the intake grate and blocked cooling passages don't produce a code. That is the awkward part of this problem: the app can rule a sensor in or out, but if the sensors look honest, the answer is under the boat. Pull the grate, look at the impeller and check the water at the outlet before spending money on parts.
More background on the sensors is on the overheating and temperature sensor page, and the supercharged models are covered on the SVHO and SHO diagnostics page.
