The GP1800R SVHO feels strong at the first run of the day and a bit flat after an hour of hard riding in warm water. That is the pattern owners describe. It concerns the supercharged GP1800R SVHO from 2017, and the same logic applies to the FX SVHO and the FZR and FZS.
Heat soak is real, but it isn't a fault
A supercharged engine heats the air it pulls in, and heat builds up in the engine and intake after a hard run. Owners of these boats report heat soak consistently. Some loss of power on a hot day can therefore be a property of the engine and not a fault you can chase down.
What you want to know is whether your boat loses more than that, and whether heat is really the reason. A scan can help sort the two cases, because the intake air temperature is one of the measured values and doesn't depend on how the boat feels to you on a given day.
The values worth logging
Take a test ride with the data logger running, ideally a stretch where you can hold the same speed for several minutes, and record these:
- intake air temperature, the value behind code 23
- intake pressure, the value behind code 29
- barometric pressure, which sets the baseline
- engine speed and throttle position
- engine temperature
- ignition timing
- battery voltage
Before the ride, switch the key on with the engine off. Intake pressure should sit close to the barometric value then. If the two don't agree, the pressure sensor or the plumbing is the first thing to question, before any heat theory.
Reading the log
Line up the moment the boat feels weaker with the other curves. If intake air temperature climbs, the power drop follows it, and ignition timing changes at the same moment, that fits heat soak. It is a pattern and not proof. If the boat feels weak while intake air temperature stays flat, look elsewhere: intake pressure that stays low at full throttle, a throttle position that isn't reaching its end, or a battery voltage that sags.
A second run after a cool-down is useful. If the boat recovers its pull after ten or fifteen minutes of idling in cooler conditions and drops again on the next hard pull, you have your pattern. If it never recovers, the problem is not simply thermal.
What the scan can't prove
Intake pressure that stays close to ambient under load would fit a weak supercharger or its clutch, a topic owners of the supercharged models discuss often. The app shows that value, but it can't prove damage to the supercharger. Neither can it see the water path, the impeller or the intake grate. Overheating can also cut power, so check the temperature curves in the same log; the overheating page covers that side.
The M-JET Diagnostic supports the GP1800R SVHO, the FX SVHO, the Cruiser SVHO and the FZR and FZS, and the logger works with any of them. The step-by-step for the power side of the problem is on the GP1800R SVHO intake pressure page.
Two logs, one from a cool morning and one from the hottest part of the afternoon, ridden over the same route with the same load, will tell you more than any single reading. If the two look alike, heat is not your problem.
