I have a Tmax 500 2009 model year, and I’m having an overheating issue I can’t quite fix, the problem only appear after a long ride on the highway, while riding on the highway the heat gauge is perfectly in the middle, the minute I get off the highway and come to a stop, the heat gauge immediately goes all the way to the max and coolant is being thrown on the ground from the coolant reservoir hose,
After turning off the bike and waiting about 5 minutes after its overheated, the temperature gauge will go back to normal and even the vent will turn off!
now that could sound like a bad radiator but I’ve changed the entire coolant system: a brand new radiator, thermostat, the mechanical heat unit, coolant flush+new coolant, none of the passages are blocked, vent works perfectly, no leaks, water pump rebuilt,
I even did a reverse pressure test to see if the head gasket is bad but everything is fine.
It is worth mentioning that if I ride in the city more of a throttle-stop type of commuting it doesn’t overheat,
no matter if the ride is 10 hours long or really fast it won’t overheat in a city ride, only at the scenario I mentioned above, I really can’t wrap my head around what is the problem since I changed and tested everything so any tip or advice would be appreciated.

Sounds like your cooling fan is not cutting in as it should when you slow down or stop. Could be a faulty fan or temperature sensor/switch/relay or (less likely) just that the coolant is not in contact with the sensor due to air in the system. Can you hear the fan when you stop? You should also be able to feel hot air being driven through the radiator and out under the scooter. Coolant boiling in this way will quickly cause further damage so it should be attended to immediately.

Bypass the fan switch, ignition directly to the motor/relay. The fan will run all the time which will be annoying but fine in the short term. If this fixes it you have the answer.

I once rode across a chunk of Russia with the fan cobbled off the horn switch. Worked fine so long as I remembered to push the button every few miles before there was traffic.

Andy