Modern Vespa

Just after some opinion before I take it off to the shop, my temp gauge has been fluctuating the past couple of days on rides of around 25-30 mins. Basically on cold startup the temp gauge takes the usual amount of time to slowly creep up to normal temp (10 mins or so). After that it will sit at the mid point if I'm cruising or at idle, but if I accelerate hard past 80km/h it will peak up to just below redline and fluctuate around that point until I ease off acceleration, where it will return to mid point again.

Any thoughts apart from "get it checked"

Low coolant, air bubble in coolant, misbehaving thermostat, scale/corrosion buildup in radiator, clog in coolant tubes, broken coolant pump.

Apart the "get it checked", you have the option to "check the coolant level yourself". Not very hard to do, and it is well documented in the manual..

The issue you describe is common for low coolant level...

Takes about a minute to remove the coolant expansion tank cover..

Correction in red...

I agree would the two low fluid comments above. I'm guessing you have leak. Make sure the bleed screw is not loose. Don't tighten it too tight. It's easy to strip the threads. Check for coolant leaks. My 250 never ran low on coolant. When was the last time it was changed. Should be every two years

Best Miguel

Yes of course - should have checked before posting I guess I thought the fluctuation was unusual. There's coolant there but it looks like it's just below the min level so I'll top it up.

Don't know when it was last changed. It gets serviced every 5000kms or so.

Right. If you can see the coolant even if it is only in the bottom of the expansion tank (when the engine is cold), that's not the problem then..

My GTS doesn't leak but being in QLD (40° and up every day) it has required topping up twice in the first 10K.

My next step if I was in you, would be to check the electrical connection to the water temperature sensor under the seat. Sometimes dust and dirt or if you have washed the engine lately could do that.. If that doesn't make any difference, then you have a possible faulty thermostat (but they normally are design to fail in the open position), or the temperature sensor itself... But check the wiring first..

Seems a bit odd. Coolant runs hotter than 40C, so then wouldn't everyone's bike require occasional top up then? It's a pressurized and sealed system, so where would the coolant go without a leak?

For start the 40° is the air temperature not the engine temperature. The coolant could escape from the expansion tank cap. All I know is that I don't have any leakage when the GTS is stationary and yet the GTS has used 60ml of coolant in 10K.. Not that much and not that odd at all..