bike dumping coolant from expansion tank

So I got my 98 St1100 about 4 months ago and the thermostat was knackered as the temp gage never got out of the white. Having gleaned a valuable load of info from you guys (thank you) it has now been replaced with an OEM thermostat and now the temp goes up to just over the cold line when ridden at a reasonable pace. Now its summer(!) in the UK and there have been a few very warm days - even around town the gauge quickly goes up to just over half way and sometimes 3/4 when the fan will kick in but never over that (so far). IS this normal?
Also it keeps dumping out all the fluid from the expansion tank- i fill it (the expansion tank) up to the mark again and after a ride- out it comes- as far as i can see the tank is empty. So i take the tupperware off and remove the rad cover and the fluid is still to the top? IF the thermostat wasn't working the bike would overheat right? I mean i have ridden to Scotland and back without it overheating- it's just the expansion tank keeps going dry.
any comments/thoughts?
P

Take a look at the hose that runs between the radiator and expansion tank as it maybe be hard and need replacement.

Mike has this one right. A Crack in the tube prevents coolant from being drawn back into the the radiator. My 01 had a Crack right in the center, both ends looked fine. Don't buy the replacement from HoNdamukong. Save a ton and go to Auto zone.

If the radiator appears full to the top when the cap is removed, the overflow hose may be OK. If so I suspect the radiator cap is not holding pressure. It could be a bad cap or any issue affecting its seal. Check the filler neck for any deformations and the condition of the rubber cap seal.


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My 92 ST11 also had the disappearing coolant in the overflow tank trick. I checked the small clear tube from the tank to the radiator for cracks per the forum but it was fine. The temp needle behaved just like yours however. Just a little into the normal range on normal days (no fan ever) or when on the road, but get into stop and go traffic on a hot day (fan kicks in) and its well into the normal range and close to overheating. (but still technically normal!)

I added a switch to turn the fan on manually. Before getting into town on a hot day Id switch the fan on and presto! The temp gauge stayed just a little into the normal range! No more needle near the overheat section!

And....

From then on THE COOLANT TANK NEVER EMPTIED! It always stayed full!

I dont know why but there it is.

Perhaps if you add a manual switch to the fan and turn it on before entering stop and go traffic on a hot day before the fan pops on your needle will behave and your coolant tank will never mysteriously empty.