Ok I took my bike out today andit stayed pretty much in the right place on the thermostat reading. I stopped my bike at a friends house and left it running. it began to overheat and water began to pour out the bottom. The reading was close to the read part of the gauge. The fan was running. After I got back on the bike and riding a bit it cooled back down.
The question I have it this. Can the thermostat not open and the sensor still work right. Also can the engine cool down alone by riding and cooling the fins with air and the thermostat still not open.
I haven't had this problem until last night.
yes, riding it can cause temp to go down, but not by cooling the fins on the engine. its the air thru the radiator that cools it down when riding. the fan alone is not always enough to keep it cool, when going low speeds or stopped.
temp sensor can read correctly regardless of the thermostats position or condition (bad or good). its on the 'hot' side of the thermostat if I recall.
if riding causes it to cool down, then look at a partially plugged radiator, causing less coolant to flow at idle (riding is done at higher rpms than idle, and therefore there is more water flow avail to go thru the radiator).
if the fan is cycling on and off properly (and if the radiator is doing its job it should cycle, even at idle), then the only thing that can really cause your issue is either a plugged (internal, or external, check the front for debris) radiator, or a failing thermostat (probably stuck partially open, or wont open fully).
I suppose a water pump thats marginal could do it, but thats pretty much a stretch, since when its ridden, it cools.
all of this is of course assuming there are no air bubbles in the cooling system