Coolant temperature rises then falls

[kobra]

I have this weird thing I'm seeing where during warmup within the first minute or so of riding the bike, the coolant temperature will rise just slightly past the middle mark, then drop rapidly down to about 1/3. As I continue riding, the temp slowly climbs and stabilizes in the middle. It pretty much stays in the middle for the rest of the ride with very minor fluctuations based on load and ambient temp.

To me that says the thermostat is remaining closed, and then suddenly opening allowing a rush of coolant to come in past the temp sensor. The purpose of a thermostat is to heat up the head as quickly as possible and maintain it, but I would expect a more gradual opening rather than overshooting.

Any theories on this or what I could check?

[(F5)]

Geez it's been a while but my 350 did just that I seem to recall.
Worry about the wear in the rear linkage bushes instead if you have a spare minute.

[evan_calgary]

I check all my therms in a pot of boiling water. They go from fully closed to fully open in a few seconds (some 'pop' open) which could be what you are seeing given the temp sensor is in the head and relatively close to that unit.

[Yellow and Black RZ]

Been a long time but this behavior sounds familiar.
Both original and after a rebuild, same thing. Get worried for a minute, check again and it's @ normal temp.

[kobra]

Thanks guys. The weird thing is I don't recall this behavior before my recent rebuild.

[kpke]

Mine do that. For ever. I would not be concerned.

[waltmil]

I noticed mine does that several years ago when I first got it. the forum consensus was, they all do that. No worries, I've had no problems and it still does that.

[kobra]

Well that's reassuring. Phew!

However my curiosity got the best of me and I took out my thermostat to test in some hot water. It only opened about 5mm (spec is 7mm). I happened to have a spare and it opened the full 7mm, so I swapped them out. Might make a difference, might not.