Is this the thermostat, or something else?

[DSA]

Hi All,

 

I've got a Y2K 5th gen bike with 22,000 miles on it. While riding today in ~56F (13C) weather and holding a constant 70-75 mph on a near-empty highway, coolant temp settled at 156F (70C). It moved very little from this point as long as I kept constant speed, and increased to about 164F (73C) when I briefly exited the highway and was going 30 mph for maybe 1 min or so. 

 

From reading around the forum, it seems like my temps are a little on the low side (though not by a lot, depending on the source). My other bike, a Blackbird, does tend to run quite a bit hotter than this, though that data point is probably of very limited value. Is it time to tear the bike apart and replace the thermostat? 

 

Thanks!

[Grum]

If you are noticing the engine taking longer to come up to temperature, as well as lower than normal operating temps, then thats a good indication of a jammed open thermostat.

Their most common fail state is to the open position, fortunately! Just like this.

[Terry]

Thanks for posting in celcius and fahrenheit DSA!

 

I agree with my learned colleague. The thermostat should close down around 78C, so anything lower while running indicates a stuck open thermostat. My test is to start the engine from cold and keep a hand on the radiator. If it slowly heats, the thermostat is stuck. If it stays cold then suddenly gets hot, the thermostat is good. 

[DSA]

Thanks for the replies! I tried this - should I be touching the metal upper part of the radiator, or the meshed part? If the former, then I do feel the radiator slowly heat up with the bike. It certainly isn't stone cold before suddenly getting too hot to touch.

 

@Grum: What does a fully closed thermostat look like? Are the springs fully compressed?

[Terry]

The image shows the stuck open thermostat from my last VFR. The gap between the upper ring and the plate above the spring is what opens when immersed in hot coolant. When cold there *should* be no gap here. The brass coloured tube is where the expansion and contraction happens, I presume there is a wax pellet in there like the fast idle wax unit has on the later bikes. When hot, the plate will move downwards away from the upper ring.

 

For a fun science experiment (when your wife is out) you can boil some water in a pot and dip your thermostat in and watch the magic happen before your eyes. Then remove it and run it under cold water and watch it close. Or, if your thermostat was like mine, just swear at it and order a new one. 

[DSA]

Perfect, thanks for clearing that up! Final question - given that my operating temps seem to be ~10-20F (5-10C) shy of where they should be, how urgent is it that I address this? I ask because it seems like a bit of an involved task, and is likely to snowball a bit (seems like a crime not to check the valves while I am in there). I've got a few other bike projects lined up, and wonder whether I should push this to the top of the queue, or whether it can wait for a bit. 

 

Will operating the bike based on the temps I mentioned in my first post cause any damage?

 

[Grum]

Doubt that you would cause any issue by not replacing it asap.

You may find you're not getting the best fuel economy, and the oil may not be working to its optimum at lower temps.