Do Thermostats Partially Fail?

Okay, here's what I am experiencing. My bike warms up as usual. The problem is that it seems to get hot too quickly these days. It's been easily running 10 to 15 degrees hotter the past month or so and I thought maybe I was just being paranoid but this morning with an outside temp of 69 degrees, it was heating up awful quick. As I sat behind a school bus, it went from 200 to 220 in maybe 15 seconds. The fan is coming on and it goes back down, but even when I got onto the highway today, it was still running about 195 in cool air. From the spring I can remember that running in cool air at highway speeds, I usually see only about 100 degrees above ambient. So, do thermostats close fully and then only open partially? It's definitely not hot enough to be stuck closed....

I just started having this same problem, and I too was thinking partially failing thermostat. Anyone here can us, that would be great!

They can partially fail... all it takes is a faulty thermostat or corrosion (which shouldn't be the case). If you are going to take it apart to replace it anyway, I would check the current one be putting it in a pot of water on the stove (not when the wife is at home) and watch as the water temp goes up and comes to a boil. you should see it open all the way by the time the water boils!


After fully reading your post... it all looks normal to me!! Also, Thermostats open slowly as the water temp rises but it doesn't take a long period of time or anything. The only way it will open all at once is if you drop it in a pot of boiling water.

One other thing I thought of, is that you may have dirt and grim in your radiators... that can cause your bike to heat up much quicker on the highway!

Yes, they can partially fail.
If your digging all the way in to check your thermostat, you may as well replace it so you don't have to go back in later, when it does fail.
Cheap insurance.

Check your coolant level. If it's low, the bike will heat up aweful fast!

I'm not an expert in this area, but what you're describing seems normal to me. I'm running around 195 on the freeway anytime the ambient temp is less than 100, even when its in the 60's outside. To follow your previous logic, if it was only 35 outside, I don't think your bike would run at 135. The thermostat would open at the same coolant temp and the bike would run at 185 - 195. Double check your coolant level, and watch that it doesn't get above 215-220 when you're at speed, and you should be OK.

It's probably on it's way out. I noticed my bike running gradually cooler before my thermostat stuck open. This was in the spring, so as it warmed up outside it didn't seem right for the bike to run cooler than the previous day...

Mine did this after I had the dealer change the coolant. I "burped" the air out of the coolant system and all is black to normal. Just can't get good service.

Put bike on side stand
Remove right fairing and radiator cap
Wrap towel around radiator opening, coolant can rise w
Idle engine until the radiator gets hot, it should be when the temp gage is 170-180
The thermostat should be opening
Rev the throttle quickly and air bubbles will appear from radiator
Repeat several times and until no more air comes from radiator
Top off coolant
Replace cap, turn engine off, and replace fairing

[image: View Post]Jovinski, on Sep 19 2008, 09:30 AM, said:

Okay, here's what I am experiencing. My bike warms up as usual. The problem is that it seems to get hot too quickly these days. It's been easily running 10 to 15 degrees hotter the past month or so and I thought maybe I was just being paranoid but this morning with an outside temp of 69 degrees, it was heating up awful quick. As I sat behind a school bus, it went from 200 to 220 in maybe 15 seconds. The fan is coming on and it goes back down, but even when I got onto the highway today, it was still running about 195 in cool air. From the spring I can remember that running in cool air at highway speeds, I usually see only about 100 degrees above ambient. So, do thermostats close fully and then only open partially? It's definitely not hot enough to be stuck closed....

I wouldnt worry about it till it actually gets close to overheating, your still operating in a normal range


I was running 20w50 dino oil a few months back and it was averaging ten degrees higher than other oils, still within specs though

[image: View Post]spud786, on Sep 19 2008, 02:53 PM, said:

I was running 20w50 dino oil

Isn't all oil Dino oil? :goofy:

Okay, so my bike overheated when a friend rode it hard in the mountains on a hot day, slow twistys, uphill, 2 up. After this, coolant was almost all gone, and it has run hotter since. 49K miles and I am on the original thermostat, but it had stuck open before. But as I have said before, Water Wetter fixed it (it is soap based). So I haven't put back in Water Wetter yet, I will try it before I tear into the thermostat.

Some of you say normal temps, but my bike use to run cooler...

Range 170 to 225 degree

This was just regular crude oil 20w50, I'd say on an average 195 degree is about my average temp(normally , where with the 20w50 I was averaging 205 to 210 rountinally, a definite difference.
seemed not to dissipate heat as well, the 15w50 synthetic I've been running the last 4,000 mile, the average temps are back down, in that 195 200 range on an average.

Just a thought, but is it possible to replace the thermostat with a failsafe design? IDK. That way if it does fail it goes fully open and not slammed shut?

[image: View Post]motorhead1977, on Sep 21 2008, 04:14 AM, said:

Just a thought, but is it possible to replace the thermostat with a failsafe design? IDK. That way if it does fail it goes fully open and not slammed shut?

no fail safe models made.

bad thermo's effect gas mileage.

some members have changed thermo's, only to have the new one fail quickly!

there are no "good" answers to this problem.