Observations of a stuck open thermostat

[Actionfigurejoe]

I tried that last night by running on the centerstand with a towel on the radiator to block airflow. The temp guage went to 4 bars which is 230 deg. measured using a IR temp guage on the radiator endcap, then to 5 bars/240 deg.
This morning it was 50 deg oat, it still wouldnt get past 2 bars in town or 1 bar on the hiway. So that didnt help.....

Tried the same tonight. Coolant began to flow at 185F measured w/IR temp guage. Three bars were also showing. A stuck open thermostat can cause excessive cylinder wear. Composite cylinders like the ST incorporates are less prone to wear than iron bore motors because of greater thermal stability. Still, it's a good idea to rectify the problem.

 

[BigBadblue]

Now to nail down the cause of the failures: Paint chips? Coolant crystallization/bad coolant? Horizontal mounting of t-stat?? Flawed t-stat design? Combination of these or something else altogether? Like I posted earlier, the RC51 and VTX1800 both list the same part# for the t-stat and I have visited some of their forums and have not found any complaints of t-stat failures. Anybody know what orientation the t-stats are mounted in these bikes? I imagine they use the same coolant. And has anybody had a second t-stat fail, and what coolant did you use on the second one?

 

[04ST1300a]

BBB,

That makes me think its somethiing else in the coolant, like paint chips as mentioned earlier.

Has anyone tried to match this t'stat with another one at an auto parts store?
Surely there is another one that will fit.

 

[Killtimer]

Has anyone tried to match this t'stat with another one at an auto parts store? Surely there is another one that will fit.

Numerous people have tried with no success I'm afraid.

 

[04ST1300a]

Does anyone have the dimensions or an old one laying around they could measure?

 

[MMOB]

Does anyone have the dimensions or an old one laying around they could measure?

It is a dual valve type thermostat; there is a second spring and disc at the end of the wax motor. One valve controls the flow of coolant into the radiator, the other valve controls flow of coolant through the bypass circuit. These are not easy to find.

 

[wjbertrand]

It is a dual valve type thermostat; there is a second spring and disc at the end of the wax motor. One valve controls the flow of coolant into the radiator, the other valve controls flow of coolant through the bypass circuit. These are not easy to find.

I found plenty of thermostats with this design but they were longer, shorter, bigger in diameter, not offset from the center-line like the ST's is or they did not have the little bubble flapper thing. Usually there were two or three things not quite right. I scoured the inventory at Kragen, AutoZone, Pep Boys and NAPA. I looked at t-stats made by Prestone, Stant, and another manufacturer who's name escapes me at the moment. I've examined literally hundreds of cage t-stats and come up with nothing close enough in physical dimensions or features to be comfortable with. Then there is the additional requirement to match the temperature rating (82C or 180F).

The only thing I haven't yet done is try a Honda cage dealer. If they turn out to be available there I wonder if they will be any cheaper than the $22 price I found at Zanotti Motors for the OEM. Some of the cage t-stats I looked at were over $40.

 

[NormanPCN]

Your right on the mark Norman. It begins to open from 176-183F and 203F fully open it is!

I don't know this motor that well, but having just looked this up in the manual is it sound to assume that 1 bar the thermostat is beginning to open and 3 bars is fully open? The fan then kicks on when coolant temp exceeds fully open position?

The service manual does give info about the fan on and off temps. Around 210 if I remember correctly. This is a pretty tight range to the quoted full open temp and this agrees with observation. The temp gauge does not move in the fan on/off cycle. On most cars with electric fans the temp guage rises a ways before the fans kick in. Don (DonD) speculated as to what the bars on the guage mean by taking the resistance specs for the temp sensor from the service manual. The specs have a resistance O1 and temp T1 and resistance O2 and temp T2. Since the temp sensor is calibrated to T1(min) and T2(max) it is reasonable to assume that bar1 is T1 and the last bar is T2.

 

[MMOB]

I found plenty of thermostats with this design but they were longer, shorter, bigger in diameter, not offset from the center-line like the ST's is or they did not have the little bubble flapper thing. Usually there were two or three things not quite right. I scoured the inventory at Kragen, AutoZone, Pep Boys and NAPA. I looked at t-stats made by Prestone, Stant, and another manufacturer who's name escapes me at the moment. I've examined literally hundreds of cage t-stats and come up with nothing close enough in physical dimensions or features to be comfortable with. Then there is the additional requirement to match the temperature rating (82C or 180F).

The only thing I haven't yet done is try a Honda cage dealer. If they turn out to be available there I wonder if they will be any cheaper than the $22 price I found at Zanotti Motors for the OEM. Some of the cage t-stats I looked at were over $40.

Besides overall length, diameter and shape of bypass valve, diameter of main valve and valve seat, and operating temperature ranges, there is also piston travel and both spring forces to take into account when identifying an equivalent part.