This spring when I went to the parts house to buy a new summer (180F) thermostat, I thought I would splurge and buy the more expensive "fail-safe" model from Motorad. Well, life this fall has turned out to be much more challenging than normal, and I had not found the time to change to my winter (195F) thermostat. But I was not too concerned, usually a 180 thermostat makes plenty of heat.
The weather here in Ames turned pretty cold last week and we needed to make a quick 180 mile trip to Minnesota. I started the '95 SLE and let it idle with the defroster and heat on high to help loosen the 1/2" of ice that had accumulated over the car. In a period of about 20 minutes with outside temperatures less than 20F, I got the car cleaned off and away we went. As we pulled out, the temperature gauge was reading about 200F from the long idle. As we drove up the interstate, the temperature gauge kept dropping, finally coming to equilibrium just over the 100F line. The air from the heater got colder and colder as I increased the heater temperature control to maximum.
When we returned from Minnesota, I removed the summer thermostat. Here'* what I found:
Wide open and locked in that position by a little brass tang. The extended idle had increased the temperature enough to pull the thermostat flange past the "fail-safe" tang, locking it open. Now, this is not supposed to happen until the engine has "overheated." I have not been able to find what temperature is defined as overheating by MotoRad, but as I cleaned off the ice, I do not remember hearing the cooling fans engage. Although I was not watching it closely, I estimate that it locked open somewhere between 200 and 220F.
Here is an image of the other side showing the temperature rating:
I Googled this issue and found others had experienced the same problem of the 180F fail-safe thermostat locking open at temperatures well within normal operating range.
I don't think I will be buying any more of these.