# Testing a car thermostat **URL:** \ **Category:** Factual Questions\ **Created:** [December 1, 2012, 9:37pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704 "2012-12-01T21:37:33Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [December 1, 2012, 9:37pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/1 "2012-12-01T21:37:33Z")
Replacing the wife’s water pump (see other thread) and I tested the thermostat by keeping it in a rolling boil for a few minutes. Didn’t open which is not uncommon but when I tested the old one which I assumed worked and it didn’t open. Two thoughts: I don’t know what I’m doing and don’t know what an open thermostat looks like (I doubt that) or because I’m in Denver and water boils at 200-205 degrees this testing method doesn’t work. If the latter, any other ways to test? ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [December 1, 2012, 9:46pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/2 "2012-12-01T21:46:53Z")
Some cars have a 212 thermostat, maybe boil it in coolant and try it. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [December 1, 2012, 9:49pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/3 "2012-12-01T21:49:01Z")
You can boil it in a 50/50 antifreeze mixture to boost the boiling point. Do you have a thermometer you can check it against? ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [December 1, 2012, 9:53pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/4 "2012-12-01T21:53:10Z")
I used to do pretty much what you did except that I would hang the thermostat into the water. I dont see how that would make much of a difference, however. Two thoughts: 1) you don’t say what type of car and what operating temperature is called for. Newer cars operate much hotter than the 1960’s 165 degree (summer) and 195 degree (winter) temps. If your thermostat is supposed to open at 250, for instance, the boiling water test is insufficient. 2) Restoring/racing 60s/70s era cars with apprpriate thermostats, we found the quality of the newer/foreign made themostats very poor. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [December 1, 2012, 10:06pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/5 "2012-12-01T22:06:59Z")
2006 Jeep Commander and a Duralast thermostat (no rating I can find). Since the thermostat is so easy to replace I’ll just install it and watch for overheating for a few days. Oh and since I’m on the subject, is there any reason to backflush the system, either at home with the Prestone kit or professionally? It’s the orange Xyrex-05 HOAT coolant so looking for rust in the coolant won’t work. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [December 1, 2012, 10:17pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/6 "2012-12-01T22:17:24Z")
The way I’ve always tested a thermostat is to install it, then test it. It removes all the temp/pressure variables. After you have installed it, feel the hose that connects to it until the car gets up to operating temp. If it’s not opening, the hose won’t be hot. If it is, there will be no doubt in your mind, because the hose will be very hot. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [December 1, 2012, 10:21pm UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/7 "2012-12-01T22:21:18Z")
> [@scabpicker](#): > > The way I’ve always tested a thermostat is to install it, then test it. It removes all the temp/pressure variables. After you have installed it, feel the hose that connects to it until the car gets up to operating temp. If it’s not opening, the hose won’t be hot. If it is, there will be no doubt in your mind, because the hose will be very hot. You mean the upper radiator hose? Sounds like a plan. ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [December 2, 2012, 12:35am UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/8 "2012-12-02T00:35:52Z")
> [@scabpicker](#): > > The way I’ve always tested a thermostat is to install it, then test it. It removes all the temp/pressure variables. After you have installed it, feel the hose that connects to it until the car gets up to operating temp. If it’s not opening, the hose won’t be hot. If it is, there will be no doubt in your mind, because the hose will be very hot. Good plan. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [December 2, 2012, 12:56am UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/9 "2012-12-02T00:56:56Z")
Yep, it’s the upper hose on every engine I’ve worked on. I’m glad that I could pass on the trick, it was taught to me years ago. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [December 2, 2012, 2:07am UTC](https://boards.straightdope.com/t/testing-a-car-thermostat/642704/10 "2012-12-02T02:07:08Z")
If you have a temp guage in the car you can simply sit in the drivers seat and watch the guage go up, when the stat opens you will see a slight drop in temp and then it will very slowly resume heating. This will occur at the operating temp of the thermostat.