160 degree thermostat opens at 175

[Redmond49]

I bought a new thermostat and then tested it against a thermometer in hot water. Even though it is stamped as a 160 degree thermostat, it doesn't actually open until about 175 degrees. Then, as the water cools, it does seem to close up at closer to 160. It's a Murray branded item per the box, but stamped MotoRad. 

 

Is the behavior of this thermostat the expected behavior? I expected it to open at 160.

[Redmond49]

Never mind, the repair manual answered this question, at least for the OEM design. 

[desoto1939]

14 hours ago, Redmond49 said:

Never mind, the repair manual answered this question, at least for the OEM design. 

Ok you found your answer inthe repair manual. Great that you have a repair manual.

 

But what was the answer to your question.  This is the time to pass on knowledge learned.  Someone else could have the same question but if you have the answer pass it along to everyone else.  We all enjoy learning especially anything about these old cars.

 

Rich Hartung

desoto1939@aol.com

[Dartgame]

I always test stats before using them. They almost never open at the temp specified, and open at a higher temp.

[Young Ed]

Isn't the idea is that they start to open at the stated temp and a fully open later? In this case starts at 160 done by 175?

[RobertKB]

1 minute ago, Young Ed said:

Isn't the idea is that they start to open at the stated temp and a fully open later? In this case starts at 160 done by 175?

 

That's what I think, too.

[Dartgame]

What I've seen is most stats do not start to open until they are well past the specified temp, and when cooling are closed well before they reach spec. temp. In this case a 160 opening at 175, wont be an issue anyway. Where this becomes an issue for me is with 195 stats opening at 205-210 or higher and then closing at 205 or 210. 

[JBNeal]

My heat transfer studies learned me that the thermostat eventually stays open in certain conditions as coolant is flowing non-stop to the heat exchanger.  At this point, the mass of the engine begins to reach a steady state temperature, and the lubricant begins to act as a coolant.  This is why most diesel engines have relatively large crankcase lubricant capacities, as they are pulling double duty for maximum engine performance and reliability. 

[Redmond49]

Hi, 

 

Here's the follow-up, it's been an interesting learning experience. The repair manual says thermostats are supposed to open and close within a few degrees of the stamped temperature.

 

I bought this car (46 Plymouth) going on 4 years ago now. It ran but overheated right off the bat. To start off, I had the radiator re-cored, installed a new water pump, new hoses, pulled the distribution tube, and flushed the block. Then I installed a new 160 degree NOS thermostat. The engine overheated anyway. I hadn't tested the new NOS thermostat before I put it in (oops), so when I pulled the cooling system apart again, I did. The NOS thermostat didn't open at all, so I threw it away and tested the original one, which seemed to open and close nicely at 160 degrees. 

 

In the end, it turned out the engine needed a complete rebuild, so fast forward to this spring. When I assembled the cooling system on the newly rebuilt motor, I kept the original 160 thermostat. While the engine no longer overheated, it did run around 175 degrees based on the gauge and thermometer readings, and the head had surface temps quite a bit warmer than my other flathead Plymouth (a 49 with a 51/52 Dodge truck block in it). The temp gauge was somewhat active between 160 and 175 but mostly on the high side. Since the thermostat was the only thing I hadn't changed out in the cooling system, I decided to try a new one.

 

I bought the "Premium Quality" Murray Plus thermostat that O'Reilly's website indicates is right for these cars. Even though it's in a Murray box, it is stamped MotoRad, so I assume it's sold under several brands. I tested it in the stove pot, and even though it's stamped and sold as a 160 degree thermostat, it opened around 175 degrees and closed around 160. I installed it anyway just to see how it would behave in the system. The car heated up to 175, then it opened and the temp gauge dropped to 160 for a moment then it started to climb back up. With this Murray thermostat, once past the initial opening of the unit, the car seemed to run at a steady and consistent 175 degrees.

 

A buddy mentioned that he had a lot of issues like this with thermostats for his GTO, and he pointed me to the Holley/Mr Gasket thermostats. I found this one with the right diameter and profile - it's not listed for flathead sixes, but I thought I'd try it out:

 

Mr. Gasket 4366 Mr. Gasket High Performance / High Flow Thermostat - 160 Degree (holley.com)

 

In the test pot, it opened promptly at 160 and stayed open to about 150. When I put it in the car, it took care of the running-warm problem and now the temps and patterns I see on the gauge are exactly what I see on my other car. If it's cool outside, the car runs consistently at 160, on warmer days a bit above that but not up to the 170-175 mark. 

 

So it only took 4 thermostats and some persistence to get to the one that works. 

Edited April 25, 2021 by Redmond49

[Dartgame]

The style stat in your link is exactly what I use. These I believe are made by robertshaw. They fit perfectly in my 52 and all the muscle era v8 mopars. They cost more, but you get what you pay...Note the size of the open area as well - lots bigger than the box store equivalent.

[TodFitch]

1 hour ago, Dartgame said:

The style stat in your link is exactly what I use. These I believe are made by robertshaw. They fit perfectly in my 52 and all the muscle era v8 mopars. They cost more, but you get what you pay...Note the size of the open area as well - lots bigger than the box store equivalent.

Looks like the same one I use in my '33 (before they went to the bypass style thermostats on later cars).

[MackTheFinger]

On 4/4/2021 at 6:21 AM, JBNeal said:

heat transfer studies

My older brother's heat transfer studies taught him to not stick his tongue to a hot pressure cooker. ?

[Los_Control]

Guess I would be happy the T-stat opens & closes, be more concerned at what temp the engine ran at while warmed up.

Naturally you want a accurate gauge. I personally want a 180 T-stat ... I think a engine needs to warm up to run well, get all the metals hot and expanded & sealing well.

Just my worthless opinion, 160 is not warm enough. 195 leaves little room for error, 180 is just right. Sad the modern cars require such a hot T-stat.

 

My little pea sized brain, I would think the T-stat itself needs to warm up open & close a few times, get into it's prime working temperature. Then function as advertised.

Throwing it into a pan of boiling water is a good diagnostic tool, tell you if it is functioning.

If you think about it. The T-stat is room temp, you bring the water to boil. The thermometer is telling you the water temp.

The water is hot, but the T-stat and the spring is just now getting to temp.

It would make sense the water is 175, but the metal mass/spring of the T-stat is cooler. It has not had time to evenly reach 160.

 

Again just my opinion and we all have one ... sometimes we are better off to not share them 

T-stat in a pan of water is a diagnostic tool while searching for a overheating issue. .... Not to tell you what temp the gauge works at.

I suppose if you warmed & cooled the pan of water several times and test ... It could tell you something.

Again I prefer to just watch the temp gauge to see if I like the T-stat temp.

[Redmond49]

I have both the service manuals that apply to the P15s. The early service manual for the 1942 through early 1949 models says to run the warmer thermostat in cases where you are making a lot of stops in the driving of the vehicle. The later guidance, from the 1953 service manual for the 1946 to 1954 cars, eliminates that guidance completely and specifies the higher temp thermostat specifically for **extremely** cold climates when extra heat is desired. (I'm thinking places like Fairbanks, Alaska and other places where it can be below zero for weeks on end). Their "best practice" guidance seems to have evolved over that 15 year period. In both cases, I think it's worth noting that the point of reference is the 160 degree thermostat.

 

 

Edited May 1, 2021 by Redmond49