Why does a thermostat fail?

[ward cleaver]

I am curious about the failure of the thermostat on my 18hp Evinrude. It did not overheat in a barrel but when I took it out on the lake and ran at full speed it did overheat. So I pulled the thermostat and it was almost filled with sand. I cleaned it and tested it in hot water on the stove and it never opened up any more than its original slightly open position even at temperatures around 180 degrees. The impeller looks good but I will go ahead and replace it as long as I have it out and keep the old one for a spare.

Question is what would cause the thermostat to fail? I understand it is a bi-metal design but just looking at it, I can't see what could go wrong.

 

[Silvertip]

Re: Why does a thermostat fail?

Why does an engine die, why do people die, why do radios die, or for that matter why does anything fail. Thermostats are not unique. They, like anything else man made or hatched can fail -- and they do. As one very wise person once said -- "Stuff happens". You don't have to overheat an engine to cause a stat to fail. They just one day decide to quit working.

 

[Cofe]

Re: Why does a thermostat fail?

I have a uncle in his 80's now.
He was a avid boater all his life.
He said that whenever he bought a small outboard, he always removed the thermostat. "Sure it will fowl up sometimes", he would say. But then he would just decarborize it. He never had an over heat problem.

 

[Silvertip]

Re: Why does a thermostat fail?

But then I'll bet he never hear of "cold seizing" did he. Thermostats control engine warm up. If you remove the stat and hammer down a cold engine, the pistons will heat up quickly while the cylinder walls remain cold. The couple thousandths of an inch of clearance between the two closes up quickly and you have a stone cold engine that is seized. A thermostat that fails closed generally causes overheat at slow to off idle speeds. One that is stuck open results in the potential for cold seizing, poor idle, higher fuel consumption, plug fouling and carbon build up. A bypass valve that is stuck shut causes overheat at wide open throttle. The manufacturers put the stat there because they consider it essential for the longevity of the engine, not because they just wanted to add more parts.

 

[Cofe]

Re: Why does a thermostat fail?

Thanks Silvertip for the info on "cold seizing". I have done some research, and now understand the concept. I just wish that thermostats were more reliable. I run one engine on a river quite often, and the river has lots of trash, depending on how much it has rained ect. I hate keeping a constant watch on the tell tale while trying to navigate. Some of the newer river boats sport a Honda twin with a self contained cooling system. Looks kinda odd to see a radiator up on the motor.

 

[dingbat]

Re: Why does a thermostat fail?

Question is what would cause the thermostat to fail? I understand it is a bi-metal design but just looking at it, I can't see what could go wrong.

The opening and closing of a thermostat is regulated by thermally active materials. The operating temperature can be controlled by material selection (more or less reactive material) or by changing the mass of the materials.

Overtime, the constant expansion and contraction weakens the material and permanent deformation of the material soon follows. At this point the thermostat is either out of calibration or outright fails

 

[ward cleaver]

Re: Why does a thermostat fail?

Silvertip, thanks for explaining the cold seizing problem. I was thinking about running it without the thermostat to flush out the cooling system. Now I realize that a hose would do just as good a job at no risk of damage to the motor.

Dingbat, thanks for the failure info. Makes sense. I assume the thermally active material is on the end of the thermostat and when it heats up and expands it presses up on the shaft allowing water to pass thru the opening at the top. And the spring makes sure it closes when the heat is taken away. Interesting that mine has failed partly open, or is that the cold state of the thermostat?

 

[Israel Hands]

Re: Why does a thermostat fail?

I don't buy into the "cold seizing" theory 100%. My 87 Looper has the thermostats mounted in diaphragms. At 2800 rpm and above the water pressure pushes the thermostats out of the way and the engine temperature drops, almost instantly, to the bay temperature. So it was designed to run without thermostats at high speeds. Maybe some engines are different?

Israel Hands.

 

[4JawChuck]

Re: Why does a thermostat fail?

I don't buy into the "cold seizing" theory 100%. My 87 Looper has the thermostats mounted in diaphragms. At 2800 rpm and above the water pressure pushes the thermostats out of the way and the engine temperature drops, almost instantly, to the bay temperature. So it was designed to run without thermostats at high speeds. Maybe some engines are different?

Israel Hands.

From my observations I would have to agree, I have an automotive temp sensor plumbed into the cylinder head on my Merc outboard and it rarely gets above 100F. My thermostats are also of the diaphram design and open completely above a certain rpm. My professionally calibrated hand sticking into the peehole water stream at WOT tells me the water temp never gets above 80 F unless I am idling for long periods. The Merc thermostat in my engine opens at 143 F, so basically the thermostat is there to prevent overheating at idle.

BTW thermostats use a wax pellet that melts at the opening temperature which allows the spring to open the valve.

 

[BLU LUNCH]

Re: Why does a thermostat fail?

The ones I pulled Saturday from my Mercury 150 looked good buy boiling them one did not open fully, no clue............

 

[Cofe]

Re: Why does a thermostat fail?

Had a engineering guy tell me---
If you're going to run without a thermostat, modify the thermostat by cutting the two upper straps and remove the guts. Reinstall the modified stat with the gasket. This way there will be a limited amount of water circulating thru the head. (Like the thermostat is wide open all the time)
Also....Let the motor warm up good before going to WOT or Light Speed (whichever comes first.)

 

[kenmyfam]

Re: Why does a thermostat fail?

Personal opinion.
If it came from the factory with a stat then it needs one.

 

[ward cleaver]

Re: Why does a thermostat fail?

The heat sensing end of the thermostat is the lower end right? And the valve opens when the shaft moves up. The cold or normal state is fully closed and the hot state is fully open at or above 145 degrees. So does the spring open the valve or close the valve?

 

[seahorse5]

Re: Why does a thermostat fail?

So does the spring open the valve or close the valve?

The spring pulls the thermostat shutter closed as the heat sensitive material cools and retracts.