W126 gasoline. Running cool all the time. Never gets over 50C. Must be that the stat is open. This doesn't really bother me except that perhaps it might run a tad rich and become less fuel efficient, but I haven't noticed that. Started in the summer and it seemed to be a good thing then. So is this the 'fails open' (would be smart design) normal design or otherwise stuck not closed? Never overheated. Besides a gasket and a new stat, do I need anything else when I replace this?

I'd shoot the upper radiator hose with an IR thermometer just to make sure the gauge isn't the problem.....

May be you have no stat fitted .Have you had the car long ,do you now if it as one fitted?.If you dont have one in i would say it was taken out because its running hot for some reason thus the cool running . It is an easy job removing the stat Some thermostat tops have an O ring around inner face and no paper gasket..

if the engine runs rich for excessive time, you essentially wear down the cylinders as the fuel washes the oil out.

It also doesnt allow the PCV system to work correctly as the oil never gets hot.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Good idea, but everything about it seems to be running cool.

I doubt it is running that rich, just that some computer type thing might be treating it as not fully warmed up. I wonder what the oil temp gets to. You there may be a fear that water doesn't boil off.

There is a stat. About a decade. It used to have a very consistent warm up pattern. You could see the response on the temp needle as if the stat was snapping open once warmed up.

So does anyone know if these fail in the open position? Ever have one fail closed? If the spring resists flow then a broken spring would fail this way, right?

Yes they can stick in open possition if spring is weak over time .Or the bridge that holds the spring come away from the stat .So then it wont shut.

yes they stick.I've learned to run one cooler than stock,not a 160 degree.I also drill one hole in the stat,it speeds bleeding the air out.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

That hole might come in handy for a closed failure, at least you get some flow at the expense of slower warm up.

I have had a Wahler fail open and a Behr fail closed...

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

obviously, you need a thermostat. i had 1 560sl that had a stuck open thermostat the client decided was ok. about 9 months later, he lost the engine. the t/stat had cycled somewhat and got stuck closed, overheating it and pulling the threads from the block for the head bolts. good luck, chuck.

My aftermarket unit (Waller I think) failed open: the arched bracket on the top pulled loose. 50C is only 122F. I think most of the automotive engineering community would say that's really bad in a bunch of durability, economy, emissions, driveability ways. Certainly better fix it before winter.

DG

Well thank you guys!