Thermostat Normally Open?
 

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.

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.

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...

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!