W123 300D Does the Coolant Thermostat Orientation Really Matter?
 

Or is it superstition? The thermostat seems to be a simple spring and valve why would the orientation matter? Is it a mechanic habit from thermostats that do matter? In fact 1 brand (waler sp?) doesn't even have an orientation mark.

I've put them in without checking and everything seems fine temp wise. Maybe it will come back to haunt me this summer?! Should I remove mine and spin it around till the mark is facing up?

Any kind of technical explanation of the thermostat mechanism and why it would need to be oriented up is most welcome!!!

(I'm not asking about the direction, that makes perfect sense to me)

There is a spring that actuates the valve. It is on one side of the stat and not on the other. The spring is supposed to be on the hot side so it will open at the temp needed. IOW yes, emphatically it matters.

The diversion stat used in most benzes is even more important since when it is open it channels water to the radiator and if closed the water goes directly to the head to get the engine up to temp. faster.

Things in a car are all parts of systems. Use them as designed for best performance.

Makes sense that the spring needs to be in the correct direction (facing the block). That I understand.
What I'm asking about is the rotation of the thermostat so that the mark is facing towards the sky. Sorry if my initial question was unclear.

https://d3nevzfk7ii3be.cloudfront.ne...hytnBpCl.large

Why?

It does make a slight difference but not much. The "bleed" hole (or holes depending on manufacturers) are there to allow any air in the system to escape into the radiator after the engine cools. They are positioned to be in the highest point for maximum effect.

Not much difference but then again, you ARE dealing with Germanic engineers.

I looked in the FSM and it does say to oriented it according to the mark. Besides those thermostats with a small vent on them I think there could be another reason.

On some of the Engines there is a temp sending unit/probe on the upper front portion of the thermostat housing (the part that the lower hose attaches to). Lining up the thermostat properly possibly keeps the arched frame of the thermostat out of the way of the probe if it happened to be long enough to make contact with the arch.

When I look at the OM 615 manual on CD it shows the temp sending unit. On my 84 300D that front Thermostat Housing has a spot on it where it looks like something should go but it is not drilled and tapped. It is a balk cylindrical indentation.

Ha now that actually makes sense. Finally an explanation Thanks! Also makes sense why it wouldn't make much of a difference especially to operating temps

Except that it makes no sense in the case of the OM617 engine, which features a vent in the thermostat housing itself.

Considering the same thermostat is used in a wide range of engines it DOES make sense from a manufacturing view point.

https://www.ebay.com/itm/Thermostat-New-OEM-OM616-OM617-Diesel-W115-W116-W123-W126-/372797371840

Forgive another dumb question but where is the vent on the OM617 housing? Also would that vent on the housing make the vent in the thermostat redundant in that case?

See page 2 https://tinyurl.com/yn39fmz3

There is a passage in the housing just outside the perimeter of the thermostat flange. (That's why there is a small hole in the thermostat gasket.)
Not saying that they never existed, but I have never encountered a OM617 thermostat that featured a vent hole. If one was so equipped, it would be redundant.

You can find all your answers in my E-book:

https://www.coolcatcorp.com/thermostats/Air%20Bleeds.html

The bleed hole is there to allow air out of the system when it's initially filled, that's it's only purpose. Some manufacturers build bleed ports into the housing. But the thermostats are generic items, so pretty much every thermostat has a provision for air bleeding.

The thermostat is powered by a wax motor, which is the copper cylinder in the base. it contains a pellet of engineered wax, which melts at a specific temperature. When the wax melts, it expands, and that's what pushes the valve open. The spring returns the valve to the closed position when the wax cools.

rs: See picture in post #4. Your picture?!

The same feature drives pop-up turkey roasting timers!

I've also never seen one with an actual vent hole hence the initial question about weather orientation mattered on the OM617... Thanks to everyone for shedding some light on this and being very helpful and answering my questions!:D