A small item among the many things that need upgrading in my 300D was the issue of running cold. My SDL had the same issue, never going much above 70. Upon removal, the thermostat on the latter obviously wide open.

Not so on the 300D. Looks solidly closed. I quickly saw that the gaskets that came with the Behr/Mahle unit from Pelican are not sufficient, not even right for it.

What I really don't get is the bypass part. I can see the usefulness of some bypass, such as the little bumpout cage thing with the stainless steel ball on my 603 stat but this one looks wide open all the time. That can't be right, otherwise you'd get what I have now, an engine that stays at about 70, and takes a while to get there. Many of you have seen this already, sorry to insult your intelligence, the badly drawn arrow points to the opening, nothing that I can see turns off the flow, ditto on the outer housing.



This photo also has the new thermostat, which does seem to be correct, and the gaskets that came with it.

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1986 300SDL, 362K
1984 300D, 138K

the hole you see gets very small inside the housing and there is a cross section of it on page 2 of this pdf:


https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/617/20-005.pdf


it's so small it could actually be plugged and yes your gaskets don't look complete.


Look for pitting on the sealing surfaces of the housing if it's pitted you may have to use rtv

Thanks. That's a good graphic. I like the idea of some small bypass, the method used on the SDL stat is good I think. Word is that it allows flow when the engine is off but the action of the water pump will close it off. I think that's right. Mxfrank said it's called a 'ball valve or jiggle valve' and shared this link about them:

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

Somehow mine is letting too much through, always stays cold. Who knows, the stat might have flaws I can't make out, maybe the old gasket was letting too much through. I'll get the correct gasket and see how the new one works.

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1986 300SDL, 362K
1984 300D, 138K

First of all, it's not a bypass, it's a vent. It's there to allow air to burp out as you fill the system. It's always open, and you probably have a tiny bit of coolant that leaks through there all the time. It's not clear to me that it has any positive effect when the system is in normal operating state, but according to the page posted by christuna, it allows venting while the engine is running (which begs the question, where would air come from if the system is full?) This isn't very different from some modern systems that rely on pinholes in the thermostat for venting:

https://www.coolcatcorp.com/thermostats/gatesairbleed.jpg

Whatever the strengths/weaknesses of the design, it doesn't explain your problem. Are you certain that the engine is really running cold? Check it with a thermometer. If it is, replace the thermostat as a matter of course.

Good points. The car looks nice but it's about 35 years old. The gasket above could be broken into pieces pretty easily. If the temp gauge is faulty, that would be good to know.

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1986 300SDL, 362K
1984 300D, 138K

Could it be there to help keep the pressure differential between ambient air and the internal system in check?

I was under the impression the expansion valve did that. Does it bleed to atmosphere?

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-Typos courtesy of my mobile phone.

No. All it does is allow air to burp around the thermostat when the system is initially filled. After that, it leaks a bit of coolant, but this is neither useful nor harmful. This graphic explains it:

https://www.coolcatcorp.com/thermostats/air%20trapped.jpg

If there was no air bleed during the initial fill process, the thermostat wouldn't open until the engine was hot enough to cause damage. With the air bleed, you can be sure of having fluid at least to the level of the thermostat.

I have no idea what you mean by an expansion valve.

Once the system is full, there's really no purpose to it, which is why the jiggle valve is a better solution.

Engine running too cool: most common is a stuck thermostat. A bad temp sender can give a false reading. Fan stuck "on". Heater valve open during warm up. Wrong thermostat installed. Thermostat flange drilled. The airbleed only leaks a bit...maybe a couple of liters a minute...not enough to be significant, unless someone has reamed it open. In a gas engine, overadvanced timing, but unlikely in a diesel.

That was my take also. The SDL is the first motor I've seen with a thermostat on the lower radiator hose. Seems like it would make for difficulties in water getting into all the necessary spots. On the upper mount, also makes sense, but seemingly less crucial. Pretty sure many or maybe most all of the cars I've owned had the upper mount thermostat with no bleed provision. I've read several times with the SDL/603 that when filling, be patient. Fill it up, wait a half hour and top it off again, do that for a while.

What you say about 'heater valve open during warm up' might be an issue. My heater is always full hot. I've had other issues that need attention so I've let that one go for now. I have, of course, heard of climate control issues. And I know a little about the monovalve, I put in one of the nototious aftermarket units in my SDL and remarkably saw an improvement. I think it was an earlier gen of aftermarket units, my parts guy said it had been on his shelf for about 15 years. What I don't know is the what and where of the valve that allows hot water into the heater core. Not sure if the monovalve is where that happens.

I was a bit spoiled with my years with the E30s. The heater on those cars is remarkably straight forward and reliable. You can get heat where you want it, when you want it.

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1986 300SDL, 362K
1984 300D, 138K

Almost all thermostats have some means of bleeding air. Sometimes it's a tiny notch or hole in the the poppet, so it can be hard to see. When the thermostat is located on the cool side (as with all Mercedes applications), it's less critical because the vent only bleeds air from the lower hose. And since many of their designs T the expansion tank off the lower hose, the vent only bleeds half the volume of the hose. It's nice that the engineering is so thorough, but a lot of folks still pull the upper hose and fill the engine directly.