Over cooling type issues w/ 1979 300d non turbo
 

My 300d has a lot of trouble warming up all the way as of late. It had been sitting for about 2 months but now I'm driving it again and temps are in the high 30s outside.

It takes about 10 mins to even reach the first notch on the temp gauge. I think that notch supposed to be 60°C. And it really doesn't get much better than that, it never reaches the 80° mark and usually just hovers in between the first notch and the 80° notch, so somewhere around 70°

All my other om617 sit at about 85 - 90 degrees.

Is the cold weather doing something weird to my coolant? Previous owner could have put too much water in it, I dunno if that would do anything.

My thermostat stuck open?? I just feel like it should reach full op temp even if it was stuck open , just take a lot longer, but it never does.

Thermostat... definitely. I had the exact problem recently. If the t-stat is stuck open the engine won't get to op-temp.

How does it stay at full temp once a good working thermo opens up after it gets to 80 then??

Not saying don't agree with u just curious

The Mercedes engines use a bypass style thermostat. When "closed", it closes off circulation through the radiator and only allow recirculation through the block. If the thermostat is stuck open, or stuck partially open, the recirculation won't happen and you'll be circulating water through the radiator. Once enough heat has been built up in the entire cooling system, then the block temp will start rising.

Time for a new thermostat.

Your theory (or feeling) is quite flawed. The thermostat, when functioning correctly, restricts flow to the radiator in order to "enforce" a specified temperature. If the engine reaches "full op temp" with no restriction, your cooling system has no reserve cooling capacity.

It's not appropriate to think of a thermostat as simply "open" or "closed." It operates through a dynamic range in order to maintain a minimum temperature.

Ya but what I'm confused about is that shouldn't after a while the coolant temp should reach full op temp even when stuck open?? Just take a very long time? But it never does.

I'm going to replace the thermo for sure and I agree that it will fix my issue.

Im just curious and confused as to how it could not heat up all the way after a long time

I understand how the thermo works, it stays closed until it reaches the specified opening temp of that thermostat. So in this case, at 80° it opens up at allows coolant to flow thru out the whole system.

So how at this point does a car with a proper thermo maintain its full op temp? Why doesn't it go back down like mine that is stuck open does??

The lower bolt on the T-stat housing tends to be corroded and stuck in there. It's only an M5 or M6 and it's real easy to break when you try and take it off. What ever penetrant and tricks you do to loosen up stuff, start early.

Two words: heat transfer. That's why the cooling system exists.

The system does not "heat up all the way" with a functioning thermostat. Why should it do so without one?

That might be overly optimistic. The thermostat continuously adjusts to regulate coolant flow to maintain the specified minimum temp.

An 80° thermostat starts to actuate at 80° but does not fully actuate until about 95°. The thermostat does not just "open" and call it a day; it continually adjusts coolant flow as necessary to maintain the specified minimum temperature.

Man I love how people on forums feel the need to prove to the world that the guy asking a "stupid" question is an idiot... And then answer the question. Lol... U just found it neccasry to throw ur little jabs into both of ur comments.

There are limits on "failure to recognize the correct answer when told."

You exceeded the limit.

Hijacking the thread a bit here, but I'm having the same issue. Ordered a new thermostat already, but will driving with the old one until it gets here hurt anything?

No.

Short term, no problem.

Long term, could cause some sludge build up because the oil won't get hot enough... at least that's my understanding.

The thermostat is a variable device - it opens and closes as needed to regulate coolant temperature. They rarely stick closed, usually fully open or work partially but won't close all the way, which will cause a slow warm-up but no overheating.

If it is absolutely to cold to drive with the T-stat stuck open, place a piece of cardboard in front of the radiator. Blocking airflow thru radiator will increase engine temps. Just be sure to watch temp gauge, and open/close airflow blockage accordingly, Be careful not to overheat engine. Up here when it gets down below about -10F we have to block radiator airflow, to get our diesels up to temp. Mostly on the farm equipment, cummins, and duramax's. The MB does not get driven in those temps.

Your comment is uncalled for. You should be thankful someone took time to answer your question. tangofox007 answered your question in his first post, yet you failed to read and process that information and stated the same incorrect T-stat "theory of operation" again.

A very good chance that replacing your T-stat will fix the problem. I had the same symptoms in my 1984 300D. In winter, it was running at 60 C. I tested the T-stat in a hot pot of water, against a new T-stat, other T-stats, an IR gun, and a mercury thermometer. It seemed to work OK, but careful testing found it opened sooner and more sluggishly (less "gain") than the others. After replacing, the engine has run 80-84 C on the dash gage, winter and summer.

BTW, many people think the T-stat regulates at an exact temperature setpoint. Not true. The regulated temperature varies with heat load, so it will settle a bit higher in the summer. That is normal for such a "proportional-only" control device which engineers term "proportional offset". The reason is that at higher heat load, it must open further for more water flow. Doing so requires a slightly higher temperature. Its response to temperature is termed "proportional gain", and yes it should settle at a steady temperature, as others stated, i.e. it doesn't constantly open and shut (though it would if the gain was too high).

My impression is that many of us here consider it very worthwhile to test a new thermostat by placing it in a pan of water, and then heating the water up while measuring the water temperature with a thermometer. My experience has been that the thermostat STARTS to open at around 80 degrees C, and continues opening further as the water reaches higher temperatures. I do this with every new thermostat.

I will ABSOLUTELY second this suggestion. I got burned by an OEM thermostat for my SL recently. Being brand new, I ASSumed it was fine, but continued overheating. When I pulled it out to test, it was open maybe 1mm at 97˚C. The new Behr I bought to replace it was wide open by 90˚C. Both had an opening temp of 84˚.

Yes, Marshall nailed it. I use both a digital thermometer and a caliper so that I can measure both initial opening temperature and full-open temperature. Note that the full stroke specification is found in the Technical Data book, and not in the service manual (that I can see).