I understand these things are either DOA out of the box or they work.

Has anyone had one crap out after a year or two?

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1981 240D 4sp manual. Ivory White.

Is not the brand name sufficient warning?
Flaky pie crust - OK. Flaky thermostat - not so much.

Just installed a Mahle Behr thermostat a few months ago. I did the boil test, and it opened correctly so I installed it. No issues so far, the car runs around 85c

Yes, I've seen stats get temperamental after a year or so. Not the norm but seen it. Probably is common but we notice it more, first being car focused and second having a temp gauge vs. a warning light.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Noticed some running temps around 78-79, And then around 100-105 underload.
Partially stuck open I’d guess.


Mine is also Mahle Behr. Installed it May 2018. Just started flaking out on me.
The Mercedes branded part is only $15 more. Think I might spring for it.


Ha. I’m referring to the Oxford version, not the brand version.

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1981 240D 4sp manual. Ivory White.

I had a genuine MB thermostat crap out on me after 3 days of driving in my 500SL. Original thermostat finally stuck halfway open (can't blame it after 36 years) and was replaced with a genuine OE one. Problem seemed solved until suddenly very high engine temps that didn't go away with bleeding. I replaced with a Behr and tested in boiling water before installing. Opened up wide. Tested the genuine MB one I pulled and you couldn't fit a fingernail under the plunger. So yes, they can fail after some use.

Don't underestimate a crusty radiator contributing to cooling problems either.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Nothing is sacred...

I suppose I could just try the same brand again.

My system is surprisingly clean. No rust or scale or whatever one finds in there. I also flush out the radiator fins every few months.

I had a year and a half of completely steady temps. Drove one way on a road trip, no issues. Coming back (same route), much higher temps -wasn't a gradual thing. It's like the thermostat got stuck.

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1981 240D 4sp manual. Ivory White.

Not on the Mercedes. On my Chevy Astro Van. I normally test them before I install them. But, the last time I did not test and it did not open when it was supposed to. But, because I had not tested it I watched the Thermostat like a hawk and the Van was still in the drive way.

Unfortunately I had to remove a bunch of crap to get at the Thermostat housing and draining the coolant again was a mess.

The problem was I could not quickly find any of my 2 glass candy thermometers nor my metal digital thermometer. So I did not do the test.

So I had to spend the time looing for a thermometer anyway and had to test the bad one to find out it was not opening till 20 degrees higher then it was supposed to.

A pan on the Stove filled with Water and a teaspoon+ of salt to keep the water calm so you can see the thermostat start to open and fully open and that also raises the boiling point.
You get some wire and suspend the thermostat off the bottom of the pan and you also keep your thermometer off the bottom of the pan and be patient as it heats up and watch.
Thermostats can be plus or minus 5 degrees and that is kind of a normal range for them.

On W123s the trapped air burping issue can have you thinking your thermostat is not opening.

When I worked as a mechanic testing the thermostat before installation was mandatory. The Boss would rather pay you for 10 minutes of testing then the time it takes to drain coolant and replace with another thermostat.

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84 300D, 82 Volvo 244Gl Diesel

I've seen several engines that almost require the OE thermostat. My 00 Cummins temp fluctuates with any (at least several I've tried) McParts tstats. NAPA works well as does Cummins. Fluctuation is so common that there are threads arguing whether or not it is "normal".

I like it rock steady so I don't worry whether the little increase will keep going.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I'm with you there. This one gave me solid performance until it didn't.

Diesel911- I also tested. SOP. That's why I was surprised when it started crapping out after only a couple years of service. Im not familiar with the metallurgy involved but it can't be too sophisticated.
You're in LA, you've driven the grapevine. When all is well, I climb that thing at around 92 max. When it isn't, it climbs to about 105 right around the time I crest the hill.

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1981 240D 4sp manual. Ivory White.

It is not the metallurgy that is the issue. It is the Wax(?) capsule that goes bad.

I lost an Engine on a Chevy Chevette on Grapevine. Only Fan Belt I have ever had break on me.

Story: I bought a new Thermostat for my Volvo Diesel from Autozone. On the Box it had made in Israel but stamped on the Thermostat was made in Germany.

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84 300D, 82 Volvo 244Gl Diesel

Reading threads over the years some peoples cars indeed seem to run hotter then others do and it is not always related to the thermostat.

Without having the Car to investigate yourself you can't tell if it could be some of the other issues like the fan clutch or the AC Condenser or Radiator is full of Bugs and dirt and so on.

I also thought that the Electric Coolant Fan was supposed to go on at X amount of coolant temp. But, I have never read up on that to know for sure (note I have no working AC).

When I tested the Electric Coolant Fan on mine direct to a Battery I found it barely turned. I fixed it and then never bothered to it back on.

One of the issues with a thermostat from the Mercedes Dealer is how long it might have been sitting on the shelf.

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84 300D, 82 Volvo 244Gl Diesel

I use motorad, as always ran lower temp,gas and diesel, being in the mountains

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

I have not personally experienced funky T-stats thought I know guys who have. This is, or can be, an "issue".

Dan

Great point re: sitting on the shelf. Didn’t know about the wax, did some interesting reading on the subject. Quite an elegant design...

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1981 240D 4sp manual. Ivory White.