I recently bought an '85 300sd with 268k.
The owner clipped some kind of high curb & damaged/distorted but didn't break the radiator.
Then he let it sit for 14 years in Mississippi, full of good coolant the whole time.
I put in a new Nessens radiator & it ran hot, the thermostat never opened.
I bought a Mahle/Behr thermostat off eBay & it got hot again. I put it in boiling water on the stove & it didn't open until it hit 220ºF.
I bought another of the same off Amazon & it does the very same thing.
I bought a Mercedes one from this site. Engine and back of the thermostat housing still got 200ºF hot. I put the thermostat in a pan on the stove & it opened up at 185ºF.
I put it back in the car, made extra sure the system was completely full & it got hot again. The back side of the housing gets too hot to touch yet the front of the housing stays cold as does the radiator.
When I pull the thermostat cover off, it's always completely full of coolant.
My first job at 16 was working at an indy Mercedes shop when this car was new. I've been working on them ever since & have owned quite a few, especially diesels.
What the heck am I missing?
Thanks y'all.

At this point, the easiest thing you can do is burp the system.
Park on an incline (nose up), and turn the heat to max. Start the car, leave the rad cap off. Let it idle until the thermostat opens (around 85C). If there's any trapped air, you'll have bled it off. See how that goes...

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1981 240D 4sp manual. Ivory White.

Good advice. Thank you.
The front of the car is up on stands & I've also tried filling it through the upper radiator hose, into the engine, until it came out the top of the radiator.
The heat works well & it does bring the temp down some when it's turned on.

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85 300SD. 88 300SE. 85 560SL.

No pic…?

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1981 240D 4sp manual. Ivory White.

Pic of what? The thermostats? or the car?[IMG][/IMG]

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85 300SD. 88 300SE. 85 560SL.

You’d alluded to a picture but it seems to have loaded now.
Not sure about the sensors… you need one of the 300D guys.

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1981 240D 4sp manual. Ivory White.

While we're on the subject, There's 3 sensors on the thermostat housing.
What is the broken one in the photo for?
[IMG][/IMG]

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85 300SD. 88 300SE. 85 560SL.

The broken sensor in your picture looks like the vacuum thermostat for the EGR system. If the sensor were intact, it would have two metal ports in a Y shape. One side would be fed from supply and the other go to the EGR. When temp makes the wax melt, vacuum is allowed to flow.

If the EGR is deleted, no need to replace the switch. EGR bypass/deletes all remove the lines on that switch...

That's my somewhat educated answer anyway.

I think the temp sensors on the T-stat housing varied over the years. One electrical sensor prevents the cabin blower from running until a certain coolant temperature is reached, to not blow cold air on passengers. I recall a plastic thermo-vacuum one which was part of the emissions system (now gone). A 3rd one might be to actuate the radiator fan, in addition to the temperature switch at the AC filter/drier. There are also 2 temp sensors on the L side of the head. One is for the dash display and recall the other is for the EGR box. Today, they would have one sensor and share its reading digitally.

When I had a "runs too cool" issue in my 1984 300D, I put the T-stat in a pot of hot water, along with a new T-stat and one for my old Mopars. The suspect T-stat opened a bit too soon and responded sluggishly with temperature, whereas the new T-stat opened snappily at 82 C (180 F). In the car, the coolant now runs at 82 C on the dash gage winter and summer. I used both a thermometer in the pot and shot the T-stats with an IR gun. The later was amazingly close to the thermometer readings, even shooting thru the water at a shiny surface, which amazed me.

My 1985 used to run hot at long stoplights, creeping close to the red line on hot days. I tried everything - water pump (2), used radiator and citric acid flushes, T-stat (2), even removed the T-stat housing to see if a rag was blocking flow. Finally, a new radiator cured it. The old one torn open had a thin film of orangish metal corrosion (?) blocking many channels. But you have a new one now. Trying to purge air by inclining the front and cracking the upper hose while running is a good suggestion. Good luck.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thank you for the relies & help.
The plot thickens. It still gets really hot with the new Mercedes thermostat.
When I turn the heat on, suddenly the thermostat opens, everything circulates, the radiator gets hot & the temp drops to 88ºC.
I turn the heat off & within minutes the thermostat closes, the radiator cools & it runs really hot again. The thermostat stays closed until I turn the heat back on.
What the heck??
BTW, it still has the original fan clutch that is stuck engaged so it pulls a ton-o-air.

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85 300SD. 88 300SE. 85 560SL.

I have seen engines exhibit this hot/cool action with a head gasket leak because it bubbles the coolant out of the head.
But this thing has no other head gasket symptoms. No extra pressurizing, doesn't blow any bubbles, no exhaust or fuel smell in the tank, no coolant smell out the tailpipe & the water level doesn't drop over night.

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85 300SD. 88 300SE. 85 560SL.

The symptoms would suggest a fault of some kind in the bypass circuit. Flow thru the heater could be fulfilling the bypass function.

I concur, I'll pull the whole housing off today but it's pretty fool proof in there.

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85 300SD. 88 300SE. 85 560SL.

Ding Ding Ding We Have a Winner. Thank you Frank Reiner.
I found it. There's a little bypass/bleed passage between the upper radiator hose & the thermostat housing.
It was plugged full of corroded pucky from both directions. I had to literally drill it out. It does same as the holes some people drill in their thermostats.
Like I said, I just bought this car and I have no idea of it's maintenance history but it was full of fresh looking antifreeze.
And the vacuum pump is completely worn out. The search is on & I'm looking to buy one or the discontinued kit.

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85 300SD. 88 300SE. 85 560SL.

Just how hot is it getting? Does it boil over or is it just the dash gauge saying it is hot? Sending units and gauges can fail. If it actually is overheating do not allow it to get above 230 degrees or you could blow the head gasket.

First I would try running it on a test run with the thermostat removed. Just as a test to eliminate the thermostat as a possible culprit. If it still overheats, remove the upper hose from the radiator and start the engine. Rev it up and watch carefully how far it shoots water out of the upper hose. If it just dribbles you have a circulation problem. Even though it had good coolant in it there could have been sludge in the system that has settled in the block and solidified, I have also seen the impellers of the water pump completely rusted off of the shaft. I have also seen head gaskets fail during long storage Just some things to try. Good luck.

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Retired
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"Without Fleet The Pride Doesn't Ride"

1983 300SD 609K + still going strong!
1970 Monte Carlo
1994 Ford F150