Ok here's a shot out of a cannon. So my 1983 240d runs a little hot even when its not summer time, I'm running around 90-95 degree C instead of being around 80 to 85C. So a few months ago i bought a new thermostat since its so easy to replace, and for everyone's info when we got the car back in 2014 the entire cooling system was replaced with new parts. However I don't know if the wrong thermostat was installed or if it became defective after a few years because now I'm idling at stoplights and driving down the freeway around 90-95C as stated before. What is peculiar to me is that I bench tested the new thermostat today and it began to open at around 190 to 195 degrees F instead of 176 degrees F.

190-195F=87-90C, 176F=80C I don't think I'm being to unrealistic when I say I would like the car to run a little cooler year round closer to what factory specs say. And I know the thermostat is not magically going to open all the way at 80C, if I'm not mistaken it should open +/-2 degrees C from 80C. But I was just curious what everyone thinks is wrong since my weekend project got really confusing, the thermostat I'm using is a Stant that is rated to 176 degree F or 80 C and that is the new thermostat I bench tested that is acting strangely.

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The thermostat is designed to help the engine properly warm up, not to control the max operating temperature of the engine. Unless your thermostat is not opening at all I doubt that it is causing your overheating problem. Make sure your water pump is not leaking and the belts are not slipping. You should also make sure the engine fan is working properly. Since your cooling system parts seem to be fairly new hopefully your radiator is not plugged up. A product known as “water wetter” may reduce the operating temperature some.

If you got it locally you could take it back for an exchange and say it starts to open too late and hope the new one works better.

This has nothing to do with testing the Thermoset off of the vehicle but after you drive awhile you might feel your Oil Cooler Hoses and see if they are hot indicating that hot oil is actually circulating through your oil Cooler.

If you have cold winters that thermostat would be good to have during winter.

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84 300D, 82 Volvo 244Gl Diesel

"So my 1983 240d runs a little hot even when its not summer time, I'm running around 90-95 degree C instead of being around 80 to 85C."

Since the manual on a 300D has that it is safe to go to 100C (assuming the proper coolant and concentration) he is not actually overheating. He is just nervious about sometime running at 95 C when it is not yet hot.

So he wanted a cooler normal running temp and presumably he bought an 80C thermostat but when tested it is not starting to open till 90C. He is not going to achieve 80C normal running with an thermostat that just starts to open at 90C.

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84 300D, 82 Volvo 244Gl Diesel

To the OP. Did you replace the Fan Clutch or check it to be sure it is working?
Did you check the coolant temp with a thermometer to see if gauge in the instrument cluster is accurate?

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84 300D, 82 Volvo 244Gl Diesel

This is very simple. The thermostat establishes the minimum running temperature. So if your thermostat begins opening at 90C, that's what your minimum operating temperature is going to be. Although Stant is usually a reliable brand, yours is bad. This board is capable of extending the discussion into infinity, but until you replace the thermostat, noting will change.

240D doesn't have a fan clutch. The fan is just fixed to the pulley and rotates all the time

Yes I have used an infared gun to check the engine temp to make sure its not overheating. I can't remember exactly what the temp was I'll check again today some time. Also I reinstalled the fan shroud recently and that made a little bit of difference at idle but one day I'm going to install a fan clutch from a 300sd since I was recommended to do so by an expert on the w123.

all the belts are tight and the water pump is not leaking since there are no leaks underneath my vehicle. Im just wondering if I should buy another thermostat locally that opens almost fully around 80C instead of 90 C when I bench test it. We bought that Stant thermostat a few months ago off rock auto so can't do anything about it now lol. Also I've brought this up before in a previous thread but I have no wiring or head sensor on my 83 240d to control the aux fan if the car overheats. The only thing that turns the aux fan on is the AC system, I thought I had an overheat protection circuit.

The thermostat rating is the start of operation temperature. The thermostat should begin opening at 80C, it will not be "fully open" until several degrees above that point. Keep in mind that MB uses a "bypass" cooling system, so the thermostat is also closing the bypass port. A bad thermostat will leave that port uncovered and you'll be recirculating hot water through the block.

Stant is a relatively cheap brand. You're welcome to keep giving them your money. If you don't want to go OE, use one of the OE suppliers - Behr or Mahle (not Meyle). They cost what, $10-15? Bench test before installing.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

As I understand it the bypass system allows coolant to circulate in the engine and radiator when the thermostat is closed. When the temperature reaches the rated open point of the thermostat the coolant circulates in the normal way by the water pump sending the coolant through the engine and radiator. A tip I learned several years ago is to turn on your heater if your car is overheating. The heater core (a small radiator) allows extra cooling for the coolant and while the interior of the vehicle may get warm, this should help lower the operating temperature of the engine.

Again, no matter how long this thread drags on, the problem won't go away until you replace the thermostat.

A fan clutch won't make your engine run cooler. The purpose of a clutch is to decouple the fan at high RPM to conserve engine horsepower. You may notice more engine responsiveness, but no cooling benefit.

Update everyone, so I tested the car at idle with an infared gun and the temp sender was at 70C at most and the rest of the head was around 50-60C. The thermostat was 40C coming out. A big problem was the wire coming off the sending unit to the left fender well had two breaks in the insulation and was damaged after 30+ years of engine heat. Tested it with an ohm meter and the resistance was quite high so I ran a new wire to the temp switch through the sheathing for the glow plugs for neatness. (my father put the wires for the glow plugs and temp sender wire in a giant heat shrink tube to keep them somewhat protected years ago.) After soldering on the mercedes connectors from the old wire I did and ohm reading again and had alot less resistance compared to the original wire. So now my temp gauge is reading alot closer to 80C now, it never goes above 85C at idle so now I'm going to do a road test soon and see what happens. Also I put in a new temp sender unit like 2 months ago, and yes a clutched fan may not do much for cooling but the minimal performance improvement may be worthwhile.

Good diagnosis and repair.

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