I have re read your post and it looks like you have done your homework before you posted.

Is your timing retarded or perhaps your car has lots of blow-by heating up the internal engine.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Ahh, okay. I thought that might be what you were getting at, but I wasn't sure because in the the FSM that SD Blue posted, it specifically says that, "It is therefore wrong to remove the thermostat for better cooling of the engine." While I never planned on permanently or semi-permanently using the vehicle without a thermostat, I wasn't sure what the objective of testing the temperatures without a thermostat would be.

The place I like to focus my IR testing is right at the area around the engine temperature sensor. Not only is it more accessible for the IR gun, it also seemed logical to test at/near the sensor reporting the temps.

From what I can tell, the engine seems to be running very efficiently. About 6 weeks ago, I rebuilt and balanced the injectors (using Monark nozzles, and getting all injectors within 3 bar of each other). I'm currently averaging 28 MPG with a 60/40 mix of freeway/city driving. I have not yet emptied an entire tank on the freeway, so I can't report on highway-only efficiency. My Mercedes-only (diesel specialist) independent mechanic tells me he's very impressed with the low amount of blow-by from my engine, especially considering its high miles.

I personally don't know how to measure blow-by and I haven't done a compression test, but the car has good power, doesn't smoke, and when removing the oil-filler cap on the valve cover with the engine hot and running, there is less blow-by than any of my 3 friends with the same engines in their cars. (How scientific, right?)

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1984 300SD
351,000 miles

Lets see:

You have a car that runs strong, gets good mileage, doesn't smoke, and doesn't overheat (you're not overheated until the gauge hits the red-see page 54 of the owner's manual). I'm failing to see a problem.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

True, true... I knew I'd probably get that response at some point, and deservedly-so. I mostly feel the same way, but I've got a pretty bad case of OCD, so knowing what "ideal" is and seeing that I'm not there really drives me nuts! And knowing that it's better for diesels to be a little too warm rather than a little too cold is a good thing still doesn't help, unfortunately.

In my experience, German cars' temperature gauges (like VW and MB) are much more sensitive than their American and Japanese counterparts. They actually go up and down as the engine temperature does! The only times I've ever seen in an American temperature gauge do anything other than stay at the dead-center middle is during warm-up and overheating! My '81 VW Rabbit Pickup always ran one-and-a-half notches hotter than center (similar to what is happening in my 300SD) and it drove me nuts in that car too! Perhaps American-made vehicle owners are happiest with the "ignorance is bliss" approach. The Fords, Mazdas, and GMs I've owned always stay right in the middle of the temperature gauge. I never worried about temperature in those vehicles. Has anyone else noticed anything along those same lines?

I've only owned the 300SD since last September, and I figured that the very cold temperatures we had here last winter were the main reason I hadn't overheated. Now that the weather is getting warmer and the hot summer is on its way, I wanted to address this "issue" before it gets too hot here.

The last thing I'm considering trying is a Shout/citric acid flush. I'm not sure of the condition of the inside of the engine block's coolant pathways, but as I mentioned before, the coolant from my original flush was very clean. What I forgot to mention is that that same coolant was green. I don't know if it was conventional green or an extended-life green. Either way, it wasn't the Zerex/MB coolant it should have been, and I don't know how long it was driven that way.

Is the Shout/citric acid flush very effective? Has anyone seen measurable results from such a flush?

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1984 300SD
351,000 miles

If it is any consolation, I have the exact same vehicle and it, too, runs close to 100C. whenever it feels like it. I, too, have replaced everything but the sending unit. Nothing affects the result.

My infrared thermometer does show a slight benefit............93C. on #6 when the gauge shows 100C.

I'm tired of worrying about it..........the engine doesn't care if it runs warm (actually preferable for fuel economy).........why should I?

With a 50-50 mix of Xerex and a pressure of 15 psi, the boiling point is 128C. and the engine can run safely right to 120C. without any issues.

American cars don't usually have "real" gauges. German cars generally do. A while ago, American car manufacturers figured out that many American drivers were worried when the gauges in theirs cars showed fluctuations in temperature and oil pressure, which were normal responses to changes in load and operating conditions. So, rather than attempt to educate their customers, the OEM's came up with gauges that hovered in the middle of the range most of the time. German manufacturers figured their customers were well enough informed to know that engine temperatures and oil pressures vary during normal operation.

BTW, my 300D normally runs about 100C, and has since I replaced a bad thermostat about seven years ago. Before that it wouldn't get above 60. I just replaced my water pump today (leaking badly), so we'll see if that changes the running temperature by much. I also discovered that my fan clutch is seized. Will work on that later.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

Well, if nothing else, I'm glad that I'm not the only one experiencing these same things. I know 100C isn't overheating, and if my temperature stays consistent throughout the summer, I probably shouldn't worry too much. I wanted to get the air conditioner working for this summer, and knowing the load it puts on the engine, I wanted lower engine temps to start with.

I guess I'll keep an eye on the temperature as the weather gets warmer. In the meantime, if there are any other suggestions out there, I'm all ears. Thanks everyone for all the suggestions and help thus far.

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1984 300SD
351,000 miles

There was one unusual event you reported in my opinion. You stated when the heater is first enabled it lowers the temperature read. Then shortly after with the heater still running the pre heater engagement temperature is returned to.

This to me indicates the new thermostat more than anything is causing the issue you are seeing. Think of it this way. You turn the heater on. This scavenges heat from the engine coolant in the engine. The thermostat shuts down to restore the heat so the temperature rises until the thermostat opens again.

Since you purchased a thermal reader check the operating temperature in the vicinity of the thermostat housing. With any flow issue or inneficiency the temperature would be held down with you continiously extracting heat from the heater core. Making the temperature remain lower .

Far too many site users have experienced using our heater cores to pull the temperature down when say having a bad radiator in a car or perhaps low coolant level but usually with low coolant level the heater core will not produce heat. Since these cars have a separate heater flow set up they might be differant.

You may want to seriously elevate the front of this car. Take off the upper rad hose and see if you can burp out some trapped air. This can have effects as well. Top off with coolant at that point if the level drops in the hose. This is the only way to properly fill these systems anyways with some assurance all the air is out. Trapped air can impact internal flow patterns inside the engine block and produce diffferent temperatures in different areas than the manufacturer intended by design. My last thought is perhaps a collapsing suction hose on the waterpump. Just feel it to make sure it is not gone far too soft with age.

What is unusual about that? That would suggest that the temp is being thermostatically regulated.

If the coolant system is overloaded the removal of heat by the heater core would result in a continious temperature drop remaining at the indicated lowered temperature. Yes what is occuring does point to the thermostat working but not properly I believe.

For some reason it is regulating at a higher temperature than desired. As mentioned could even be as simple as trapped air. I do not think the new thermostat is bad particularily but do suspect the temperature readings at the thermostat housing will be less than at the area of the temperature sensor on this particular car. If not the thermostat is bad or installed backwards.

I appreciate all the help, advice, and suggestions so far.

I'm going to try a known-good thermostat and see what happens with the temperature. While I'm not overly concerned with the current temperatures, I'm hoping to get it down at least a little.

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1984 300SD
351,000 miles

I figured I should update this thread.

I purchased a known-good thermostat from qwerty (thanks) and installed it last Saturday. I have been using the car every day for a week now in a wide variety of temperatures (28F to 75F) and at the temperature has never risen above the 9 o'clock position on the gauge (about 88C). The gauge typically shows between 80C and 85C, so I'm very happy to be back to where I should be with regards to engine temperature.

So, it appears that the two brand new thermostats I purchased and tried initially were both operating at the upper end of what I assume is their margins of error for manufacture.

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1984 300SD
351,000 miles

Thanks for closing the loop on this one. It is frustrating that we cannot trust new parts to operate correctly. I am glad you found the solution.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).