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

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 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

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

maybe replace that one with an OEM one, 16 lb comes out to about 1.1 bar. the OEM specifies a 100 cap (1 bar). worth trying since you've done/tested everything else. the 100 cap will make the cooling system run at a little lower pressure

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL

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.

What is the best source for the 1 bar (14.5 PSI) cap? The ones at Auto HausAZ are 1.2 and 1.4 bar. Napa and AutoZone only show 16 PSI caps (1.1 bar).

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1984 300SD
351,000 miles

There is nothing wrong with a 16 PSI cap.

I'm starting to be fairly suspect of the thermostat as well. While it seems like it's working, it just seems to be doing it's job at a consistently higher temperature than what it should be.

I don't remember the exact numbers, but the temperature at the thermostat housing was lower than on the cylinder head. The highest temperatures were (naturally) on the head.

I'm pretty sure all the air is out of the system, too. I don't get any temperature spikes while driving that would indicate hot air passing by the temperature sensor. While refilling the system with coolant, I initially had the front end of the car up as high as it would go before the tailpipe contacted the ground. (I work at an exhaust shop, so I have access to lifts and such to enable that type of front-end elevation.) I ran the engine till it reached operating temperature (thermostat open) with the heat on high and then lowered the front end of the car until the front tires were about 5 inches off the ground. This enabled me to get a more accurate coolant level reading on the expansion tank, and I burped/topped off the system for about 20 minutes.

Having done that, I'm pretty sure all the air is out. Also, I'm 100% sure the thermostat is installed correctly.

I've been reading a bit about complaints people are having with thermostats not opening at the correct temperature. It seems like many of the 80C thermostats being sold are simply not opening at 80C, but rather at the higher end of the product's "rating tolerances." I honestly can't remember the particular brand of thermostat I've got in my car at the moment, but I think it's a Wahler. Do the OEM Behr seem to be more accurate and consistent at opening right at 80C?

Thanks.

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1984 300SD
351,000 miles

This is also somewhat amusing. Out of curiosity, I've been searching for the 70C thermostat using the Mercedes-Benz part number, but it seems virtually unavailable for purchase anywhere, but AutoZone has the 160F equivalent (71C) available for order direct from their website (Duralast/Thermostat (3586) | 1984 Mercedes Benz 300SD 5 Cylinders 3.0L Turbo Diesel | AutoZone.com). Interesting, but expensive. Too bad it's a Duralast.

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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.

I have a known good Wahler thermostat if you want it. It kept my 300D at 85 degrees under normal operating conditions. (I replaced it on principle in conjunction with a new radiator installation.)

Thanks for that. That actually does offer some consolation, as do all the other posts of people with 300D/SDs running at or near 100C. If I wasn't so OCD, I propably wouldn't even be worried in the first place... Interestingly, the FSM says that the water/coolant mix comes from the factory at 55%/45% by volume, respectfully. While a 5% difference in watever vs. coolant isn't much, it does suggest that running a mix of slightly more water than coolant might be advisable.

I tend to believe that my higher temperatures are part of my increased fuel economy as well. My engine runs great -- much better than any of my friends with the same engines, and I get the best MPG to boot.

I get similar results in my 7.3L F-350 with a higher 203F thermostat. Putting in the higher temperature thermostat in that truck immediately raised the fuel efficiency by 2 MPG.

PM sent.

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1984 300SD
351,000 miles

A higher pressure cap will not make the engine run cooler, it will only raise the boiling point of the cooling system. It also puts unnecessary stress on the hoses, heater core, and radiator. The coolant that's pouring on to the ground from a ruptured hose at 15 PSI is not cooling your engine at all.

I run my signature car and my 2003 Dodge at 7 PSI with no issues at all.

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'83 300D, 126K miles.