Diesel coolant temperature running hot

I replaced the thermostat in my '85 300D turbo because the car was running too cold - typically at 70 degrees C, and it would only climb to 80 degrees when sitting in traffic. I put in an OEM MB thermostat rated for 80 degrees C, and refilled the system with yellow MB coolant, and let the car idle with the reservoir tank cap open to purge any air trapped in the system. The temperature gauge climbed to 90 degrees, and after driving the car for a while, to a steady 95 degrees. I let the car sit overnight, added coolant to the reservoir to the proper level, and drove the car at various speeds for about 1/2 hour. The coolant temperature remains steady at 95 degrees C. My '83 300SD with the same engine indicates a steady 90 degrees C. The engine in the 300D does not appear to be running any hotter and has good power. Should I be concerned about this? Is it possible to get a bad thermostat that open at too high a temperature? Should I try another one? Tip for replacing the thermostat: Place a plastic bag over the alternator so that when you unbolt the thermostat housing, the coolant will not drench your alternator. This happens even after you drain the radiator. I already learned that lesson on the 300SD. Is there a coolant drain plug on the engine block?

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Diesel Engine Specific Aid

engine block draincock

according the the chilton book it does. on the right hand side. will have to investigate further.

Engine Block Plug, Coolant Temp

I also recently performed a thermostat replacement and coolant flush with my 1982 300D. (I thought it was previously running a bit on the hot side though, not the cold side.) From what I gathered from the various sources I checked, it seemed as though the engine block drain plug is "more trouble than it's worth" to deal with, as more often than not it's frozen in. One recommendation I tried was to jack the front of the car up a few inches while doing the work to help the air get out of the system. I'm not sure how much effect raising the front end actually had, but I haven't had any temp spikes or anything to indicate air in the system. That said, although I haven't had any temp spikes or any obvious weirdness, my engine temp indicator also displays a fairly constant 90 degrees C, as it did before the new thermo, fluctuating only about 5 degrees up or down, kind of similar to yours. Even in heavy stop-and-go San Francisco Bay Area traffic it never seems to get much above 95 degrees C, which is "somewhat" reassuring. Once warmed up to at least 70-80 degrees C after starting, my engine has pretty decent acceleration and growls along at high just fine on the freeway. But I have wondered whether the temp range is too high just as you are wondering, since overheating can be potentially devastating to our beloved 617 turbodiesels... Anyone else want to weigh in on the topic?

Ok, I need some advice here.

Ok, I need some advice here. Just to make sure that the new thermostat is not opening at the wrong temperature, I installed another new one and the temperature is only a couple degrees less. Still somewhat high (about 95 deg C). My 300SD runs at constant 85 deg C with the same type 80 deg thermostat, identical engine, and identical Behr radiator. Next, I am going to install a new temperature sending unit. I do not think the problem is the fan clutch because the temperature is constantly on the high side even when moving at fast highway speeds, where the fan clutch does not need to function. Water pump? Not likely - it is not leaking, so unless the pump has a plastic impeller which has partially disintegrated, this will not be the problem. I do not think any MB diesel water pumps were ever made with plastic impellers. Is this true? So this leaves the radiator as the primary suspect. It looks pretty old. Would it be a good idea to have the radiator flow tested by a radiator shop or should I just get a new one? I am aware of the consequences of a broken radiator neck failure while driving.

Same experience here

Hi, Mine's an 84 300D-T I just changed hoses and thermostat with the parts and excellent instructions from here. Old T-stat with green coolant, ran 80C all the time, and took a long time to warm up. When I did my work and filled the car with coolant, it was parked in a part of my yard that slopes back and to the right, so the car was low in the back and on the right side. It's a very small slope so I thought, "no big deal". But when I test drove, it warmed up super fast, and ran up past 100C within a few blocks. That ain't right. So I pondered whether I could have installed the t-stat backwards, etc....then I went and read the instructions again, especially the precautions about bleeding the system. This morning I parked the car in another part of the yard where the right side of the car would be higher, and sure enough, the reservoir level dropped 2 inches, and it took another 1/2 gallon of coolant. I jacked up the front right, just to be sure. Now, with the new t-stat, and M-B approved yellow coolant, the car runs 90-95C steady and warms up pretty quickly. I also cleaned the banjo and related tubing and switch-valve per Ken's Performance book and now the car goes like a cat with it's tail on fire. What a difference! The banjo was OK, but the switch-valve was clogged with soot. I couldn't be happier. Many thanks to Ken and Kaia, for the great service and information