Got it solved, but I can't tell you exactly how because I did a lot. Ran up the same canyon today, AC on max and the air temps around 94. Coolant temp never got above 105 and stayed around 100 most of the time. Here is a list of what I did:

1. Water pump. It wasn't needed, and I debated taking an original part out for an aftermarket, but I did since I was in there working. I still have to og, and it was in good shape.

2. Thermostat, replaced with mercedes part. Tested the old one in a pot, and it seemed to work properly

3. Radiator flush. I did a more gentle one first, but when I was changing the water pump I could see some white build up left over, so I went with the CLR radiator flush and left it in there for an hour.

4. Water wetter and new coolant.

5. I added an electric fan to the oil cooler. After looking at the factory oil cooler, i couldn't see how it would get any decent airflow. I could be wrong, but I wired an electric fan with the trigger coming off of the 12v from the condenser fan. At idle, I can feel a ton of hot air being pulled out of there, and I assume it acts as boost not a restriction at speed. I am considering changing the trigger to coolant temp, and not the fans, because its my understanding those fans only kick on with the AC, and it would probably be better to have the oil cooler fan kick on anytime it gets hot.

All in all Im pretty happy with how it turned out. The car would have never made that climb before, I had to cut the AC each time it crept up to 110 because I could tell it would keep going.

For some reason the engine won't start on the first crank since I did the work. I can't imagine its related. The car seems to be idling a little rougher than normal too, I can feel more vibration in the steering wheel too. Im guessing something with fuel delivery, but I have some reading up to do.




Thanks for the help, and on to the next project.

on my 95 e300d which is a similar engine, I was running a bit over 100 on mountain passes with ac off at 65 deg F. I started with new coolant, water pump, thermostat, and radiator. When I pulled the radiator I realized the problem. it was full of deposits so much that the coolant in the top wouldn't drain out the bottom (328k mi). That helped with temperatures at higher speed. The clutch fan I did later helped in town by a few degrees. I would start with verifying that the radiator isn't plugged, and go from there.

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1990 300d 2.5 turbo
1995 E300d

I don't know that '91 Mercedes diesels have a cat/s?

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'06 E320 CDI
'17 Corvette Stingray Vert