Have this (new to me) 86SDL. Unlike the 300D I had before the temp will come up to 80-82C if I am around town or driving below 50. However on the road it runs to 102C for periods, and drops back, then runs up again. The 300D just came to temp and stayed there. I am wondering if this is an indication of a faulty thermostat, or perhaps the fans are not coming on as they should. So far since I have had the car our outside temp has been 0-25F most days. It also starts well when plugged in, but requires more crank time and runs rough the first minute or so after a cold start, could this be a glow plug problem??? Gave the 300D to my kid and am thinking of asking to trade!!!

Check the thermostat. When the temp goes as high as 102 and drops down, can you hear the fan working? I would let the car idle on the driveway with the hood open and check to see that the fan clutch is working when the temp gets high. I had a similar problem in my old 190e. on the highway, temps would rarely rise above 80 but if I was in traffic, the temp would shoot to 102 and then drop. I found that the thermostat was stuck open. Good luck.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold

If you let it sit and idle the temperature will never go above 60C. I have not had it in warm weather so I don't know if that would remain as such in the summer. But now you have to be on the road running to warm it up. Took a longer trip the last couple of days to do a little duck hunting and it maintained abour 98C and the heater/defroster work fine.