Hi, I have a 1985 300D that is running warmer then it was when I put it in storage last fall. It has 260,000 miles, 105,000 which I have put on. 60,000 miles ago I replaced the radiator whith a Bher, installed a new water pump, fan clutch, and I recently replaced the thermostat.
The temperature climbs to 3/4'ths up the gauge when hill climbing, then falls back down to half way up the guage when decending hill. During turnpike driving it is half to a little more on the guage. I had the cooling system flushed and it made no difference. I think the heater circulation pump is leaking a bit, and my "new" radiator seems to be seeping around the edges. The water pump may be weeping over night. It did it for a few nights but seems to have stopped??? I squeesed the top radiator hose with car ideling and felt no real surge when I released the pinch. Could these be causing the pressure to be lossed? The car starts and runs fine in all conditions. When it was cold it would stumble for the first five seconds then even out. I think it needs glow plugs, as I have never touched them. Any thoughts would be greatly appreciated. Thank you in advance.

Have you replaced the thermostat?

Have a great day,

Just a word of warning. Check the temp that the new thermostat ACTUALLY opens by submerging it in a pan of water on the stove top.
We buy 80 degree thermostats. Some open as low as 60 and some as high as 105. Shoddy quality control, eh?

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You can get farther with a smile and a gun than you can with just a smile.

1984 300D
1985 300CD
1980 300SD
1993 SL500

laird, your description of what happens in hill climbing and t-pike driving sounds pretty normal to me.
if your temp gauge goes up nearer to the top (120 deg) and stays there then it's time to really worry!!

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Replace the radiator cap.
spo out