My 300sd is running a little rough at idle. The temp is normal like 85 c. When I am running 75 80 mph on the expressway for about 20 min the temp goes up to very close to 120 c. The needle also jumps up and down like between 95 and 120 c. What could it be?
John

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1983 Mecedes 300 SD with 383K plus motor and trany rebuilt within the past 50K.
1984 Mecedes D 254K
Planning to make both veggie cars

Replace the temperature sensor in the head and clean any dirt/debris out of your radiator.

A flickering needle suggests a bad electrical connection somewhere.

Like Lance said, replace the temp sender, or at least pull it and clean it to make sure it had good ground. You might want to check the wire to the gauge for continuity as well.

Thanks ya'll I will replace the sensor today. Should I repalce them both?

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1983 Mecedes 300 SD with 383K plus motor and trany rebuilt within the past 50K.
1984 Mecedes D 254K
Planning to make both veggie cars

Does it go smoothly up to 120? or does it go up to 85-95 and then start jumping around? or does it always jump around?

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

only when it goes over 100 does it jump around

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1983 Mecedes 300 SD with 383K plus motor and trany rebuilt within the past 50K.
1984 Mecedes D 254K
Planning to make both veggie cars

Replace the sender, VDO is notorious for going bad, also check replace thermostat and flush system. Check the ground of your car as well as your dash panel.

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99 Gurkha with OM616 IDI turbo

2015 Gurkha with OM616 DI turbo

2014 Rexton W with OM612 VGT

The sensor is almost 22+ years old, it's amazing that any of them still work!