I just purchase a 1985 300d Turbo. No matter how much I drove the vehicle, street or highway, the temperature on the gauge never reached 80 degrees, it would go a tick above 40 degrees. I just replaced the thermostat with a Whaler OEM 80 degree one and now the car idles at about 90 degrees, in between 80 degree hash mark and the next hash mark on the gauge which is 100 degrees? When driving on the freeway, the gauge reads about 1/3 up from the 80degree hash mark. Is this normal? The car runs so much better now! FYI, the thermostat I removed was a Behr OEM with 80degree temperature on it, I guess it might of opened too soon because when I removed it, it was in the closed position.

Thanks!!!

Normal operating temp is just slightly above 80. The temp is very precise unlike newer cars so it will fluctuate. As long as it stays below 100 and closer to 80, you're doing just fine.

Dkr.

Sounds normal to me......about 1/3 between 80 and 90 is where my 83 300D gauge is when operating normally

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
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1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

Search for my post. Similar deal, but I "took it to ground", testing the T-stat it water at various temps and my temp sensor accuracy (can't help it, I am an engineer). Problem turned out to be the T-stat, though on cursory glance it appeared to work OK.

my 84 sedan guage was reading right about 60 when fully warm. so i replaced the t-stat with a new 80 degree Behr, and it is still reading 60 when warm. i'm thinking it's a faulty temp gauge. could it be? and if i ground the instrument cluster could that help? thanks!

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Will



72 280SEL
85 300CD 288k
00 E320 Wagon 4-matic 100k
11 E350 Wagon 4-matic 45k

73 VW Westfalia Camper
67 VW Westfalia Camper

66 Porsche 912

The thermostat can get stuck open or closed, although you could also have a blockage in the radiator somewhere or a bad reading on your gauge. I would recommend first shooting the radiator in several places with an IR gun when the car is (or should be) up to operating temperature. You can then proceed from there. I would recommend staying away from instrument cluster troubleshooting until you're absolutely positive that is where the problem is.

Dkr.