My 300 D seems to operate at the upper level of the temp gauge (100 C). When the car is at a standstill it quickly rises to the 120 C mark. Once it is moving again it returns to about the 100 C mark. I'm thinking this is not good. Is my radiator the culprit

80-85 I think is considered normal. your temps are very very high... you should check your coolant level, make sure your water pump is circulating coolant... flush the system and replace said coolant and if that does not help. Replace the rad.

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Generally speaking, heating at a standstill is caused by a lack of air flow. You could have a problem with the fan clutch. A restricted radiator will allow the engine to run cool at idle, but will not be able to dissipate the heat when the engine is under load, even though it is moving through the air.

You'll have to do some basic coolant troubleshooting:
- check level
- check to see it is building pressure
- Make sure temp gauge and sender are accurate (verify with laser thermometer). 120*C is too high.
- When it is warm, feel the radiator hoses (just be careful). Thermostat could be stuck
- make sure coolant is flowing when the engine is on (water pump)
- check for any leaks
- Check fan clutch
- Check if aux fan is working, check fuses to make sure it hasn't blown

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

It's kind of a Catch 22 - when the gauge reads 120*C, the fan should be blowing full tilt. You can't miss the sound. When the fan's blowing full tilt, the gauge won't read 120*C.

Sixto
87 300D

I forgot about the Fan Clutch and Aux Fan....oopsie. I know when the outside temp is 90 and above my temp never gets above 85 with the ac blowing...

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Depends on which model 300D we are dealing with.

There's a 300D without a clutch fan?

Sixto
87 300D

I beg to differ........I can hear the fan and the gauge is at the 120*C mark.

Radiator, defectve thermostat or cracked head are possibilities that come to mind on the 603. Technically it could also be a head gasket. Just do not fixate on it as it is a smaller possibility

. It is not an absolute test but check to see if there is pressure in the system after sitting for eight hours.(coolant hoses are hard) This does require that the cooling system can hold pressure of course.

Clutch fan or aux fan?

Sixto
87 300D