Hello,

my 300SD is nearly overheating at high speed driving (i.e. 75-80 mph). Temperature is 100F outside, with AC on (R134), gauge at 110-115-120C, so that I have to slow down to 65 to prevent overheat. New water pump recently, thermostat couple years ago. Also replaced radiator few years ago. Car runs fine at 55-60 mph (95-100C), and similar at idle/city driving. Shouldnt a car normally cool very nicely at high speeds?

What could explain this?
could this be the fan clutch..it is original.

I would tend to condemn the radiator. But, I would replace the thermostat first. Constant hot running damages the thermostat so I replace one on each repair anyway (they are cheap).

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Another thng that may have been overlooked, is the radiatior cap itself. Since its off to the side on the resvoir, and it probably didn't get replaced when you got the new radiator, it warrants examination.

Also non MB rebuilt waterpumps are known for not having the close tolerances that MB requires, (although I would think that this would cause more of a problem at idle, unless at high speeds cavitation occurs).

keep your cool!
-Larry

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03/83 300D 184k
07/73 280 160k

Actually
1) The radiator cap was replaced last summer, it did help some.
2) The water pump is an MBZ original, i junked an aftermarket replacement I bought.


maybe this is normal in southern california where it is so hot...although I would still think the car should run cool on the highway even with A/C and temperatures at 100F outside?

I'll probably switch the thermostat out over the weekend for a new one, maybe it is sticking?