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300SD
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Ok, when I got my '85 300DT two months ago, the car seemed to be running a little cold. Right around 75deg C, and temperature would eventually get to 80-85 deg C, but not higher, even with A/C on. Also, the auxiliary fan would come on when the gauge temperature reached about 85 degrees.

It looked like the thermostat was sticking open, so I replaced the thermostat with a new one (same 80 degree rating, brand name). The engine warmed up quickly, but the temperature got to 95 degrees C and stayed there without fluctuations. I did not hear the auxiliary fan come on at any time.

I got another new thermostat (again 80 degree rating, brand name), and put it in. Same warm-up time. Same steady 95 degrees. No change. Did not hear the auxiliary fan to come on.

Installed new gauge temperature sending unit. No change in temperature readout.

Took out the last thermostat and drilled four 1/8 inch holes in it (similar to what dieselgiant describes on his site). The warm-up time increased, but eventually temperature got up to steady 95 degrees. Again did not hear the auxiliary fan come on at all.

I drained the radiator and drained the engine block. There was no sludge anywhere, and the coolant was very clear. I installed the original thermostat that the car came with. Now the engine runs at steady 80-85 degrees C, but takes a while longer to warm up. This thermostat has no holes drilled in it, and it is an 80 deg C rated Behr-Walter thermostat.
Surprisingly, when the indicated coolant temperature reached 85 degrees C (only when idling for a while with A/C on), the aux fan came on.

Anybody has any logical explanation for this? I ran out of ideas.
 

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'84 300D/ dakota benz trk. Sold... miss it. "88 260E '91 190E 2.6 '89 Volvo 240DL "85 300D
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1,252 Posts
mine done this same thing. i put in a new t stat and only drilled one .125 hole. we have done this on hot rods for years, and mine acts perfectly now. only time i have heard aux fan come on is while sitting a long time.
 

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European 1974 350SLC ,'78 300CD& '80 300CD sold , '81 240D SWMBO's Car '84 300CD & Euro
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672 Posts
Didja Check The *Actual* Coolant Temp. ?

Like with a candy thermometer inthe open filler neck ? . one of my three Mercedes ('80 300CD) does this same B.S. and it's driving me crazy ~ I've swapped out T-stats , drilled and non drilled , tried running only a restrictor plate , various gauge heads and senders , the damned coolant never goes over 200° F no matter what but the _gauge_ reads 95° ~ 100° C regardless of ambient temp. , speeds or loads......

I can lay my hand on the radiator top tank or upper hose at any time and when the dash gauge is reading 100° C I can open the filler cap, no boiling....

I just flushed it with the <magic> Ma 'Benx dealer citric acid and new water pump & housing.....
 

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W-1-2-3 Go!
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Ambient temps also play a role here. It may have been cooler the other days and hence no aux fan input. Hotter days would make the fan come on more I think.

The use of AC will make condenser warm, giving the cooling system a harder time. This will increase the temp at the switch and trigger the fan. If the condenser isn't hot enough to trigger the switch, the fan won't come on. So if you're using the AC when it's cold, it won't turn on.

Were all these done in idle?
 

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European 1974 350SLC ,'78 300CD& '80 300CD sold , '81 240D SWMBO's Car '84 300CD & Euro
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672 Posts
Who Are You Asking Here ?

:confused:

karugs said:
Were all these done in idle?
 

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W-1-2-3 Go!
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If the tests weren't done at idle then that might account for the aux fan not running because of additional cooling. If they were done at idle they should have turned on (talking to thread starter) if the temp rose.
 
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