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Discussion Starter #1
I have an 81 300D that whenever I drive over about 60mph for 10 minutes, the temp gauge steadily rises from it's normal position at 80 to around 90 without the a/c on. Is this considered a "normal" range? I know redline is 120 but could my thermostat be functioning incorrectly? If I reduce speed to 30-35 the temp drops to about 83 or so, and even sitting in traffic it's hovering around 85. I haven't heard the aux fan come on, but that may be only related to the a/c. Any suggestions?
 

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1991 300 SE
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18,534 Posts
90 C may be normal for your 81. Yes, it is a little higher than most experience, mine never makes it above 85 C in stop-and –go driving in summer with the A/C on.

You should be experiencing the opposite: hotter in stop-and-go and cooler on the highway. Your aux fan s/b on by the time it’s at 80 C with the A/C on. The T stat working incorrectly may cause the problem. When did you have the system flushed and filled? If its been a long time the may be deposits, air, or the coolant is no longer as efficient as it once was – a system flush and refill may solve the problem.
 

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1980 300D
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I've only been driving my car for just over a week. I have not even exceeded 60mph yet and I haven't seen the temp gage go over 50 yet. Granted there may be other issues, but would that be normal???
 

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1982 300D Turbo
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300K300D Let your engine warm up to full temp and if it does not go past 50* then dont drive it till you get it fixed. Your wearing out parts really quick that way. Diesels dont like running cold at all, just my .02 [^]
 

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96 E300D
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The original equiptment thermostate will start to open at 80c and will be fully open at 95c. Most engine that aren't pushed hard will stay around 80c but a very hot climate or hard run will push it higher. If neither is your case then I would flush the cooling system.
 

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Discussion Starter #7
I don't think I am "pushing" the car, but 65 mph it creeps up to the line between 80 and the 120 redline. I assume that's 90??? It's never gone any higher, but I live in the south and it's been about 80F outside and humid. No a/c running though. Stop and go it's just above the 80 most of the time. So a flush may do the trick?? THanks
 

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W-1-2-3 Go!
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That line is 100 deg C. Same as your car, my car did that on freeway runs, even higher when I went past 65mph. I changed my thermostat and it went between that line and 80 deg C (90 deg C). Thermostat was also broken...
 

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96 E300D
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You can pull out the thermostat and check it wil you flush but if you don't know when it has been flushed before than I would flush the system anyways.
 

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Discussion Starter #10
If I flush the radiator, how will I know if the thermostat is bad? Where is it located? When flushing, is there a drain at the bottom of the radiator? Any advise, I've never flushed coolant before, just added more....[:D]

Thanks!!
 

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1984 300D Turbo
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300K300D - 4/23/2005 1:01 PM

I've only been driving my car for just over a week. I have not even exceeded 60mph yet and I haven't seen the temp gage go over 50 yet. Granted there may be other issues, but would that be normal???

that would be a clear indication of a t-stat stuck open.
suggest you replace it soon as diesels do not like to run cold.
 
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