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Old 01-26-2011, 09:48 AM   #1 (permalink)
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Date registered: Sep 2010
Vehicle: 1985 300D Turbo
Location: Westen Oklahoma
Posts: 24
84 300D Thermostat Change and results

Ok,

I have had this car a few months and driven it about 8,000 miles. It has 256,000 on it now.

It has alway run cold. And mean cold. Never above 60ºC. That worked good for the summer and fall but these cold temps now makes my 1 1/2 hour drive in the morning rather chilly in the car.

First I replaced the mono-valve. No difference in the heat in the cockpit.

So I figured the PO had take the thermostat out since the car was located in South Texas.

I located a new thermostat at NAPA. It was an 80ºC rated.

So I dug into changing the thermostat truly expecting not to find one in the housing. But there was on there and it was a 75ºC rated thermostat. I installed the new one and now the car runs about 90ºC.

Noted changes included better heater, quieter running at interstate speeds (3300 RPM) and smoother idle (no more loping). And better fuel milage. I'll report back when I do a calculation at the pump this evening.

I have attached a pic after running at 75 MPH for about 70 mins.

Is that the normal temp?

Thanks, Jim
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84 300D Thermostat Change and results-photo-jan-26-11-27-00  
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Old 01-26-2011, 12:45 PM   #2 (permalink)
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I cannot add to this, but I am in the same boat you were. I am almost certain that I have a 60c thermostat. I have the same symptoms. Over the next few weeks I have off of work, so I will be replacing the monovalve, thermostat and of course the antifreeze. Ill be keeping an eye on this thread to see what other more experienced members say.
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Old 01-26-2011, 03:55 PM   #3 (permalink)
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Whippy, your thermostat is probably stuck open. I don't think a 60C thermostat is available for 123s.
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Old 01-26-2011, 05:35 PM   #4 (permalink)
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Vehicle: 1984 300D (235,000 miles), 2001 E430 (78,100 miles)
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Post New Thermostat New Temp

I replaced the coolant and the thermostat a couple of weeks ago. Regarding the thermostat, it was like-for-like replacement (Behr). However, I now get different temps.

Before, it would come up to 82C. Under load up-hill it would go up to 100C (maybe a bit higher during the summer).

Now, it comes up to 88C, and on the same hills goes up to 98C.

So again, then; 82 - 100, now; 88-98. Seems to be controlling to a tighter temp band. Maybe that'll translate to better fuel efficiency.
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Old 01-27-2011, 05:22 AM   #5 (permalink)
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Typical germans, all needles should be exactly in the middle at idle (including the gas tank to be really OCD). You should never see anything over 94 unless you have the ac on in 100+ degree heat and you are racing up mountains (kind of a joke in a w123).
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Old 01-27-2011, 06:02 AM   #6 (permalink)
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Your thermostat starts to open halfway at 80c so, on a hot day, 90c reading should be expected.

If you want it to be cooler, try using 71c. Upgrade the auxillary fan to a bigger w124 single fan model so it cools faster and covers a larger area. Replace the thermoswitch. Mine used to have 105c rating. Changed it to 85c.

RESULTS: Early AM or Evening - 77-79c. 11Am to 4PM or on hot days 82c-87c
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Old 01-27-2011, 10:31 AM   #7 (permalink)
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Thanks for all the replies. I think I'll leave the 80C in for now and if I still own the car by summer I may look into putting in a cooler one.

I bought this car for a purpose and that project is about over. I most likely will sell it. I just don't need three vehicles. It has been a great car. I drive it 170 miles a day for 5 days a week and no problems. Anyone interested email me and I'll send you pictures.

By the way, when I filled up last night it came to 22.5 MPG. Thats about normal for me so no increase in milage with the new thermostat.

Jim
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Old 01-27-2011, 10:35 AM   #8 (permalink)
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Thats pretty low MPG. I get 24ish in mostly city driving and I wail on that poor go pedal.
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Old 01-27-2011, 10:53 AM   #9 (permalink)
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Yea thats what I thought. That MPG is doing 75 MPH with the compressor running. Maybe a valve adjustment might help haven't done one yet. Either way it's way better than my big chev Duramax diesel. It only gets about 17 (ON A GOOD DAY DOWNHILL AND A TAIL WIND) but I'm hooked on the power. Great truck.
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