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Old 12-01-2004, 01:17 PM
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Join Date: Oct 2004
Posts: 5
Question W123 First winter. Engine temp too low?

I just picked up my 1985 300D (California) this summer and it has been great so far. Over the summer it was running a steady 85c-95c depending on A/C usage and traffic didn't seem to matter how hot it was outside.
Winter has now set in down in Texas and I can't get the temp up to 80 even with the A/C blasting. My normal commute is 15 miles each way and it now warms up to about 70c and stays there even on longer drives.
Everything else seems to be in order all fluids and filters are fine.

Is this normal behavior in winters or might there be something wrong?

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Old 12-01-2004, 01:19 PM
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You will need to change your thermostat, it's bad...
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Old 12-01-2004, 01:20 PM
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Join Date: Jul 2004
Location: Annapolis, MD
Posts: 650
You should be running 80+. Sounds like the PO removed the thermostat. Without it, it'll never get up to proper operating temp.
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer
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Old 12-01-2004, 02:02 PM
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Join Date: Oct 2004
Posts: 5
New Radiator

Thanks. I'll take a look tonight. I think it has a new(er) radiator so maybe they didnt' put a new thermostat in and it finally broke or it's missing altogether.

Last edited by evblazer; 12-01-2004 at 02:14 PM.
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Old 12-01-2004, 03:14 PM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,558
Just replace the thermostat, it's opening too early. Every used diesel I've purchased in Texas ran too cool in the winter.

'Stat replacement on the om617 is about as easy a task as exists.

- JimY
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Old 12-01-2004, 07:14 PM
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Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,320
You need to get up to 80 or so to prevent excessive wear. The normal 617 thermostat starts to open before 83 and should be open there. I believe for a winter thermostat they use a 92 degree on that is used with gassers to lower exhaust emissions, but never lived where its real cold so I leave the standard one in place all year round.
You may not have a thermostat in fact. Or it might be stuck open either way you need to get it replaced.

I had a thermostat (in fact its still on my '79 300TD) that my mechanic friend drilled a hole in it to bypass the thermo flap in it. He said it burped the engine. I always back-fill the head and engine to prevent having a burping issue with any of my Benz's but I went along with his great modification to an old thermostat I let him sell me
I have seen new (stock) Behr thermostats with a small burp hole and a bb in it, so I figured it was OK even though he made the hole pretty large IMO. That car always ran cooler than my other turbodiesels. I thought one reason was because it has a three row radiator in it (special order and unfortunately I can't find another one like it!) but perhaps also because of the larger hole in the thermostat. Since that is my hauling wagon and I like to take long trips across the desert and in mountainous places, I left it in.

If it gets real cold with any diesel, you can place a piece of cardboard across most of the radiator opening. In the 123's is simple, raise the hood stick a thin piece of cardboard between the front of the radiator and the AC condensor. I used to do that for short trips to get the car warmed up. Doesn't hurt a thing. Older Big rigs have a series of blinds that open and close with a bowden cable to keep the cold from cooling their oil too much in the winter where chill factors are severe. Diesels need to get warmed up good to run properly. 70 degrees isn't real cold but it isn't the right operating temp either.

1971 220 (gas) 4-spd manual 106441
1979 300TD w/ 85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 260200
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!
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Old 12-15-2004, 11:29 AM
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Join Date: Oct 2004
Posts: 5
Thumbs up Fixed!

Put in a new thermostat I got, old one seemed to open earlier then the new one in hot water stove testing.
Also replaced flushed and rinsed the green fluid the PO had put in with his new radiator and now have the proper G-05 in there. I also believe I found a ground out wire coming out of the water pump which I reinsulated.
Don't know which fixed it up I think all is well. My operating temp is now about 90C. It comes up in just a few minutes and stays there so I think I'm all set.
Thanks for all your help!

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