To add to the round of summer "my car runs hotter than it does in the winter!" threads...

My 82 300CD is running around 90-98 (hasn't hit 100 yet, but I keep it under 70mph because of temps) degrees on the highway during the summer, with temperature increasing with higher engine RPMs or higher outside temperature. This problem cropped up recently when temperatures shot into mid 90s. During the winter and around town, it runs the usual 82-85 degrees. I might add there is no AC to turn on.

I know that 100 degrees is an acceptable operating temperature, at least as far as the integrity of the OM617 block/head/headgasket is concerned, but I'd still like to get back down to the 85 range. I looked in the coolant expansion tank, and I see a yellowish color, which should be either MB or G-05. I also "burped" the radiator, and didn't really get very much air out. From previous threads it seems like this is most likely a thermostat issue?

I could either drill holes in the one I currently have, or replace it with a new 80 or 75 degree unit (which I would prefer, something seems wrong about drilling holes in the thermostat). I've also heard of people having problems with "new" thermostats not being 80 degree thermostats... and the three available on fastlane aren't indicated as being a certain temperature. Is there a reliable source for 80 (or lower) degree thermostats for these cars?

Thanks.

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver

I got my t'stats from here http://*************** no problems, I wouldn't drill holes in it either. They're 80 C's.

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83 SD

84 CD

No.

You are looking at the function of the thermostat from the wrong direction. The purpose of a thermostat is not to keep the engine cool; it is to maintain a specified minimum temperature.

When considering drilling holes, remember that you have a bypass thermostat.

Back wash the face of the radiator from the engine side to remove road debris. Clean the condenser fins if there is one. You are describing a circulation issue not a thermostat issue. Partially plugged radiator comes to mind. A good radiator shop can test for cold spots on the radiator with an infared temp gun. Cold areas mean plugged passages within the radiator core. Have the pressure cap (radiator cap) tested to verify proper pressure. and report back

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

1984 300d Turbo
Replace The Fan Clutch $168.00 1-2 Hrs 10 Mm And 13 Mm,clean The Plastic Fan Blades, Diy, It Stays At 80 C

Do everything I said which is basically free labor. If no change, I agree, replace the fan clutch.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

If you have a jumpy t-stat it's probably just the guage.

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1980 300D 300K KM - parting out
1979 300D 375K KM - parting out
2 300Ds for sale

The fan only works to cool at low/idle speeds. On the highway, the natural airflow from the moving vehicle cools the radiator.

Clean out the radiator and A/C condenser.

I presume that the fan clutch plays more of an important role than just at idle. Why else would it be a clutch? I found that when I replaced this part it made a significant difference.
Along with power washing the radiator, using an MB OE T-stat and making sure all radiator fins are straight, a properly operating fan clutch will add to the air being forced thru the radiator because it is sucking the air thru the radiator at almost as fast rate a rate the engine is revolving.

I performed this procedure because I didn't like my temp gauge sitting at 95 on the highway in 100 degree days. A properly maintained cooling system including all components operating properly, no matter their relative importance, is essential. Yes, the engines can withstand the high operating temps but what is the long term cost? I care not to find out.


I also recommend Redline Water Wetter product. It adds cooling capacity to the fluid. It takes a while to show a difference but it does work. My engine temp has not risen above 90 since I did this job. And we drove 1100 miles over the 4th of July thru some pretty rugged Northern Maine and Atlantic Canadian terrain.

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1984 300D

How do you clean it out? Compressed air? Power washer? Seems like the latter would be too much and bend the fins.

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1985 CA 300D Turbo , 213K mi

If the washer nozzle is held at a good 6-8" from the radiator, no fins will be bent if you are spraying straight, which is a good idea. The more dangerous aspect to this operation is holding the nozzle too close; you may punch holes into the radiator if you are not careful.

I removed my radiator, laid it down flat with a piece of wood propping up one end. I power washed from back to front and front to back then again from back to front. So much garbage came out of it. Well, I live in NE and there are lots of bugs.

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1984 300D