My 83 300D has been running warm. 90-100 range when cruising at hiway speeds. I have flushed the radiater several times with no change. It has not over heated and cools again during stop and go traffic. What's up?

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

Replace the thermostat.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

It's either the thermostat or the gauge. Sometimes the thermostat cures the problem........usually, when the gauge is reading slightly high........it doesn't.

See if you can borrow an infrared temperature gun and shoot the cylinder head.

If it's running 180-190F. the gauge is the problem.

If it's running close to 200F., then I'd change the thermostat.

But doesn't the termostat fail in the open position? Wouldn't this make it run cooler?

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

not if its stuck.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

Any engine gas or diesel that runs warmer the faster you go generally relates to slow circulation or a restriction in the air flow. Assuming the radiator is not partially clogged internally, I would verify the A/C condenser in front of the radiator is not restricted from bugs and road debris. I would also flush water from the engine side of the radiator forward to force any debris from the face of the radiator core. Its possible but unlikely that the thermostat is stuck partially open. If its over 4 years old, it wouldn't be a waste of money to replace it just out of preventative maintenence.

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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.

I have this symptom myself, on mine it's an inaccurate sender, I don't worry much about it (because I know it's off)! BTW, 90 degrees C. isn't that high for a 195 degrees F thermostat-could just be slow to open, some members have also reported trouble with worn down/corroded water pump impellers that cut down on the flow somewhat.

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1982 300DT 190K (Diesel Purge + synthetic oil=smoothness at last!!!)
2004 Ford E-350 6.0 L PSD 227K
2006 Dodge Ram 3500 SRW HO Cummins 4X4 48RE 42K (brute force tow vehicle)
2005 Scion xB wife's rolling pop can
1993 GMC Sierra C3500 6.2 142K

Ok, yesterday I installed a new thermostat (176degree) and flushed again.
still hot.....100+. yes it was hot, I could smell it, but no boil over. any more ideas?

__________________
81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

Are there bubbles in the coolant reservior when the car is running? (could be a bad head gasket). Also check the lower radiator hose to see if it is soft and may be sucking flat at higher RPMs.

Check your belts too.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

Most likely you've got air trapped in the engine somewhere. Either fill through the upper radiator hose with the car nose-up on a steep incline or wait for it to work its way out. I've had the darndest time with t-stats but I've finally hit on one that works after going through four. I ordered a 175F from Autozone and knocked out the 1/8" plug on the plate that opens and closes and now it runs steady at 82C whereas before it would run from 90-100C. By the way, it took me two days to get the air out and for temps to settle to 82C after putting the good t-stat in.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo

I dont know how other benz engines are but on my 190D I can loosen the temp sender where the upper radiator hose meets the head. I fill it until coolant comes out then tighten the sender. Seems to get all of the air out.

I did fill through upper hose till coolant came out radiator. Belts are good nothing slipping. I'll check hose tomorrow.

__________________
81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

What if I just took the thermostat out? would it run cooler?

__________________
81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

Dont bother. Just replace the thermostat. It sounds like a big job but its a 15 minute deal. Takes longer to change your oil.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42