I was changing my antifreeze with the new green stuff and l left my t - stat out just to see what would happen. The 1983 300sd was running fine before and seems to run fine but 10 degrees cooler now.
My thinking is if the its burning the fuel right (mpg is the same) no smoke, cooler running will give a longer life to Tranny and trubo.

Am I doing something wrong?

I am the original ownner of the car with 232,000 miles on it and live in 80 degree year round temps.

THis has been discussed a lot (try the search feature ).... and pictures of the shop manual showing the flow directions, etc have been posted.....
It is designed to have the thermostat IN it....and you should keep it there.

It seems the first thing to do when your diesel starts to run a bit hot is replace the thermostat. It helped mine by 10 degrees. I suspect yours was bad.

You should keep it in.

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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

I have "heard" that leaving the thermostat out can allow the coolant to circulate too fast to cool as it goes through the radiator. But this does not always seem to be the case. I have tried this in the past on other older (1953 Chrylser New Yorker hemi) and found that it merely postponed the overheating

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead

you replaced with new green coolant?

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have no worries.....President Obama swears "If you like your gun, you can keep it

Off topic, but how much skydiving do ya do anyway? I went a couple times back in the mid-80's. Awesome experience!!!!!!!!!!

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)

rickg, I learn to skydive and scuba in the army (our side) in 1970 and have been a parttime instructor in both after I got out in 1975. I have logged over 6,000 scuba and 2,00 skydives. It's a sickness like diesels.

However, does any one know if running without a t-stat in the south hurts the car - not on any search.

You must run a thermostat. The disk at the bottom of the thermostat closes off a bypass when the thermostat opens. Without the themostat in place a large volume of the coolant will just circulate in the block and never make it to the radiator. You can modify a themostat so that it is always open, but there is little benifit in the long run.

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Bob D.
Parrish, FL
1 SDL, 1 D, 1 TD, 1 Mog

If there were a benefit to running without the thermostat, it would be documented in the manuals. These cars were tested/operated in all parts of the world and the thermostat is designed to provide optimal performance.

'No thermostat' is a wives tale perpetuated by anecdotal information from people who have removed a defective thermostat and improved cooling. No thermostat is better than a stuck thermostat. A working thermostat is better than anything else that has been devised. Leave the engineering to MB.

The MB thermostat was designed to operate in a specific range.

Assuming you have an 80C thermostat

Below 80C the coolant is pumped directly through the engine bypassing the radiator completely.

Between 80C and 95C the thermostat allows coolant to freeflow both through the radiator and directly through the engine.

Above 95C the thermostat forces the coolant through the radiator before it goes to the engine.

If you run without a thermostat you may be just fine, for a while. But if your ever hit a situation where the engine temp starts to rise (due to insuficiant cooling).

So you really should put a thermostat in there. They are cheap and there is absolutely no benefit to not using one.

As a side not. MY wifes 300D the thermostat was stuck partially open (akin to running without a thermostat). It never overheat. But it took a good 30 minutes for the engine to reach 80C. Since replacing the thermostat its getting much better fuel economy since the engine is reaching operating temperature in 5 minutes now.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.

Thank you for the information, I shall put it back in.