I'm now convinced otherwise.

Too many members have run without a thermostat and the result is always cooler operating temperatures.

The reason is as follows:

Yes, it's true that the bypass is open without the use of the thermostat. However, the thermostat provides such a massive inline restriction to flow, that the removal of same will provided additional flow to the radiator, even with the bypass open.

AFAIK, there is nobody who removed the thermostat and witnessed higher operating temperatures.

From what you state above, it sounds like your Alternator/Generator isn't working either. If this is the case, how are you starting the vehicle? Also, without a working alt, how would your electric aux fan function? If you don't have a working alternator, I would get that fixed as soon as possible so that your aux fan will work.

For your head gasket, if you have no coolant in the oil, check for external leaks from the engine. Is your water pump leaking? The cheapest fix is the thermostat, so you could start there. Replace the coolant with a 50/50 mix as this will provide maximum cooling and protect you from freeze down to about -30F. Make sure you use Mercedes coolant or Zerex G-05. Don't use regular tap or hose water for the mix...use disstilled water.

Make sure you have the proper pressure cap on the coolant expansion res. They are cheap as well.

Also, find out if the previous owner has replaced any other parts in the cooling system like the radiator. Inspect the radiator for debris, damage, leaks.

Thanx again for all the advice, folx. I took it out on a really hot day with no thermostat, and it went to 80 C, after a 15 min. warmup and 10 mile drive.

I then put in a known good thermostat, and repeated the same journey, and it ran closer to 90C. So i guess having a t stat definitely makes it run warmer all else equal (The weather was actually a litter cooler the day i ran with the t stat and the car ran warmer).

Plan to take it on its first road trip this weekend. Any advice?

sir edmund

I agree! Same with my 300D. In cooler ambient temperature (early mornings, evenings, or when raining) water temp. gauge readings are higher compared when outside ambient temps are 90 deg F and above, which the temp. gauge reads lower. I think this is normal, the thermostat is compensating along with the ambient temp. and the engine temp., because our water temp gauge (if still accurate) are pretty much like a thermometer which really reads the temperature changes according to the variations of engine loads. Unlike those newer model cars, which their water temp gauges are pretty much like an idiot light in three positions: cold; normal; and over heat. I am not convinced that these types of gauges reads the the actual temperatures of the engine cooling system for the fact that the gauge's pointer doesn't even move as if it is nailed to its normal position under all engine loads and varying conditions, no matter how long you're stuck in heavy traffic, driving a long uphill, or driving it hard under load with the AC at full blast. For me, it is just not possible.

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'85 300D Turbo - CA Version

the temp guage will only read the temp at the point in which it is located. it is a part of an entire cooling system that was designed to work together...with a thermostat.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Hi folx, thanks again for all the advice. I drove the 300D to Vermont and back and some locally, and the temp seems to hoover around 90C. I assume this is about normal, but i wish i could get it to run a little cooler. When i had no thermostat in it, it ran at about 80C.

sir edmund the green

For a diesel engine, too cool of operating temps is not necessarily better. You will be inviting problems of carbon buildup if the temps remain too low.

I would say you are at an ideal temp. We only dream of consistent 90C temps around here this time of year.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Just an opinion, but consistent 90C is too hot ... but only by a bit ... my experience after owning a few MB diesels of various kinds and by reading a zillion of these sorts of threads is that 82-85 is the most common experience - lots of issues at work, though ... if 80C without tstat, and 90C with tstat, I would try another tstat from Phil's OEM connection (what a pain to replace, though) and see what happens.

On the other hand, the diff between 82 and 90 is about 10 percent ... is a consistent 90 better? Don't think so as everyone's would be designed and run at that temp ... but, I don't think they do ....

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg