Too Cool?
 

Is this too cool for a diesel and if it is too cold what am I doing to the engine by running this cold? By the way these pictures were taken after about 30 minutes of freeway speeds and the ambient temp was around 60. At night or in cold conditions the engine runs even cooler.

If the sensor is accurate, that's too cool. You should be running 80-85. Maybe you don't have a thermostat.

Mine runs just a hair above that. Gets up to nearly 100 if I'm climbing a long hill.

Hey Mp,

I'd like to see your dogs. Got any pics?????

The water temperature sensor technology in these cars is not exactly a scientific-quality instrument. The sensors have their issues, the wiring gets oil impregnated, and the instrument panel has its electrical connection problems.

So don't be too worried if your gauge needle is two needle-widths lower than what you might expect it to be.

A better question is, does the car warm up to that indicated temperature within five minutes of highway driving? If so, your thermostat is working. The exact temperature it opens at is subject to the instrument you use to read that temperature.

If that were my car and it displayed normal warmup characteristics, I wouldn't worry, I'd be happy. :)

Ken300D

Chris Bell,

I would change the thermostat. The most common failure mode I have experienced, actually the only one, on MB gas and Diesel automobile thermostats is sticking open and then running the engine too cold. I just went through this on the 1988 300E last weekend, which was made a bit more interesting by the fact that the thermostat housing cover was plastic and in roughing it up while changing the thermostat (stuck in the housing like it was superglued) the hose connection nipple broke. Had to wait until Monday to get a new, aluminum replacement housing cover. Added some drama and rework, not likely applicable to engines with aluminum thermostat covers from the factory. Engine in that car now runs at exactly 82* - 85 * Celsius, and all the coolant stays inside the coolant system. Jim

What I find most interesting about that pic is that the speedo only goes to 85. The speedo in my 240 goes clear to 100, and I know it can't even go that fast. A 300SD can pretty easily go over 85, can't it?

There was a time period in the early 80's where US Federal law mandated that all vehicles had to have speedometers that indicated no higher than 85 mph.

You'll also see Mercedes before 1985 with no third brake light. Federal Law required them in 1985. The third brake light for pickup trucks came along later, like 1990 or so - not sure.

I'm going to guess and say the idea was to prevent people from "seeing how fast their car will go". :)

But then the aftermarket speedos became available.......

Aftermarket third brakelights were the rage for awhile too.

Ken300D

I've always hated the whole third brake light thing. Unnecessary if you ask me. I love that my 240 doesn't have one.

Yeah, it's interesting to see the different "fads" that government regulations have gone through. Like my '89 Nissan 240SX that had the seat belts on the little motorized track. I know that was required for a bit there.

I should trade speedos out, since the max speed on the 240 is somewhere around 85 anyway, that 100 just sits there teasing me...

If it were mine I would replace the thermostat. These are designed to fail open.

It is cheap and easy to replace and you will then know where you're at with it.

My $0.02,

I think it was 81 and/or 82 only that M-B's had 85 mph speedos. Even the SL's.

But back to the thermostat issue- my 84 300TD ran the same temp as pictured, until I replaced the thermostat (about a year and a half ago).

Now, after a new Behr radiator, water pump, a recent (overflow tank) cap, I am getting consistently higher running temps, like an avg of 90, with pulls to 100 C on long uphills, etc. With all of these new components, I really want to see the car run at about 82-5 (as others have mentioned) as it is not even hot out yet.

Simply try a thermostat? (It's the only piece of the cooling system that isn't brand new at this point!)

-Thanks-

JAS

Interesting but my '85 300D doesn't have a third brake light, never had one and never will.
I do have a 125 MPH speedometer.:D

3rd brake lights came in across the board in 86, all cars. There are no 123's with a third brake light from the factory.

Chris,
I assume that your car used to run a bit hotter and recently is now running cooler. If it is a new car to you then it may very well be just an inaccurate reading. If the problem is new then I agree with the thermostat likely being on its way out. Sometimes they can get a bit flaky before they stick all the way open. The good news is that it isn't usually a big deal to replace them. If you do it yourself I recommend testing the new Tstat in a pot of water on the stove since there have been a number of instances of bad, new out of the box Tstats reported here. As far as doing damage is concerned, running for extended periods at below normal operating temp may cause a bit of carbon buildup but given that you are not too far off proper temp I doubt you'll have any problems that a good hard drive won't clear up. Nonetheless, if your gauge is accurate you will want to fix this before it gets worse.

By the way I think the cyclops brake light officially got added starting in 1986 since my '85 doesn't have one. I am not fond of the way they look either but the insurance data indicates that they do help. Unfortunately that's not a good endorsement of the driving abilities of the average American.

My TD would run 90-100 all the time. All I did to fix this was flush the system with clean water 3 times to get all the grime out. Once I put a fresh 50/50 mix in, it's hasn't yet past 100*c...under 100mph... ;)

I also tested the thermostat in a pot of water on the stove (With the t-stat not touching the metal) and it opened exactly 83*c.