Diesel Engine Running Too Cool? 78C Bad?
 

My first post: this must be addressed somewhere in the old threads but I couldn't find it. My 82 300SD (617?) is running at about 78C. Now, I'm in Massachusetts so it's about 35F outside and I have the heat on. Turning the heat off only brings it up to 80C. I remember in the summer, the car usually ran at 90C with no AC on. Is it ok for the running temp to be so affected by the weather and heater usage and is 78C just too low for good economy and engine protection?

I'm mainly asking because this is my first winter with the car (but not my first Mercedes, previously 1967 200D). I bought this car knowing I was inheriting someone else's problems so I count on everything needing to be replaced. I know I could just go ahead and replace the thermostat but I want to take a break because I've already done so many other things lately (tie rods, center link, valve adjust, ALL filters, oil cooler hoses, seat switches, air cleaner brackets, steering stabilizers, auxilliary fan, etc.) Thanks!

Welcome aboard. Assuming no one's gone and swapped engines, your 82 300 SD does have the OM 617 engine. The proper thermostat for that engine is one that opens at 82 degrees, I think. I'm sure that you shouldn't be running @ 78 if the engine is fully warmed. If your temperature gauge is accurate, its time to check the thermostat. When the engine runs cool, you lose some mpg and increase engine wear. The themostat is a very easy swap out if it needs to be done.

Get someone with a laser type temprature probe. See what the motor really is at when it reaches operating temp. Too many variables, like thermostate opening early, temp sensor going out of tolerance to a temp guage that flat out reads wrong.

Would not be the first time a temp guage was wrong.

If you really want to know the engine temp you will need to get an infrared temp tool. The temp sending unit can read off, although it usually reads low with age. Even then it is still very possible that it reads low.
I know, I had a shop run the infrared on mine, the temp guage shows it running a bit low, about 80C. Did it a no charge. Showed the actual temp was 85C.

Mine did that a while back and it was the thermostat that was still original. It took me about 15 minutes to change it and now it runs nice and warm (not too hot ofcourse)...

the thermostat itself is like 10 dollars or 12 so it's not biggie.

As I mentioned on another thread, my '82 300SD runs at a steady 60C, according to the gauge. I have a new thermostat on order, and this time I'll test it on the kitchen stove before I install it. The nights have been getting pretty cold in Southern California these days!

"I know I could just go ahead and replace the thermostat but I want to take a break because I've already done so many other things lately..."

What follows is just one opinion... :D

While 90C is a bit high, and 78C seems a bit low, there's no immediate danger, and your engine will keep running while you take a well-deserved break.

Watch the gage to see if it goes even lower when outside temps dip further.

Find/borrow an IR temp sensor to follow up on other posts, and to double-check your gage's calibration.

Best Regards,
Jim

Oparrott,

A large piece of corrugated card-board can serve as a very good temporary improvement for the winter months. Usually only need to cover the drivers side of the radiator, just slip it down between the A/C condensor and the radiator. Should work fine in your area until spring, but don't forget that its there!!

My TD would NEVER get above 60*c in the winter (even with a new thernostat) until I blocked about 85% of the radiator surface with a piece of cardboard.

Remember, keep the oil cooler clear DO NOT cover it up, ever! Just a warning. :)

Thanks for the help!
 

Cardboard?! Alright, at least we're not afraid to try things around here! :cool: I think this is my kind of place. In my garage, there's room for pieces of cardboard AND IR thermometers. Low-tech, high-tech, it's all good. Thanks for the multifarious advice (really).

79C is fine, my OM 616 turbo comes with a 72C thermostat and at 41F ambient outside temp, it is running at 79C. During 45C+ summers, it goes up to 90C.

oparrott,

I'd like to know how you can read 78*F on the dash temp gage. I can't read mine that close. Seems to me that it would have to be a digital gage to be able to be read that close.

Gages are rarely perfect anyway. They might be plus/minus 5 degrees. If you have enough heat from the heater, your engine is probably running hot enough. Yes outside temperature will affect the engine temperature.

P E H

would it matter that much if the oil cooler were blocked w/cardboard in the winter if the engine isnt getting too hot and you're running synthetic?

thanks.

Unless it's -50*f outside I think it would be safer to leave it open. With the cooling jets under the pistons, even with a cool engine, the oil can get pretty hot. The oil filter housing has the thermostat at 110*c!

Well, it's Centigrade on my dial and by 78, I just mean "right below 80 with visible space between." "78" is just a concise way to say all that, even though I know it can't be that accurate. ...Yeah, I don't think I'm getting quite sufficient heat from the heater so I am planning to do something about it. If I don't get an IR thermometer for Christmas, maybe someone will give me a piece of cardboard at least :) !