Operating Temp for OM617 - Page 2 - PeachParts Mercedes-Benz Forum

#1 01-09-2009, 03:27 AM

Quote:

Originally Posted by jt20

Your diesel will drop in fuel efficiency if you are running too cool. ie.. heavily modifying your thermostat or removing it.

It will also overheat if the thermostat is removed. Unlike most g@ssers, the t-stat in these engines is a mixing valve. When it opens it blocks coolant flow into the engine block and redirects it to the radiator. Without the t-stat, the block and radiator ports will always be uncovered and coolant will take the path of least resistance (back to the engine block).

jt20 · #2 01-09-2009, 03:36 AM

once the entire system is pressurized (head / block) the pump is not fighting any force other than moving the water

I cannot agree with the overheating w/o t-stat

#3 01-09-2009, 03:44 AM

Its basic hydraulics, every substance on earth flows the path of least resistance. Since both the radiator and block would be unobstructed by the t-stat, water would be flowing through both. But, cooling performance is significantly reduced since some hot water will always be recirculating back. If you want the most heat removal capacity on hot days, the best option is an unmolested and properly functioning thermostat.

Read the attachments. If Mercedes says its a bad idea, its probably a good idea to believe them since their R&D budget is a wee bit bigger than any of ours.

jt20 · #4 01-09-2009, 12:15 PM

thank you. By reading those attachments I have come to the understanding that the radiator is, indeed, the path of least resistance.

Why else would their R&D staff decide to use a t-stat to LIMIT flow through that region?

If the block and head were less resistant to flow, how would coolant ever circulate through the radiator to cool them under normal circumstances? It is only when there is too much heat, that the flow through the block and head is limited by the bypass valve so that MORE cooling will occur by sending coolant through the radiator in greater proportion.

ie.. IF you begin to overheat the engine w/o your t-stat, you have no chance of cooling it off b/c there is no mechanical means of directing flow based on engine needs. But until then, you are over-cooling the engine due to the nature of the system.

It appears that the t-stat is the dam, and once OT is reached, God opens up the flood gates to nourish the weak under-cooled. But the lord taketh as well, he reduces the flow through the block by proportioning the flood to the areas that need it more.

#5 01-10-2009, 05:48 AM

Clearly you don't understand how it works and are very resistant to learning how it works. I'll drop the argument right here because there is no point wasting my time in drawing it out any firther.

oldsinner111 · #6 01-10-2009, 06:29 AM

I agree with Forced on that.I built a 351c once in a mustang.We tried running with out a stat.It ran hot,because coolant never had time to cool.

Johtotahti · #7 01-10-2009, 09:15 AM

Thanks Forced. Those images set the record straight for me about the role of the thermostat in these fine diesel vehicles... One thing that is appreciated about these cars in colder climates is the warmth they offer the driver (in comparison to modern vehicles) and I can now see one of the reasons how that comes about (as the radiator is only used in cold weather when "need be").

t walgamuth · #8 01-10-2009, 09:49 AM

The stat in a benz is not an on off valve like a garden hose. Off on is how a traditional american made car stat works. With an on off valve you can cool fine without it but the car will run too cool under light loading.

A benz stat is a diverter valve. Its more like a switch in a rail line. When the engine is cool the stat diverts all water back to the head without ever going into the rad. When its hot it diverts all water to the rad and none to the head directly. It goes to the head after passing through the rad in that case.

If you remove the benz stat you will allow the water to seek its own course and it will send about half directly to the head and half to the rad. Thus when its really needing cooling its only cooling half the water and the car will run hot.

If its cold outside and the car needs to warm up half the water will continue to go to the rad and it will not warm up.

Drilling holes in the stat will only reduce its effeciency. A good stat working the way the system is designed to be operated cannot be beaten.

If you don't understand look in your repair manual for a diagram of how it works.

jt20 · #9 01-10-2009, 04:47 PM

good point.

I do not wish to run w/o t-stat. I am simply supporting my claim that the engine will OVER-cool until a critical point is reached.

Others have said that it will UNDER-cool.

All b/c a member mentioned that he modified his t-stat to flow more.

rcounts · #10 01-11-2009, 01:09 AM

All I can add to this depbate is that if your t-stat won't close completely you will run too cool. I just swapped mine out today. Before the swap, with half the radiator blocked with cardboard (a temporary measure to try to raise the temp) it never got above 70*C (160*F) in city traffic, and spent a lot of the time down closer to 50*C (120*F) on city streets.

After putting in the new one it runs right about 85*C - 95*C (185*F - 205*F) on city streets. Haven't had it out on the highway yet. I was getting about 24 mpg around town before. I expect that will go up 1-2 mpg now that it is operating at the more efficient proper temperature.