6.9 overheating issues

[Styria]

Presently, there is on ebay a 6.9 listed for sale in Victoria. I have looked at the photo of the engine bay, and noted a replacement 85 degree switch to activate the auxiliary electric fan. Now, some of us would be aware that, from the manufacturer, the standard switch fitted cuts in at 100 degrees to assist in cooling and keep the engine/water temperature at the recommended level. For some time, I have recommended that 6.9 owners should fit the 85 degree switch to assist in cooling during the summer months. It all sounds pretty good and straightforward, doesn't it ? Well, I have been wrong, according to a friend of mine, a fellow 6.9 owner.

He maintains that it is absolutely inappropriate to fit the switch that comes in at the lower 85 degrees. Having regard to the fact that the thermostat does not open until the temperature reaches 87 degrees (thermostat specification), the water in the cooling system does not circulate and reach the radiator. Therefore, if anything, he maintains that the coolant in the engine itself is far higher than it should be, and all that is achieved by the auxiliary fan is to cool the radiator coolant itself but, because the thermostat is closed, that coolant does not circulate right through the system. Accordingly, he maintains that the engine will run too hot.

He also expresses absolutely no concern if an engine is running at 100 degrees - boiling point. The engines are designed that way, and he maintains that one can run an engine for hours at that temperature without any undue concern for overheating. He and I have some great discussions, and with him being an engineer, he is able to properly read and understand details provided by the manufacturer in their workshop manuals. Anyone have any thoughts on this subject matter ? BTW, I run the 85 degree switch in Gleaming Beauty. Perhaps I should change back to the standard item. Regards Styria

 

[Andrew280SEL]

I'm no expert on this, but from reading the old manuals it always fascinated me that MB always seemed to suggest one should not be concerned about temperature unless it approaches the red zone.

Which, on these W116's etc as you well know, is at the very very top of the gauge

So it wouldn't surprise me if they were built to withstand 100+ degree temps.

 

[Oversize]

Unless you're running straight H2O with no radiator cap, 100deg is not the boiling point. And I'd imagine MB would give customers a far greater safety margin than 15deg (100-85) anyway....

 

[sean sherry]

My understanding has always been that the pressure cap say 7 lbs raises the boiling point from 112 to about 120. Allowing an engine to run safely up to say 100 %. The higher the Temp, within reason, the bigger the bang to drive the Piston down via greater gas expansion.

 

[CraigS]

Boiling point with water only is 100 degrees C, but with coolant, this is raised to approx. 128 degrees C, so an engine running at 100 degrees is not a problem - if anything, it is more efficient.

If you are not running coolant, aside from being an idiot, you are causing harm to the competing elements in an engine i.e. cast iron/aluminium.

 

[Styria]

Well Craig and Mark, the question I pose relates to that switch itself. Should one stick with the standard 100 degree switch fitted as standard, or is it of any use to run/use the 85 degree unit ? I should point out that it was a very well known company of long time Mercedes mechanics that advocated the use of the lower temperature switch to help minimize possible high temperature running or overheating. Regards Styria

 

[Michel]

From the explanation relating to the coolant and its properties for lowering the boiling point, I believe you would stick to the factory settings, noting that the factory researchers and technicians at MB are slightly more learned than US AND that company you mentioned.

 

[CraigS]

And I suspect because of a less than pristine cooling system. If everything is in order, which means radiator, water pump, timing, sensors, thermostat, cylinder heads, block, there SHOULD be no reason to change it, but of course, not all systems are pristine. I think it is better to find the root cause of any problem, - if there is one, rather than changing something to overcome a problem.

 

[Andrew280SEL]

Or worse, those mechanics who just remove the thermostat altogether

 

[Styria]

Okay, let's go around it another way. Say everything was to be perfect, as perfect as can be with an almost forty year old car, and that is extremely difficult to attain, if the temperature gauge on a hot day exceeded 100 and reached about 110, is there any benefit likely to accrue by having the lower temperature value switch.

I understand, of course, with that prevailing high temperature, and the thermostat fully open, and the auxiliary fan going like mad, is the temperature likely to go down further when compared with the same set of circumstances, but the 100 unit fitted ? Regards Styria

 

[BenzBoy]

Okay, let's go around it another way. Say everything was to be perfect, as perfect as can be with an almost forty year old car, and that is extremely difficult to attain, if the temperature gauge on a hot day exceeded 100 and reached about 110, is there any benefit likely to accrue by having the lower temperature value switch.

I understand, of course, with that prevailing high temperature, and the thermostat fully open, and the auxiliary fan going like mad, is the temperature likely to go down further when compared with the same set of circumstances, but the 100 unit fitted ? Regards Styria

Possibly not - and thus you find that with cars that have marginal cooling at the best of times people fit a second fan or a larger radiator. I recall my mother complaining in the 1970s that her V8 Holden would overheat with the air-conditioning going and the ambient temperature reaching 100 F (remember this was way out West where the grasshoppers rule) - the solution from the dealer was to fit a larger radiator from a truck and the problem was solved. That is, the cooling as designed was marginal even when all was working so the problem could only be solved by increasing the radiator capacity and hence the cooling capacity. I don't know of a modern car, or any Benz of any era, that has such as issue.
Regards,
Brian

 

[Oversize]

MB redesigned the M100 cooling system to suit Aussie conditions hence the deleted oil cooler (??), bigger radiator and aux trans cooler pump. Perhaps the factory engineers decided the oil cooler wasn't required with the lower compression engines? Pity the mods didn't extend to the interior materials to make them last longer in the harsh Aussie climate....

I agree to find the source of the problem and fix that. However this is all assuming you're using the car as the factory intended. As with recommended oil viscosity which I'd suggest is applicable only to sub-100K km engines, did they really intend the vehicles to last this long? The characteristics of the engine will change over all those years.

Something else to think about is a racing or dedicated burnout car where significant mods have to be made to ensure the thing doesn't self-destruct. A factory setup just won't cut it under those circumstances

 

[Helmet]

From the explanation relating to the coolant and its properties for lowering the boiling point, I believe you would stick to the factory settings, noting that the factory researchers and technicians at MB are slightly more learned than US AND that company you mentioned.

Second part sounds good to me. , first sentence should read raising the boiling point

 

[PvD]

If the thermostat operates at 87 degrees C this will keep the engine above 87 degrees C when it is cold including freezing/snow conditions. The thermostat not only operates when the car is started from cold and heats up but is operation continually. Using a 85 degrees C fan temperature switch will only make the thermostat close to keep the engine temperature above 87 degrees C and wear out the electric fan. An engine with the correct coolant and the pressurised radiator cap can run quite happily with a 100 degrees C fan switch.
My car is an English import and it would be interesting to compare the radiator and other cooling systems (oil cooler, transmission oil cooler) with an Australian delivered car. The car has never overheated, then again it does not hang about in traffic on hot days.
Peter

 

[Michel]

Second part sounds good to me. , first sentence should read raising the boiling point

I stand corrected Dr Helmet
Raising is the verb I meant to use.

 

[Styria]

I stand corrected Dr Helmet
Raising is the verb I meant to use.

TK is now going the imaginary professional route - the Godfather, the Prince of Arabia, the Professor and now the Doctor - Helmet, congratulations on your elevated status. Regards Styria

 

[sean sherry]

Back in the olden Days as my Grandchildren would say...... No pressure caps and probably no Anti Freeze Coolant to elevate the Boiling Point of water, how did engines fare ?
I remember all the old Cars running around Sydney in the fifties boiled at every rise in the road. Hissing steam at the side of the road was a common sight. I never remember a Car having over heating problems during my apprenticeship in wet and windy Ireland. So maybe it is generally a hot climate issue.,,,,, Particularly British Cars up to the fifties ????
I don't think they even had thermostats ,, just a Radiator Blind in the Winter to keep the heat in !
Heat was never a problem , just the lack of.

 

[BenzBoy]

Back in the olden Days as my Grandchildren would say...... No pressure caps and probably no Anti Freeze Coolant to elevate the Boiling Point of water, how did engines fare ?
I remember all the old Cars running around Sydney in the fifties boiled at every rise in the road. Hissing steam at the side of the road was a common sight. I never remember a Car having over heating problems during my apprenticeship in wet and windy Ireland. So maybe it is generally a hot climate issue.,,,,, Particularly British Cars up to the fifties ????
I don't think they even had thermostats ,, just a Radiator Blind in the Winter to keep the heat in !
Heat was never a problem , just the lack of.

Ahem, I have driven the PII on a day of 38 degrees C and no overheating issues - just Redline in the water - not thermostat and the variable grille vents open wide.
Regards,
Brian

 

[SEL_69L]

As I understand it the temperature sensor is measuring the coolant temperature at the cylinder head, not at the radiator.

 

[aussie 6.9]

I run an 85 degree switch in my car . Out here it can get very hot, like44c plus. I am the second owner of an Aussie delivered car and it has a supplement to the owners manual warning drivers to keep a close eye on the temp guage when travelling at high speed.So maybe the engineers realised Aussie is hotter than the northern hemisphere.