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6.9 Overheating

Started by ramiro, 26 July 2022, 10:20 AM


This might be something to check out:

From the 100.985 engine manual:

18-120 checking the thermostat in the oil filter

insert temp probe in oil supply tank
let engine run and observe temp
at 80-85 deg C a definite increase in temperature must be felt by hand in the oil cooler

note: due to the temp drop between the oil filter and oil supply tank, the temp of the oil in the oil supply tank will be below the opening temp of 95+-4C.  fully open at 110 +-4C

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,


OK.good work.! You must be sick of this. :'(
If you have a blockage in the system it will be a partial one... or if the water pump is not delivering  the fluid still flows but the longer we run it the less fluid is presenting to the radiator so it slowly gets warmer and warmer. Sure the radiator feels warm and even and we are getting temp differential but gee it getting hot if you run it long enuff.
As I see it, the system from the outlet of the water pump thru the radiator and into the water pump inlet is working fine.
I suspect that the flow in tho from the water pump is not equal to the flow out.
So that leads us to testing the water pump as per my suggestion at the foot of my last email.
The history of that new pump is unknown. So for instance the running clearance of the impellor to the casing could me miles out.
The output should be considerable.
Let's know how you.


The oilcooler thermostat is working fine , when the engine is hot the oilcooler also gets hot it then has ~ the same temp like the oil sump.

I had the waterpump 3 times out , first i just put the used in , i had the impeller of to change the seal and set the clearance according the manual , then i had the the pump out again because the bearing was bad and changed it out with the one from the corroded pump and set the clearance again but a little bit tighter.

The temp sympthom was always exactly the same even with the corroded impeller.

The  radiator is not just warm the whole top tank has engine temp when the engine is hot and the whole bottom tank has ~ 15C less.


Well I reckon you've established that everything from the thermostat, hose, radiator, bottom hose is all OK.
 That means the "cooling system" tests ok but  the  "circulation system" remains an unknown quantity.
We know the heater circuit works coz the heater is comfy  cozy and we can feel that the return hose is hot. Yes?
Basically the fluid is arriving at the the radiator header tank too hot. Time to test the output of the fabled water pump me thinks.


I see no reason for bad circulation , drove today when we had 36C and with that weather also driving normal the Temp stays at 98 , what i am wondering is that the oil temp was also around 98 everywhere i measured, i always though oil runs hotter than the coolant.

Also the Radiator has 94 C at the bottom tank opposite the outlet , and 90 at the outlet with such outside temps , so even less than 15C difference ,  if the water pump would not circulate the bottom should be much colder then the top logical thinking.

I also had to the top hose off while the engine was running some time ago when i was flushing the coolant system and the pump was clearly pumping.

What i still don't understand is why is this Car behaving exactly like mine :
In the same conditions my temps is the same like in the video thats why i was thinking that a 6.9 runs so hot.


I think it even states in manual that higher temps must be considered "normal" depending on driving and ambient conditions of course.
For example if you are doing a hill climb on 30C+ day for sure the needle will go close to red, but then should come down once you return to more normal conditions.
I always say focusing on what the needle is DOING rather than the absolute gauge reading is the key.
Does it come down or not?  Of course if it's pegged in the red and stays there you are in deep shit.
1977 450 SEL 6.9 - Astralsilber


Quote from: ramiro on 26 July 2022, 01:40 PMThe Fan turns off over ~3000 RPM so at 180 it should not make any difference and under 3000 i can clearly hear it.

Can you elaborate on "turns off"?  is should definitely slow down though.

Are you using a 87 degree thermostat?

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,


QuoteDoes it come down or not?  Of course if it's pegged in the red and stays there you are in deep shit.
After it went a little above 100 it just came down to a hair below the 100 mark at slow driving

QuoteCan you elaborate on "turns off"?  is should definitely slow down though.
Offcourse its not turning off completly but you can hear that the fan spins slower

Yes i am using a 87 thermostat tried a 80 and it made no difference at all.

Thats the most frustrating thing so far eveverything i change makes no difference at all (2 Waterpumps , 2 Radiator recores , timing chaing wrong by 1 teeth , different thermostats , flushed the coolant system several times)

There is still the transmission that's also using the radiator, it's shifting perfect, i read that a bad converter can also cause overheating somewhere but as far as i know i don't have any signs for that.


So we have exhausted the obvious and the not so obvious over a period of time/tests and tribulations.
From all of this it would be fair to say it indicates it is running about 15 deg C above the norm.
That includes unusually hot weather...and some hard punting as well as idle..dawdling around town. etc
So across the operating range of the vehicle, it doesn't's always indicating hot.
As you say it is exhibiting the same characteristics at hi speed as the 6.9 in the video......I note he hardly had the aircon on?

I doubt you have tranny problems.
The car pulls well and generally seems to have plenty of top end.........I would be staggered if a worn tranny transmitted enuff heat to the cooler to influence the engine temp.  It would have to flare and slip like crazy and that is  not happening.......

My thoughts turn to excessive backpressure.  Worth eliminating........but this would reflect in sluggish performance. particularly a willingness to rev she all good in that respect?

Now final test. Remove the radiator cap from the reservoir...ensure there is enuff coolant in it to just cover the drain feed to the radiator header.
Start the engine and either drive it until it gets to normal operating temp OR let it idle but either way we want it to get to 87 indicated and then see what happens when it rises beyond. So as it gets warmer it should creep up to  a 100....the fluid will expand but not fill the reservoir until it finally boils.....if indeed it does. If and when it boils, what is the indicated temp on your gauge?
Leave the aircon off and pick a day when the temp is perhaps hi 20's.
What is this going to prove?   
Well so far, from what I've read [yes I' might have missed it] there are very few symptoms that the engine  is actually getting hot...
Yes I know you've had the IR gun all over it but they are largely unreferenced readings.

You have reported pinging at 180kph: but not that it is boiling and puking fluid everywhere.? An engine getting this seemingly hot should not be holding its coolant.
There's another odd thing........does the engine get "heat soakback"  when turned off?. ie boils after it stops and vomits fluid everywhere.?

Would you humour me and explore this avenue? ::)


No when i drive fast there is no Limit to the temp it will go to the red mark i just slowed down when it had 115 , but the car in the video also would go further in temp i think at least is was going up till the end.

QuoteMy thoughts turn to excessive backpressure.  Worth eliminating........but this would reflect in sluggish performance. particularly a willingness to rev she all good in that respect?
Well it got to the rev limiter in last gear at 250 according to speedo so i think engine perfomance is fine, allthough in 2. gear it feel slugish above 4500 but i thinks its normal.

I am pretty sure the temp is real , i can see it staying some time at 87 when i drive slow as it should.

When i turn the engine off the temp creeps up but i never turned it off when it was above 100(because it usually cools down to around 100 when i go off the highway and drive slower) so it had no reason to boil out.
Yesterday after it went above 100 the Radiator hoses were very hard  and even after 2 hours they still had pressure left, when the engine stayed below 100 i was able to take the cap off without anything comming out just a tiny little bit of pressure.


Something screwy here. The engine should build 12 lbs of pressure or more........when you pull into your driveway after a thrash and the temp has dropped from 115  or whatever to a 100; and you pop the radiator cap, it should erupt volumes.
All you're getting is a bit of pizzle.?? boiling water .no massive release of pressure...  and no pressurised water???

It has all the characteristics of not really being hot... yet? ::)
What happens when you drive it with the radaitor cap simply not on?


Hello again ,
I did change the headgaskets now because i had enough , the rear part off the block was full with rust and the valve guides and seals were completly done and 1 exhaust valve also had a crack.
The car finaly has a perfect idle and there is no more blue smoke from the exhaust.
But my temperature problem is still fully there slowly i am out of ideas what still could be tested i did the the testdrive with a afr meter hooked up it was a little rich so that couldn't be the problem , it behaves exactly like before, and yes it is really getting hot everything thats has to do with cooling is over 100C when the gauge shows over 100C also the AC fan is running.



The first 3 pictures are fascinating, but I cannot tell where in the engine you took them. Is it possible the entire block passages are full of rust and thus have poor ability to transfer heat? The picture of the piston head looks odd.  Is that rust deposits on it?
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,


OMG!!!..........what a job. That block must be full of crud. Try running the water pump with the top hose diconnected and see if it's shifting the fluid for astat.


The first 3 picture are the top cooling passages from the 4. cylinder that was the worst place , the front cooling passages were clean.