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6.9 Overheating

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


Sadly thats also my conclusion but was hoping for a miracle.

As far as i saw the head gaskets kits ar available at mercedes for ~300 eur per side.

But i will still drive the car for a bit before i tear the engine apart , as long as its not over 30C(luckily not so often in germany) the engine temp is tolerable.


Don't give up yet!

You said you did a compression test about a year ago and one cyl was down about 1bar/14psi?
Do you remember which cyl it was?

Also on the matter of pinging. How is the timing of the engine?  checkout OK?


I checked the timning also , it is fine pretty sure the pingin was because of the vacum advance and the high temp.

3 and 8 had around 10 the others had nearly 11 , but that was before driving .

I pressurized the coolant system and will look with a camera to maybe find some moisture in the cylinders hoping to confirm the head gasket problem.


ok...No 8 has a reputation for a slow leak across the head gasket to the back of the head.
So let's start there.

On a cold engine, remove no8 plug and look for any signs of moisture.
Is this plug noticeably lighter in colour?
Nothing untoward?
 When you next take the beastie for a run, get it good n hot then remove No 8 and observe .
 Still nothing?
While she is still warm put no 8 plug back in BUT disconnect the plug lead.
 Disconnect no8 injector line and start her up. [you might want to collect the fuel ]
Give it a few revs etc ..let it run for a minute or so then remove plug no8 again.
What have we got?


I had all the Plugs out and pressured the system for multiple hours in the last days and there was no water in the cylinders when i checked with a endoscope.
After that i drove the car 50 and on the wayback i drive >= 180 for 3 km, before that temp was stable below 90 (20 outside) and after that went up to 98 and when i let off  it was back to 90 after 2 km.

Weirdly the was no real pressure after that drive in the coolant system i was able to take the cap off while the engine was still running with out anything coming out.

While it was still running i made a test with the gasket tester and it turned slowly green after ~ 5 - 10 min.

So for it me it really seem like a super small headgasket leak but i don't understand why there was no real pressure i know that my new radiator cap is not 100 % sealed but it takes 1 hour to go from 1 bar to 0,3 when i pressurize the system and the car never pressed any fluid out the cap.

I am actually thinkin about putting a pressure gauge on the coolant system and check the pressure when i drive fast to see if it changes on load.


Weirdly the was no real pressure after that drive in the coolant system i was able to take the cap off while the engine was still running with out anything coming out.

I have extracted the above quote coz you are right...something weird is going on here??

When the system is hot and therefore pressurised, lifting the radiator cap from the reservoir should cause the fluid therein to expand in volume; such that when even mildly warm, the fluid could possibly overflow. When it is really hot [ as you have described ] you would have to exercise extreme care with the cap for fear of getting scalded by red hot fluid blowing out of the reservoir.

Yet as you have described it, nothing?

This raises the possibility that the radiator line to the reservoir is blocked. Is it possible that the top of the header tank is low?.ie it is low on coolant .

The reservoir looks full enuff coz it's can't actually see what's going  on in the radiator because it's a sealed system.
I don't wonder if this is the problem because the reservoir should be ebbing and flowing all the time...and blowing it's innards out when you release the cap after a good run.

Worthy of a look.


The Engine was not real hot at that time it had normal 90C so the coolant was not boiling , lines are not blocked and the radiator hoses were soft so not under pressure , i can only thing that the coolant expanded 98 C and when it cooled down to 90 it contracted so no pressure anymore.

I am just not understand i have a small headgasket leak were are the gases going when there is no pressure , when i putt the tester on i can see a small buble rising every ~20 seconds but i am not sure if thats air because the coolant is expanding when the engine heats up.

Drove around with a pressure gauge attached to the coolant system and it only goes to ~ 0,3 and stays there also on full load and after i turn off the engine.


With engine fully warmed and the radiator cap removed, are there any bubbles of any size at any time interval in the reservoir?   ie well after the coolant has expanded.

Similary, is there any measurable signs of fluid consumption.? measured after several cycles of getting hot..then cooling down..then getting hot etc with the CAP on. We are not looking for a lot here as clearly any possible leak is we measure it when it's cold compared with when it's least 5 cycles.


The Tester had a small bubble every 30 - 60 seconds but i am not sure if thats expansion because at idle the engine temp slowy creeps up to nearly 100C.
I drove the car ~ 10000 km with this problem and didnt notice any coolant loss but i always worked on the coolant system so i would max say without working on it was 2000 km.


...mmm interesting. So the car will overheat {indicated 100 degree] when left idling?
 So it's not related to the rate of work that the engine is being called upon to do.
 That is,  at a 180 kph that donk is working hard and the temperature rises accordingly
[ to be expected but not to the degree that it indicates.]
At idle at 20deg C ambient temp, the temp should not indicate any more than 80-85.

Basically it overheats sitting still!!?

I feel there's a clue here.

1. Have we double checked the thermostat..............have we run the motor without the thermostat to see if it still overheats? See later point.

2. Viscous coupled fan. Does it feel free-ish when cold but binds when hot?

3. The fan belt is nice and snug?

4. bottom radiator hose to water blockage?

5. I think you have treble checked the radiator...core gets warm evenly from a cold start.....reflects a temp differential of about 15 degrees from header to bottom irrespective of whatever the temp gauge says?.

6 . Confirm that the aircon fan cuts in at 100 I presume it will cut in after a protracted idle with the aircon OFF ?

7. Are we getting full circulation in the cooling system?.  ie confirm that the heater circuit is working.

If YES to all of the above, then our attention turns to the new water pump.
 This may seem a little unorthodox.....Combine with Q1.
Disconnect the top radiator hose at the thermo housing. Remove the thermostat . Check as per Q1.
Start the engine....heater on...warm or cold....and observe the should be considerable and respond to engine rpm.
There will be enuff fluid in the radiator to last about a minute...ample time to determine that the water pump is delivering the goods. You can always place a hose down the throat of the top rad fitting to extend the test but you should get the picture pretty quick.

OK.after all of that, what have learnt?


Viscous coupled fan. Does it feel free-ish when cold but binds when hot?

6.9 dont work that way. They turn slower when hot.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,


.err think that one through again..?


Can you please elaborate?
What do you mean "slower when hot"?
W116 1977 280SEL & 1979 280SE
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Well after i left it idling in the sun at 33C with Aircon on for 30 min it got to 98 .

1.Yes i tried a 80C thermostat and tested them both in hot water , no difference at all.
2.I am sure that its running because it's making more than enough noise
4.No whould the Radiator get hot if there would be a blockage ?
6.Yes it does
7.Yes its working very good

I now tried 2 Waterpumps the Sympthom are 100% identical , and one was badly coroded the other not, also why would the Radiator getting hot when i would have a circulation problem ?

@viscous fan
As far as is know the viscous always runs but turns down to 1200 rpm when the engine goes above 3500 rpm and mine does exactly that.



reading the 100-985 manual

up to a speed of 3500 rpm the engine fan will run proportional to the engine speed, whereby a fan speed of 1900 rpm results

as engine speed increases further the fan speed will drop to about 1000 rpm and max of 1200

It should be very stiff when cold. if it freewheels at all it's shot.

increase engine speed slowly, fan speed must drop, which is heard clearly at a maximal speed of 3500 rpm

Also make sure you are using a 87 degree thermostat.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,