Author Topic: 6.9 creeping coolant loss  (Read 473 times)

Punker

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6.9 creeping coolant loss
« on: 29 June 2020, 03:01 AM »
Hi guys,

as reported I changed the radiator hoses and the thermostat on my 6.9. Unfortunately I am now losing cooling water. About 1.5 liters per 100 kilometers. Pretty stupid. I haven't found a leak although I suspected the control of the automatic air conditioning (it's always a bit wet there).

Does anybody have a tip how I can proceed to find the leak?

Thanks for your tips on this annoying topic
Bjorn

1978 Mercedes 6.9 # 4616 - black/black leather
BMW R 25/3

raueda1

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Re: 6.9 creeping coolant loss
« Reply #1 on: 29 June 2020, 07:47 AM »
If your leak just started when you changed hoses I doubt it's a coincidence.  Clamps need to be surprisingly tight or they absolutely will seep.  Loosen, wiggle the hose around to reseat and retighten.

Other than that?  Leaking heater core or heater hoses?  I had a kind of always-wet floor situation and that's what it turned out to be.  OTOH, 1.5L/100km is a lot, mine was much smaller.  Still, worth exploring.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: 6.9 creeping coolant loss
« Reply #2 on: 29 June 2020, 09:22 AM »
Maybe also replace the radiator cap, they are super cheap.
Daniel
1977 450 SEL 6.9 - Astralsilber

Punker

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Re: 6.9 creeping coolant loss
« Reply #3 on: 29 June 2020, 12:36 PM »
many thanks for your tips . I have already replaced the radiator cap. I will check the hoses. Is it actually possible to remove the control unit for the automatic air conditioning and kill the connected hoses?

Plan a longer tour to Berlin in 2 weeks. Then I can't lose that much water.

Greetings
Björn
1978 Mercedes 6.9 # 4616 - black/black leather
BMW R 25/3

raueda1

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Re: 6.9 creeping coolant loss
« Reply #4 on: 29 June 2020, 04:54 PM »
many thanks for your tips . I have already replaced the radiator cap. I will check the hoses. Is it actually possible to remove the control unit for the automatic air conditioning and kill the connected hoses?

Plan a longer tour to Berlin in 2 weeks. Then I can't lose that much water.  Greetings
Björn
Can you clarify?  AC doesn't interact with the cooling system at all.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jed

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Re: 6.9 creeping coolant loss
« Reply #5 on: 29 June 2020, 07:52 PM »
I think he is referring to the heater servo for the ACC...yes that can definitely leak.....
1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?

revilla

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Re: 6.9 creeping coolant loss
« Reply #6 on: 30 June 2020, 04:44 AM »
Yes, 1.5 litters/100 Km is a lot!
But coolant liquids are made to leave a white trace once dry. So it should be easy to identify the leaking spot.
If you suspect the acc servo, yes, you can isolate it (by-pass) for your trip. There was a recent post about it. I’ll try to find it.
Better to find the leaking point(s) and fix rather than the frustrating path of changing parts left and right in a trail and error approach. That can be expensive and you might find yourself chasing shooting stars. Not good.
Cheers
Robert
W116 1977 280SEL & 1979 280SE

revilla

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Re: 6.9 creeping coolant loss
« Reply #7 on: 30 June 2020, 04:50 AM »
Found it.

https://forum.w116.org/mechanicals/ac-nightmare/msg146314/#msg146314

If you need details on how to perform the bypass let us know.

Robert
W116 1977 280SEL & 1979 280SE

Punker

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Re: 6.9 creeping coolant loss
« Reply #8 on: 30 June 2020, 11:40 AM »
Many thanks for the recommendations. In the next few days I will have time to tackle the problem with your recommendations. I'll keep you informed.
Many greetings
Bjorn
1978 Mercedes 6.9 # 4616 - black/black leather
BMW R 25/3

PosedgeClk

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Re: 6.9 creeping coolant loss
« Reply #9 on: 30 June 2020, 07:05 PM »
I am surprised that no one has suggested the fluorescent dye. There are fairly inexpensive kits whereby you purchase a dye and a UV illuminator and trace down the leak shortly after running the dye through the system.
1979 450SEL 6.9

Randys01

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Re: 6.9 creeping coolant loss
« Reply #10 on: 01 July 2020, 01:01 AM »
how about a little look into the motor?

Trapo

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Re: 6.9 creeping coolant loss
« Reply #11 on: Yesterday at 03:00 AM »
how about a little look into the motor?

Randy's suggestion is sound.  I found coolant dripping into the oil pan through a hole behind the water pump. If your oil measuring stick comes out "mayonnaise" color as Randy called, you found your leak.
A Mini racer but an MB fan at heart.
My first MB: 220 1954
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My third MB: 190 1961
W210 320 S, W210 270 CDI, 190 E
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