It appears my trusted 300SD (84) has developed a coolant leak which I believe is originating in the bowels of hoses and connections around my monvalve. I cannot see any leaks there, but under the car it is obvious that the coolant is coming down from that area. Before I start surgery I am wondering what I should expect or look for. What other gizmos can be leaking in this area. I have already tightend hoses and that did not help
I have plenty of heat, recently replaced monvalve.
Any thoughts. I am off this week, so what better way to spend my time off than fixing up my MB. My wife will understand I am sure......

Yes there is a meriad of hoses at the firewall. But lets make sure it is those hoses. First, when you run the AC system at full heat does the air have a sweet smell? Hopefully not. That means you have a leak in the heater core inside the dash. The leak would drain at the tranny tunnel just as the AC does with the moisture, this is usually not a problem in the SD. But lets eliminate that first.
It could simply be that a hose clamp is loose. I just had this problem. You can tighten the clamps with a flat or phillips screw driver put using a socket driver is best.
I can't remember from my SDL days if you have to remove anything to get to all the clamps. In the W123 this is necessary, the battery has to come out to get to 2 of them.
So tighten ALL the clamps first, cheapest solution. Also check the mono valve for leaks and possible cracks in the housing. also the hoses may have developed a leak. Check them for leaks/cracks.
Then we will go from there.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Seems like we have the same problems, haha! Check the auxillary water pump. Mine has been leaking for some time now - can't find the time to replace it (or a break in the weather). Does it leak only when cold ie: after you have driven it a while and park it overnight?

Dave and Benzboy
here is some more info, I was able to visualize the aux water pump, pulled it up and felt around and could not see any leaks. There was a SLIGHT amount of moisture present but the puddle under the car , after it is parked overnight , would indicate, at least to me that this may not be the source of the leak. I even ran it till hot and did not see any leakage. I have it parked and will check later
My heat does not smell sweet and seems to work OK
When I looked under the car the fluid seemed to be coming out of a black flap like structure about mid point directly above the bell housing the part number on this flap is 1269970181.
Im lost all the hoses are tight and fluid drip free.
ANy ideas???? HELP, I am afraid to drive it

Forgot, I also tested the aux pump with 12 volt and I heard it spin.

There's a small rectangular-shaped plate on top of the heater valve(monovalve). 4 small screws fasten it down. Make sure all are snug. I fought this same battle a few years ago. These screws need to be snug.

Consider pressure testing it when the engine is cold. You may suprised at how many places it's leaking. I had one that was leaking at the o-ring for the theromostat housing and I never would have found it without the pressure tester. No telltale white signs.

Great news, not the heater core but the thermo valve that controls the fluid to the winshield heater element. I did not even know this car had this set up but a VERY KIND lister (Phil) sent me information that put me in the right direction. Had to remove wiper arms and air intake to visualize, but there it was, leaking away. I love this list, such a great service, now to find the part!