I thought I had a head gasket leak at the passenger side rear corner based on the grime there.

It was a hunch but it turned out it was just a dirt/water mixture that had dried leaving some dust behind.

Then, I thought I had located it when I saw fluid on the passenger side heater hose(between the engine and the airbox compartment).

I bought some new heater hose and crawled underneath to get a closer look.

There had been coolant running down the hose for quite a while for sure but the trail started way above it.

I then notice that it led to a bolt that holds a bracket that is attached to the steel heater line that leads from the engine near the elbow.

It turns out the bolt was loose and that there was supposed to be another bolt holding the bracket on but that bolt was missing.

The missing one attaches away from the engine block but I don't recall exactly where as I type this.

I took the the existing bolt out and it looked like it had been in the presence of water/coolant for some time.

It had some crust and rust all long the threads. The rust was just on the surface though.

I tightened the one bolt I had and I can see no leakage today after driving about 200 miles.

The bolt attaches aft of the thermostat housing but not directly to it.

It looks like it bolts to the same part that the t-stat housing does.

What is it and why on earth would coolant leak from there?

The bolt looked like it had an aluminum crush washer between the bolt head and the bracket.

__________________
1993 W124 300D
-297K on the clock as I type this.

In the Cyrillic Pictogram below...
I BELIEVE #275(or it might be #281) is the "Pipe" you're referring to and #287 is an Rubber o-ring
which when the bolt #293 is loose allows a leak ? (Maybe)

'On my engine the other end of #275/(#281) has a "Y" shaped Hose,the other two ends of the hose go to: 1. Another pipe that connects (eventually) to the bottom of the Expansion tank.2.The "Crossover Under the Engine Pipe" that completes the Windshield Washer Tank Heater Loop.

Much,Much,Much ,TOO Much:
1.Heater
2.(Automatic side to side of the Chassis Air Vent Hoses and Pipe),
3.Windshield Washer Tank Warmer System Loop.
HOSES!
['Just when you think you've found ALL of Them...Peeking around a
corner of the engine compartment...IS Another!]
'Knowing now how much footage that little electric bosch auxiliary water pump moves fluid through...I'm impressed!

AND then there's the Damn Diesel Fuel Pre-Heater/Thermostat System and it's attendant HOSES!

Attached Thumbnails

   

__________________
'84 300SD sold
124.128

The bolt I'm talking about is #293.

It definitely is strange how that loose bolt allowed a coolant leak.

Maybe it just allowed one of the pipes to torque itself loose enough to spring a small leak.

I will keep an eye on it.

I was actually worried that a head gasket leak appeared near there and the coolant just managed to find it's way out at the loose bolt.

Thanks for the diagram.

__________________
1993 W124 300D
-297K on the clock as I type this.

#293 holds the pipe firmly against the backside of the Water Pump allowing the
O-ring #287 to seal the pipe to the Water Pump Opening.
If #293 is loose #287 doesn't seal the end of the pipe.
'Hence your leakage.

Thanks!
I now know about #287 and can order Two for replacement!
Seal Ring #012 997 5148
(The same O-ring seals the fuel preheater exchanger to the Block on the opposite side of the engine!)

__________________
'84 300SD sold
124.128

Could this be the o-ring I ordered with my water pump but didn't know what to do with once I replaced the water pump?

__________________
1993 W124 300D
-297K on the clock as I type this.

Neither the EPC or the RU site give dimensions (of the ID/OD or thickness)...

__________________
'84 300SD sold
124.128