Now here's a mystery for the really brave...

My vehicle is a 1985 300CD-T (Calif.). Mileage is 195,602.
Issue: Dried residue from coolant appears on the transmission pan and the support panel for the driveshaft's intermediate bearing. I have attached three pictures.

Why do I think that this stuff is dried coolant residue? Because it has the whitish-bluish color. The new M-B coolant that I use is blue colored.

When does this residue appear?
Only after driving the vehicle on the freeway for 10-15 minutes and around 60-65 MPH.

How much coolant disappears from the coolant reservoir?
Negligible. The fluid level barely appears to drop.

All of the original cooling system hoses were replaced in July of 2011 at 178,394 miles.

I have checked the tightness of all of the cooling system's hose clamps and tightened a bit more if it seemed okay to do so.

I have used a dentist's mirror to look completely around and underneath all cooling system hoses, metal lines, and connections and have found no sign of coolant leakage anywhere.

My current guess:
Coolant is leaking from around an exterior and rear (near the firewall) portion of the head gasket from a very small spot that only leaks when there is high pressure in the cooling system (read: highway driving at highway speed). The coolant then leaks downward, blows onto the underside components (i.e., transmission pan, support panel for the driveshaft's intermediate bearing) and dries.

Other thoughts?

Attached Thumbnails