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

     

I am no expert on coolant splatter, but it doesn't seem to me that you would have such a widely dispersed pattern from a drip at the rear of the engine.

M best luck finding coolant leaks has been with a pressure tester. With the car sitting still, you won't have the splatter issue. High on my list of suspects would be the radiator tank seams.

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Perhaps Iis condensed water from running the air conditioner. I agree the splatter looks suspect. Keep checking the coolant level I would worry if it drops significantly after a week.

I'd also suspect the lower seam on the radiator.

The upper seam on mine has intermittent small leaks and creates a similar spray patter on the upper components under the hood, sprayed around by the airflow there.

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Hey, thanks for all these great replies!

Now then, a little feedback...LOL

I replaced the radiator with a new Behr unit and also replaced the auxiliary water pump at 178,394 miles (July, 2011 - the same time that I replaced all of the cooling system hoses). I have checked the auxiliary water pump for leaks and detected none. I have not inspected the radiator and its seams with a fine tooth comb, so to speak. I will do that.

Doing a pressure test had slipped my mind. That is a great idea! I will probably use a local M-B shop to have them do that.

I will say this: A few days ago, I scrupulously cleaned the area where the dried coolant was everywhere I found it, let the engine idle for about 20 minutes, and could not find a singular drip. This must of had to do with the fact that the cooling system was not fully pressurized. Correct?

Great observation about the pattern of the dried coolant being too dispersed for a drip to be originating at the rear of the engine. That is the benefit of other minds thinking about this!

As for running the air conditioning vis-a-vis condensation: I have not been using the air conditioning...

Where I live, just about any chain auto parts store will lend a pressure tester. It's just a matter of replacing the coolant bottle cap with the tester fitting and pumping it up. Then listen and look for leaks.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Ah...Good to know! I'll stop there tomorrow and check it out!

If and when I finally track this dang leak down, I swear, I'll be happier than a pig in...well, you know...

I would guess the water pump, the coolant is dripping out of the weeping hole on the pump....dropping down and then splattering back. It may not be to the point of no return and that is why it only shows up on a high speed freeway trip...

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A few thoughts:

1) What color is your washer fluid? Mine is blue...

2) If it's really antifreeze, I'd suspect a leak inside the heater core. This could easily find it's way out the a/c condensate drains, in the right general area.

Good point. Well, I will go take another look at the water pump and weep hole.
However, I replaced the water pump and gasket at a very recent 193,360 miles (November of 2013).

The color of the washer fluid is clearish and slightly yellow. I pulled up the reservoir and found no evidence of liquid underneath or around the reservoir.

I hadn't considered the heater core.
Where might I check inside the car for possible leakage of this unit?

Should be smelling the hot coolant in the car if the heater core. That's with heat on of course.

Okay.
Well, I haven't run the heat in a while. I will do so and report back.
Thanks!

I borrowed a pressure tester and my results were hoses not tight enough at:
1. Metal line off of thermostat housing
2. Hose at top of radiator
3. Large hose at bottom of radiator
As I mentioned above, I replaced the radiator fairly recently and I think that I was overly cautious about not over tightening the hoses to the radiator.

Anyway, I tightened the three hoses. Then I left the system pressurized for about 30 minutes. The pressure dropped about 1-2 PSI over that time. Unfortunately, the problem still remains.
I took the car out onto the freeway twice and both times, there was plenty of dried residue on the transmission pan and intermediate bearing mounting bracket.

My guess at this point is a heater core leak.
I currently, have some other climate control vacuum problem so when I tried to blow heat, I just got cool air. Incidentally, I recently replaced the mono valve unit, not just the insert, for a cool $380 from M-B.

Is there any way to check for damp carpet around the heater core at the passenger's feet (that is where the heater core is, isn't it?) or elsewhere for a heater core leak?

Also, is the fact that the pressure dropped 1-2 PSI while I left the system pressurized with the tester for about 30 minutes indicative of the leak?

Not necessarily a big enough leak to be of any consequence. If you stopped the visible coolant leaks, you have likely solved any problem worth worrying about.

Also, the heater is valved on the downstream side, so it doesn't need to be "on" to leak.

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