Ok, So I'm in denile..
 

No, not the river...
This hasn't even started to sink in yet.... I was just doing a cap/rotor job on my new 1991 300E and while twisted sideways trying to get the bottom bolt on the cap off I thought my overflow reservoir looked funny...like kinda dark and muddy...oh S#@%!!
This was something that I wasn't concerned w/ at purchase because I had the paper work in my hand of the head having been done less than one year/25k mi ago. And it was all shiny and everything...
So my question is: what happened?!?! Is there something that I can't think of that would be an issue other than the head? There are no leaks visible and the compression is totally top notch. Was the head warped and not planed when the head job was done? It was an indy so I can't complain to the dealer....
I know that I'm probably f%@&ed and that I should clear out a block of three or four days on the calender to deal w/ this, but I'm hoping that there's some less involved thing that could have gone wrong.
Any hope will be greatly appreciated. Any despair will be accepted and quickly followed w/ a round of gin martinis.
Thanks.
-Matt-

Calm down. If oil had gotten to the cooling system before the previous work, it'll take a good amount of work to clean cooling system. Check the archives for the full cooling system flushing procedure which includes deoiling, descaling and cleaning the reservoir. If oil in the coolant returns after a good cleaning, go ballistic.

Sixto
95 S420
87 300SDL

As Sixto suggests, it could well be left over oil in the coolant reservoir from the earlier head gasket failure. These reservoirs are very difficult to clean out due to all the internal baffles. After replacing the head gasket on our 300TE a few years ago, I spent considerable time removing oil from the coolant reservoir (and the rest of the cooling system). When I purchased my 190E early last year, it was obvious the head gasket had been replaced, and the reservoir not cleaned properly. On removing the reservoir I found that some compartments in it were still full of thick oil. Since the reservoir was also leaking slightly at the filler neck, I decided to replace it (The 300TE actually ended up getting the new reservoir with the old one from it going into the 190E since they were the same part number and the TE is our car for long trips). Fortunately, all the oil was from the previous head gasket failure as I had suspected. Replacement is a viable option for a badly oil contaminated old coolant reservoir (they are not prohibitively expensive).

Not true! It is a coolant recovery tank. When the system cools a vacuum may develop, and when the cap vacuum relief valve opens it will draw back liquid as long as the bottom of the hose in the recovery tank is immersed in liquid. If there is a leak, a vacuum is almost bound to develop when the coolant cools after operation.

There are two different cooling system design philosophies - one with a "free surface" and the other without. A system with a free surface is thought to purge air more readily, but many OEMs design cooling systems without a free surface and some seem to swap back and forth between both design philosphies. With the vapor vent line from the top of the water outlet to the top of the supplyh tank, the Mercedes design should vent vapor very readily. My own experience is that the system purges air very well after a coolant change.

When I change my coolant I add water to the recovery tank and then siphon it out to remove any debris, but it always seems clean. Has anyone actually removed the tank from inside the fender. It seems that the fender would have to be removed to do so(?).

If you slightly overfill the system it will expel any excess into the recovery tank, and that coolant will remain there and be drawn back into the system if it ever develops a vacuum.

Duke

I've never actually seen the recovery tank in my 190E, but there is a plastic splash panel at the rear of the wheel wheel, so since I'm about ready to do a 15K maintenance and a coolant change, I think I'll remove the panel and have a look see.

As the coolant heats up it expands and compresses the vapor dome in the pressurized supply tank. If the coolant gets hot enough to form vapor bubbles in the block or head the 15 psi pressure relief valve will open and vapor and maybe some liquid coolant will be expelled. The vapor is some combination of air, water, and glycol and the latter two components will cool and condense to liquid. Of course, any air will escape. If the system develops enough vacuum as it cools to open the cap vacuum relief valve, the condensed water/glycol mixture from the recovery tank will be drawn back into the system. This way coolant is not lost if it is temporarily expelled from the system due to temporary high operating temperatures, which is what would happen before the general adoption of coolant recovery systems.

On my car the hose disappears behind the inner fender to the mysterious recovery tank. I usually apply vacuum during a coolant change and just get gurgling, so there is very little liquid in it. I then use a thin neck funnel to pour about a pint of water into it and then siphon it out. As I said previously, the liquid appears clean and free of debris.

Duke