I suspect I have this same problem on my '90 300 E, because i'm leaking coolant (only after long drives) and I'm getting brown sludge in my radiator.
However, no coolant appears to be entering the engine block. I'm basing this on checking the oil dipstick which I assume would show the coolant if indeed some was entering the engine.
This same exact problem was fixed by a dealer here in Houston roughly 5 years ago (replaced head) for $5k. My car now has 195k miles on it.
(shrug) I'm not paying another fortune to fix this problem if the head is cracked, and no coolant is entering the engine chamber. I'll simply continue to just add coolant at this late stage of this car's life.