Hi trueog,

Since you were unable to trace any leak but can smell burning coolant...1 more suggestion.
This is from my experience on my 190E....

For a 4 cylinder 190E, located on the block just in between the headers, there are 4 stoppers or plugs for the water jacket as some call it. it's the size of an average coin. Mine was badly corroded and was leaking out coolant. But I'm not sure on the M111 engined c220.

Other possibilities include..leaking head gasket..which can allow coolant to seep into the combustion chambers..but if you are at this stage, i doubt that you could drive further without overheating the engine...

But to save you the trouble I agree with Manny's and Ethan's suggestion to bring the car to a good garage. Unless you really know what you're doing, it is not worth the risk to DIY. You might end up with a bigger bill.

However I still place my bet on the leaking radiator thou....