MY 89 190E 2.6 that I recently sold has a bad exhaust valve. IT wouldnt idle right. I bought a new valve (30 bucks), with a headgasket set and valve seals, and did the job for about 300 bucks myself. Its not too hard. My mechanic (worked for mercedes for 15 years) charges about 750 to do a head gasket on a 103 six cylinder (300E). I think that is very fair. Alot of people I see get taken, but doing this job on these engines is relatively simple.

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K