I had replaced the belt tensioner spring and damper, which stopped the belt from getting shiny spots from it rubbing on itself, but the tensioner and pulley were tilted due to bearing wear, so it was time to replace them as well.
The fan is difficult to remove. First, the shroud has to be unclipped and then pulled back over it. The radiator should be protected with cardboard, and then I use a special stubby 8mm Mercedes Stahlwille hex key socket on a Harbor Freight serpentine belt tool. There is barely enough room to fit them on the fan bolt.
Then I use a cheap pulley holder tool from the auto parts store to keep the fan pulley from turning while I loosen the fan bolt. I really dislike having things designed to use special tools. It is more common the newer the car.
I installed a new valve cover gasket a little over a year ago, and it's already dry rotted. It's a DPH brand. Probably should have spent the extra money for genuine Mercedes.
I had the tensioner out before, and when I put it back in, I used red thread locking compound. The bolt seemed really stuck, so I carefully heated up the area where the bolt threads into the timing cover with a torch, which allowed the bolt to come loose.
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DON'T MESS WITH MY MERCEDES!
1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles