You cannot change the rear main crankshaft seal on the Mercedes with out removing the engine. The seal is held in place by 2 pins so there is no way to rope a new seal to the old and pull it through. You really have to remove the engine from the car. You must remove the upper oil pan to replace one-half of the seal. Then you must remove the crankshaft from the block. Now you can install the seal, which I believe we paid less than $4 for from the dealer. Now you’re confronted with the dilemma. Do I replace the bearings since the cranks out, shouldn’t I replace the ??? While the engine is apart.
With the crankcase vented straight in to the engine compartment, this relived enough backpressure in the crankcase to stop the quart a day leak. However, it did cause about a quart of oil a month to blow a very fine mist of oil on everything under the hood. At this point, I was very happy because you cannot really drive a car that leaks a quart of oil in less than 20 miles not to mention the mess it leaves when you park it. But a quart of oil a month out the breather hose is a breeze to clean and could also just be renamed from crank case ventilation to automatic corrosion control system another Mercedes engineering first.
I had some luck using a product called trans X on transmissions and looked for a similar product for engines. I don’t recall the brand but one product was very specific it could only work on old dry seals and gasket material it could not rebuild your engine. I spent 8 bucks on a can. I added it to the oil with the breather hose still disconnected I drove the car for the recommended 250 miles then changed the oil and reconnected the vent tube and no more oil leak. Since then I have had 3 other occasions dealing with different Mercedes W116 and W123 rear main seal leaks where this method has been very beneficial and saved the cars from the scrap heap.
It cost $6 for enough oil leak fluorescent additive to find all the leaks on several cars. Cleaning up oil grease and gunk is great it especially makes working on them more pleasant. However, do you want too properly detail your engine or do you want to find and fix the oil leak ASAP. For $6 you will know definitively where the leak originates in under 5 minutes leaving, you time to actually fix the problem rather than chase I choose to buy the florescent oil leak detection additive .
Dave S