Mercedes had a secret they didn't want anyone to know. Before releasing their legendary OM-617 diesel engine, they disguised it in an ordinary taxi and sent it into the wild. No press releases, no announcements, just a covert mission to prove durability. That undercover taxi clocked over one million kilometers without breaking down. This wasn't just testing. It was extreme engineering validation. When Mercedes finally unveiled the engine in their W123 models, they confidently called them million