Q: I accidentally put diesel in my Mercedes gas tank.

How much damage you did depends on how strong the mixture of diesel was and how far you drove. If the tank was nearly empty you may have messed some stuff up. Getting the car running may not be a big deal. You drain the fuel tank, change the fuel filter and put in fresh gas. If the car then starts and runs, then it is also a good idea to change the oil; certainly not a thousand pounds worth of work. But you will likely have problems down the road. The diesel can damage the non-metallic components of the fuel system and cause them to fail prematurely. So your car may not be very reliable until you’ve had a few failures and changed a few parts. The fuel pump and the injectors and any other component that is exposed to the fuel may fail before it’s time. The quote they gave you was probably to do a comprehensive repair in which they could guarantee you would not break down later on because of this problem. If you like to take your chances, just go with the fuel and filter change and take the problems as they come. But if you are troubled by the idea of breaking down on the road, you should go with the comprehensive repair. If you would like to get another opinion, you can contact Your Mechanic to have a technician come to your home of office to check out your Mercedes and advise you as to what should be done.

Unfortunately, it would be difficult to make an assessment from a distance. In my 30+ years of working on vehicles, I have experienced "misfueled" vehicles that responded to a complete draining and cleaning of the fuel system - fuel lines disconnected and flushed, fuel tank cleaned and reinstalled,etc. I have also experienced others where components like fuel injectors and fuel pumps required replacement. Fuel systems can be temperamental vehicle to vehicle. I had a fuel pump fail in one of my personal vehicles some years back and not even due to misfueling. I replaced the pump and fuel filter as a matter of course. The vehicle started and ran fine. Within 10 minutes of my drive home, the check engine light began flashing and the engine died. Turned out that even with the pump and filter replacement, debris from the failed fuel pump made it forward and lodged 2 fuel injectors wide open. Two cylinders flooded with fuel, causing a bent connecting rod in one of them.

Unfortunately it may wind up being trial and error as to which components respond to a cleaning and which require replacement.