Just experienced the worst car-buying experience thus far. From where? CarMax. We've been looking for a GLE300d for quite some time, for a good deal, and finally found one at CarMax. Went there, checked it out - it was in excellent condition with a very clean carfax report. The next day I go to complete the purchase. Filled out all paperwork and paid for the car in full, and while we were waiting for them to detail the car and fuel it up, the manager comes out with some horrible news. One of the techs filled the empty tank with gasoline ...and started it up and drove about 400 feet. This morning I went to a local MB dealership and spoke to their diesel mechanic and he urged, if it's still possible, to back out of the deal. He also mentioned, to properly clean the car up, it can cost $10-17k - and that it voids the manufacture warranty that's still active. So I went back to CarMax and returned/cancelled the sale. CarMax said they're going to send it to the MB dealership and have them fix it.

I'm reaching out here to hear your guy's opinion. How bad is this really? The car was a very good deal and with great options, and they're fairly rare here in the U.S.

A few gallons of gasoline on top of many gallons of diesel? No big deal.
A tank full of gasoline then driven? I'd pass on that car. You were looking for the GLE when you found that one; another one will come along. Gasoline is a solvent, not a lubricant. High pressure fuel pumps, injectors, etc. don't like solvents. I'm curious: Did the CAR stop at 400' or did the tech stop the car after realizing his mistake?

I'd pass on this as well. Basically requires new everything in the fuel system: filter pump, fuel rail, fuel injector, fuel filter, fuel tank. And since "voids manufacture warranty" definitely pass on this vehicle.