Wrong fuel in a BMW? – yes, we have done a few …… hundred.
BMW’s are fantastic cars, they are also rear wheel drive, and have special tanks (saddle tanks) that are made into 2 cells and sit atop the prop shaft in a saddle configuration.
Note that if you put a suction line down the filler neck (covered in blue tape) you would not be able to access the fuel which is on the far side from the neck, this fuel is only accessible by either going into the tank from the circular access hatch (under rear seat) or by bypassing the fuel management system and making the car pump the fuel from the far cell to the near cell. as you might expect doing this takes some savvy.
It is on a car like this that an amateur fuel drain mechanic will become unstuck, we get called out to BMW’s that “have already been drained” fairly often, the reason is that the previous “technician” was not aware that there are 2 compartments, and to do a complete flush it is necessary to drain both.
Up until the late 80’s a BMW tank was very simple to drain indeed, because they were steel they had sump plugs, just like the one in the oil pan. and after a misfuel all that you needed to do was undo the sump nut and the tank would drain itself out, but with the introduction of plastic tanks the sump plug was done away with on BMW’s, you may be in luck if you have a very old one, but otherwise you will need to access both fuel cells and drain it out completely,
BMW’s are a very hardy car and they can tolerate some amount of petrol in diesel, however as much as the engine can suffer it, the petrol turns the fuel lines, fuel filter, rubber seals, and plastic fuel tanks into a goo, and even a small amount of petrol in the tank will coat the sides of the tank in petrol fumes which will also contribute to the goo effect.
BMW dealers have been known to ask for upwards of 10 thousand pounds to rectify cars that have been driven on petrol, in my opinion this approach is profit driven and not entirely necessary.
My advice to anyone who owns a BMW that has been misfueld is to get it drained out by a professional fuel drainer. 99% of the time that is all that will be needed, and that includes cars that have been driven until the wrong fuel has caused the car to break down.
Don’t listen to your dealer! you are under no obligation to have costly repairs done and the scare stories you are told about warranty invalidation are untrue.