Originally Posted by kenmar
I was thinking about picking up the E39 M5 that was posted here a few months ago (the one with the caramel interior
). But I came back to reality after I did a bit of research and found out that it would get the same fuel economy as my full-size pickup ($100 fill-ups!). Admittedly, you can get around the fuel costs by having another DD (saving the M5 as a garage queen, which doesn't seem like a bad idea), but the real deal killer for me was the engine carbonization problem and the costly remedy. AFAIK, there is no guaranteed way to prevent the problem or to repair it without incurring labour-intensive dismantling.
If you drive the M5 properly, you can keep it in the 11L/100 km range no problem.
The carbon problem doesn't affect anything operation wise and there is software to make the check engine light go away, hardly a deterrent of any kind.
Sounds like you wanted reasons NOT to get one, so you picked two that are actually not reasons at all.