I test drove an E320 CDi the other day so that I would actually know what one drives like. When I checked under the bonnet I noticed some oil on the top of the undertray but it looked like possible spillage when topping up, nothing really on the engine. At the end of the short Autobahn run, where we never got above 70mph, I checked under the bonnet for issues. There had been a small explosion from the oil filler cap, which appeared to be secure. The dealer reckons it wasn't secured properly though. Whilst I'm not worried about this particular car is this something to watch with this engine and what couls cause it?