Occasionally my Signum suddenly starts running really badly. This occurs suddenly, at various speeds and driving conditions. When I stop, the engie cuts unless I keep revving it. It starts up immediately though. The next day, it's as if nothing was ever wrong. I had it in at the dealer, who couldn't find anything wrong, the computer showed no faults and they checked all the hoses etc. Done the Sherlock Holmes bit and it seems worst when I've just filled up. Strongly suspect a fuel tank venting problem. To confirm this, I waited for the fault to occur, pulled over and took off the filler cap. After about half a minute, the engine started to run smoothly again and I drove on with no further problems. Since then, I've undone the fuel filler cap before and after every journey. The problem hasn't re-occured. Can anyone tell me how the tank is vented? Is it via the filler cap, or is there a tube from the tank? If it's the latter, what is the routing of this tube?