Take it a 150mph blast down the motorway for a min or 2, get the exhaust hot....then let it roll down the gears on overunn (at night) an u'll see
This is what happened to mine (not up to 150mph mind you). My mate was driving my cos and I was in his 200sx. As he was cogging it down to stop at the lights a MASSIVE flame shot out. it was there for what seemed like ages too, I thought it was never gonna go out.