I am really new to Nitro RC, and I mean really new, like I just ordered a Losi 8ight T and havn't even got it yet new, but I have a butload of experience with motorcycle tuning and engines. When finding an air leak on my Banshee I will spray wd40 or equivelent around until the RPM's Drop. It would probably be easier if you had someone to help hold the car so you could spray around.
If the car doesn't wind up without giving it any gas then it doesn't sound like an air leak to me. Normally an air leak would make it rev up without giving it throttle. Good luck, let me know what the problem is when you figure it out.