An engine being new really has nothing to do with whether or not it will have a leak. traxxas isn't really known for their high quality control standards. especially with the TRX engines. I have had a few 3.3's and a 2.5 and they had air leaks even the new ones. it can be a good idea to seal and enginebefore you ever bolt it all up and run it.

I will agree that is a good idea to check for exhaust leaks and cutting some holes in you body will help keep temps down too. does the engine over heat when the body is off?