I bought this 1999 Firebird at 150,000 miles on the dot. I noticed it had a borla in it and was pretty excited to see in was dual exhaust. Once the winter came by I started to notice fumes were only coming out of one side, the driver's side. I searched underneath to check for rotted piping or any severe leaks, I didn't find anything at all. Give me some opinions on what it might be, please and thank you.

 

We'd have to see it to be sure, but I imagine it's not a crossflow muffler. I'd just not worry about it.

show us a picture or describe it better. Im guessing you have a single pipe that goes into a muffler, and two pipes come out of it?

 

both pipes are attached to the muffler and i'll provide a picture later in the day

 

There is a borla system where the passenger side rear pipe is bolted to the exhaust just befpre the muffler with a common 3 bolt flange. These have a blocker plate inside that you can take out at the track. With it in there there's no flow out that side, without it you have an unmuffled exhaust.

This is that system