Exhaust not making it to headers. instead comes out of engine bay
I have an 88 tbird with a 5.0. It has dual exhaust with a single cat, and glasspacks. Was running fine until the other day i heard a huge vacuum leak on the highway. All of a sudden full throttle was 35 mph. The car filled up with exhaust fumes. Once i got it home, i took off the upper intake manifold to check all vacuum lines, fuel pressure regulator, pcv, etc was all plugged in and secure. I checked all surrounding hoses and pipes for leaks or looseness. The exhaust isn't even going to the headers because they aren't heating up. No fumes come out the exhaust because it's silent in the back. instead all you hear is a huge vacuum leak (hissing) and all the fumes come out of the engine bay, what looks like along the firewall. I have NO IDEA where it's coming from, and have had 2 other guys take a look but they can't figure it out either. My assumption is something is blocking the passage way for the exhaust and it's coming back up to the engine and releasing somewhere it finds. When the car first starts up, it sounds like the exhaust goes all the way through to the end, but after about 1 second it changes. I know something must have come off because when i give it a lot of gas then let off really quick, i can hear a rattle almost as if a loose screw were bouncing around. I've taken 4 hours to inspect along the firewall to find anything, but i can not. Any guesses? All help is much appreciated.