Backstory: Back in June I slid off a rock and landed the transmission on another rock which pushed up and broke all the motor and transmission mounts. At the time I thought I landed on the muffler as all of a sudden the exhaust got really loud (if there's enough interest, i'll post the video). The after trail inspection revealed that I had dented the transmission pan in (no ill effects on the transmission and have already replaced the pan) and somehow mangled the flange and gaskets on the passenger side exhaust manifold and y pipe. It appeared to run fine and I drove around 1,000+ miles in its condition. Then it started throwing upstream O2 codes on the affected exhaust I decided to do something about the leak. When I inspected the underside of the flange I noticed one of the studs was broken, or the nuts were loose. I tried to tighten them and broke one of the studs (the nut was already half way off) so decided to wait to take the manifold off and replace them. When I pulled the O2 sensor out it was completely black and fouled. Obviously running rich. So as a temp fix I got some copper gasket maker and slathered it all around the mangled flange it held for 100 miles before the pressure build up enough and essentially blew it off. I took it to an exhaust shop to inspect the rest of the exhaust as it had been banged up enough and shoved up to the point where it was touching the frame and transmission mounts, as well as the muffler having several dents in it. They told me there were a lot of crappy welds which were leaking, the passenger side exhaust manifold had a leak between it and the block, the flange, and the coupling between the muffler and y pipe. I had also asked them about the welded in cats (after market put in by the PO). The diameter of the pipe changed was reduced into and out of the cats on both sides. I asked them if that was a problem and they told me it wasn't. They quoted me $650 to bend the entire exhaust back into shape and fix up some of the welds if I was able to replace the studs in the manifold/y pipe flange and leak between the block and exhaust. That's were we get to today. I have taken off the exhaust manifold and saw a LOT of carbon build up within the pipe.

TLDR: landed the transmission on a rock, shoving the drive train up and mangling the passenger exhaust manifold to y pipe connection. Upon further inspection revealed some spotty welds and protentional exhaust diameter change issues.

Some things to note: The vehicle is a 2002 with a 2003 4.6 swapped in it. I don't know if they modified the exhaust or if there has to be changes to the manifold to deal with the higher displacement.

so my questions are:
1. With the condition of the y pipe, is it worth buying a new exhaust? I'm in Cali so cats are $$$ and limited on options.
a. I can get the current one bent back into shape, and possibly welds redone, or buy CA legal aftermarket/OEM, or try and find an alternative
2. Is there an exhaust manifold difference between the 4.0 and 4.6?
3. Is there a way to clean the carbon off the y pipes?
4. How does back pressure and other factors influence performance in the D2's exhaust?
5. I hear somewhere that sharp welds or quality/material of rods used can cause issues? Not sure how true this is or if its all bs.
6. Any notes, concerns, or ideas? I know very little about exhaust and need a lot of help with this!

I know I need to replace the studs, y pipe gaskets, and manifold to block gaskets.

To note: This is primary a trail rig + my daily when it works (I use other cars but enjoy driving this the most). I'm not worried about the looks of the exhaust as its only a matter of time before it gets dented again.

 

This is a couple of days old already, so I’ll put in my $.02. The 4.0 and 4.6 are the same blocks, so the Y pipe should be the same. I know California is very strict on emissions, but I can’t really comment on that. I know from a Dakota I used to have that plugged cats are a huge power robber. People have had threads here about testing them for plugging with a vacuum cleaner, but I haven’t seen a definitive way to test them (eg, at “x” volume of air in, you should see less than “x” psi of pressure).
Harv

 

You mentioned they are the same block, so they would have the same manifold? I don't visually see a difference and haven't found a difference online. I was under the impression that higher displacement needs a bigger intake/exhaust to allow more air in and out? Either way I think I'm just going to buy replacement gaskets all around and see how it runs. I will take it to the exhaust shop to have it re centered as its rubbing against the transmission and frame in some places.

I greatly appreciate the help!

My biggest question right now is if the carbon buildup on the exhaust is a problem or not?

 

For the carbon, you could get a flex honer for a drill to clean it out. I tend to clean and replace anything I touch mine. Autozone sells a 2" one for this exact reason.