I accidentally went over a raised manhole cover which bent down one of my cats. The bend caused a hole in the flange which made the exhaust leak so I took it to an exhaust shop and the guy chopped off the flange that connected the exhaust to the cat and just welded a pipe instead. Now ever since that my car has been rattling like crazy when idling. Also when I step hard on the gas the car kicks a few times. Anybody knows whats the problem? The cat is still bent but no apparent leaks.

 

Sorry, I have no idea what is wrong with your cat, but I will share an interesting bit of info here. Cats are warranted for 80K miles (long as they are not physically damaged), but if the baffles fail and your Infinit's cats start making noise, Infiniti says they are not warrantable as long as they are still functioning as designed. What does that mean? Your cats can be rattling to high hell due to a manufacturing flaw in the baffles, but Infinit willl not replace them under emssions warranty as long as they are not throwing a hard code. That info comes straight from Infiniti Corporate, he true dogs that they are.

 

A rattle happens when those plastic that holds the muffler is taking off and is welded metal to metal doesn't give the cats room to shake if it needs too.

 

The mufflers were never touched. Nothing was ever taken off just simple chopped off and welded on.

 

If you hit a cover hard enough to break the flange there's a decent chance other components of your exhaust could have moved as well.

 

The cat got caught on the manhole cover which bent it which caused a hole to open up in the flange. So as of now 1 cat is hanging lower than the other.