On my 98 s70t5 I had a partly clogged cat.
Although my exhaust was quiet it was limited in horsepower.


I got sick of not being able to pass another car confidently so I took it
to a shop to replace the cat with an aftermarket. They said they also welded on 3 new hangers. The rest of the exhaust is original.


They welded in the new cat and installed the two 02 sensors.


Now the car has a low drone at idle and low speed and low acceleration.


Do aftermarket cats cause drone or is there something else going on to cause this? Could the exhaust be too tight that it resonates?
Is it the new higher than stock flow rate causing the baffles in the muffler to cause the drone? In the stock cat is there a built in resonator? The guy at the exhaust shop, when he was looking at the 02's was saying it
might have a pre-cat (where the one 02 went it) it was like a step before the main body of the cat?


Sorry for all the questions but I'm trying to get an understanding of where the drone is coming from before I approach the shop.

 

The cat would not create a drone.


They likely created a leak when they replaced it. The cat hooks at the front to the exhaust manifold and it has a ball socket connection at the bottom that connects it to the reset of the exhaust system. They may not have used the liquid exhaust gasket for ball/socket which could cause a leak.


I don't recall where all the hangers are, but I think they would have to take the exhaust down to weld in new hangers. That in itself could cause a leak anywhere.

 

Thanks for the reply...


I ended up taking it back and they checked it over and found no leaks.
He was saying that since its high flow cat that maybe muffler baffles were worn creating the drone. He suggested to put in a resonator which I said ok.
The resonator reduced it a lot and now its quite bearable.
All in all its not as quiet as stock but I do have my performance back which my goal.