Exhaust Leak
 

Just installed an FT86SF UEL Header and can hear a bit of a hissing noise. Had to remove the midpipe in order to shimmy the OEM header off the car, a piece of the flange from the midpipe to the front pipe was corroded and seems to have broken off. After sanding it down and reinstalling the midpipe I still hear a bit of a hiss (although a lot less of one). Any ideas as to how I can see if there is in fact a leak or I’m just being paranoid? Also wanted to replace the gasket again since it seems to have frayed and wanted to know my best options.
Cheers.

Probably just paranoid. Monitor your fuel trims. If it's fine and you hear a hissing noise, it's just the thin metal on the header.

An exhaust leak will leave a trace over time, called carbon tracking. Where the exhaust is escaping it will leave black soot.

To aid in location, and do this at your own risk, you can hold the flame of a lighter to the suspected leaking flange. The flame will respond to escaping air.

To help seal, you can replace gasket. If sealing surface is very rough and won't seal they make products specifically for applying to gaskets to aid in sealing. Some are extreme high temp RTV, others a tar like paste.

I use leak detector spray, it’s an aerosol. MOT testers use it. Best to use it whilst headers are cold, I.E when you’ve only just started the car.

Hope this helps!!!

Or just use soapy water to find it.

Is it the OEM exhaust? Is it coming from the donut gasket? Why take the whole midpipe off instead of just the overpipe?

OEM frontpipe mated to a Perrin 2.5” catback. Donut gasket was removed during install and replaced with a regular gasket. Had to disconnect both of them to get enough room to get the header off.

As in take off midpipe and remove the bolt holding the FP to the car in order to shimmy it back. Taking it off completely was a hassle

if its after the 02 sensor its just cosmetic

yall have weird ways of detecting exhaust leaks

I usually just use my hands and ears.

You very likely do have a leak. When disturbing the old exhaust system, especially when rusted, you have a good chance of not mating it back together as well.

Definitely replace all gaskets.
Inspect the exhaust flanges to make sure they're still flat and good. An older flange that's semi-eaten away by rust can become thin and weak. It can easily warp when tightening the flange bolts back down.


While post-O2 sensor exhaust leaks are mostly "cosmetic" it still sounds obnoxious and ruins the experience.