Recently my 1999 Subaru Legacy has been producing a lot of noise at idol and on acceleration. After some probing, I found an exhaust leak underneath the cabin. The leak is from a degraded gasket between two pipes in the exhaust system.
This instructable will detail how to repair the exhaust flange gasket between the rear catalytic converter and the Catback. A video is included at the end showing the major steps of the repair.
What the heck is a Catback?
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Step 1: Put What Where?
The top image is a diagram of an exhaust system from a car much nicer than mine. For this instructable we will focus on the joint between the rear exhaust pipe and the midpipe. The Cat-back refers to all exhaust components behind the last catalytic converter.
The joint between the rear catalytic converter and the cat-back is connected by two flanges with a spring and bolt kit. The springs allow the joint to flex, which is important, a rigid joint would crack/fail during normal use of the vehicle (no give).
The problem I am experiencing is two fold. The gasket between the cat-back and the midpipe is completely worn out. Additionally, the flange on the midpipe is completely rusted through and not serviceable. Therefore in order to fix this I would have to buy a new midpipe (with catalytic converter and all its precious metals) for about 200$, alternatively I could have a muffler shop weld on a new flange (I wish I could do it myself but welding is above my skill set).
What I decided to do is cut off the old rusted flange on the cat-back side, clean up the pipe, replace the gasket, and use a split-flange to connect the midpipe to the cat-back.
Step 2: Parts List and Tools
Exhaust flange gasket - sometimes called a donut gasket
New exhaust spring bolt kit - Can reuse old spring bolt kit if it's not rusted out
Split Flange - Make sure the inner diameter is large enough to go around the midpipe. The bolt holes also have to line up with the bolt holes on the cat-back flange.
washers- you'll see
I am listing the tools that I have on hand and what I used them for. If you don't have one of the power tools ask your folks, friends, or neighbors. Heck, they may even help out.
Wheel ramps / jack stands / pull front wheels up on the curb - APPLY EMERGENCY BREAK!
Dremel tool with metal cutting wheel - used to cut rusted flange off midpipe and spring bolt kit if rusted on. An angle grinder would be best but a handsaw may work in a pinch.
Dremel 3000 w/ grinding attachment - used to remove rust from midpipe. A good wire hand brush would do the same.
Rubber/plastic mallet- you never know when it will come in handy
Eye, ear, lung protection
I'll give you a minute to collect all of the above. Ready, lets do this!
Step 3: Disassembly and Clean Up
I suggest spraying the spring bolt kit and the heat shield bolts with penetrating oil a couple days before the job.
Raise the front wheels up. If you use a hydraulic lift make sure to use jack stands. The jack can fail and lower the car, right onto you!
Remove any heat shields in the way and the spring bolt kit. If the bolts are rusted on, cut them off.
Remove the old exhaust flange gasket and any other debris present. Pay attention to the gasket orientation as the new one will need to be placed in the same orientation.
Cut the rusted flange off the midpipe. The part of the flange closest to the pipe should still be solid. It is important that there is a raised ring of metal on the midpipe, this is what will hold the split flange onto the midpipe.
Remove any loose rust from the face of the cat-back flange and the midpipe with the Dremel-3000 wire brush. You can use a file to smooth out any sharp points left over from the cut off midpipe flange.
Step 4: Reassemble
Insert the new donut gasket.
Assemble the new spring bolt kit (or old if salvaged). Note: The split flange will be a little farther away from the cat-back flange due to it being behind the old flange. If the bolts are to short (due to the spring tension) us a "C" clamp to compress the springs and attach the bolt.