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Leaking Exhaust Fumes into Cabin – Broken Flex Pipe (MK4 Jetta)

I’ve been driving for about a year now with an exhaust leak in my cabin which was quite annoying during the winter months.
Basically exhaust fumes were leaking from a cracked flex pipe and would creep up under my hood and then into my cabin.

Symptoms of a broken flex pipe (or a hole anywhere along the exhaust) are leaking fume smells around the cabin as well as little to no pressure build up
when you cover the tail pipe. In fact you’ll probably hear the fumes escaping through the crack if you block the tail pipe.

Note: My vehicle is a 2.0L gas engine (BEV engine code). The Diesel variants have a different size flex pipe so make sure to measure it before buying/installing another one.

This is a relatively big job… But will save you about $1200 (without labor) because the flex pipe is sold together with the catalytic conventor.

A word of caution.. This involves grinding out and smoothing the catalytic convertor and this is a one of the harder jobs to do due to rusted bolts, limited space etc.
Then you have to weld the new flex pipe to the cat and the pipe piece that goes into the exhaust manifold.

The OEM VW flex pipe is not sold separately as it is already welded to the catalytic conventor. If you want to fix the flex pipe without any work you have to buy a whole new catalytic conventor! To save yourself a hefty sum, you can buy an after market flex pipe and install it where the old one was.

If the leak you experience is indeed from the flex pipe, then follow below:

To start, first locate the flex pipe. Lift up the car and you should see it as seen in the photo below (Click the images to get a full resolution image):

IMG_1477

I pulled back the protective shield to get to the leak and found the crack as shown below:
IMG_1469

I then cut out the old flex pipe so that I could get a rough measurement of how big it was. Once you cut it out there is no turning back!
My flex pipe measured to be around 10 inches long with a diameter of 2 inches. Be sure to measure how long your flex pipe is.
I went out and bought a comparable generic flex pipe from our local parts store. It cost me $60 for a brand new flex pipe.
Make sure the flex pipe you buy is stainless steel

Next, remove the entire catalytic conventor from under the car by loosening the clamp bolts that hold the resonator and catalytic conventor pipes together.
With a bit of wiggling and turning the catalytic conventor should slide right out of the clamp. Don’t forget to disconnect the oxygen sensor that is connected to the cat.

Handle it with care as well since an O2 sensor is quite pricey.
Be careful with the cat! It is still perfectly fine and we do not want to damage it. Handle with care!

Grind the end piece off that was originally from the broken flex pipe. You will have to do this with the pipe that also connects to the exhaust manifold as well.
Getting the pipe off of the exhaust manifold was hard because the bolts were rusted out and would not come out without a fight.

When grinding make sure to only grind off the piece that was part of the flex pipe. Do not damage the pipe underneath the welding! See image below too see what must be grinded off.
IMG_1487

After removing the cat and grinding the ends down to only have the pipe, the new flex pipe was welded on. Unless you know how to weld or even have a welder available. I’d suggest taking it to a shop that can do this for you. Be sure to get a stainless steel flex pipe as an generic metal one might not weld as well and will rust faster.

The following are images of my finished result. I used a TIG welder to connect the flex pipe:
IMG_1489
IMG_1488
IMG_1490

With the new flex pipe welded on, the entire catalytic conventor assembly can be put back in place… but first I recommend you check one thing; the holder for the resonator pipe.

I am unsure of the correct term for this piece so I will refer to it as the “holder”. This is what a typical holder should look like:
75841_x800

This is what mine looked like:
IMG_1493

My holder was completely rusted out! There was nothing holding the pipe in place to absorb vibrations. I took the old rusted holder and welded a square piece of stainless steel with two holes through it so that a 50mm clamp could fit through. I then put the clamp around the pipe and through the square piece and fastened it together with new bolts as the old ones were rusted out. I am hoping this self made holder lasts me for a while. I had to make my own holder because again, VW decided to make it part of the resonator assembly so if you want an OEM holder you are going to have to buy a new resonator.

This missing holder could be the reason for the flex pipe cracking because the stress from the entire pipe twisting and bouncing could have, over time, destroyed my flex pipe.
Be sure to check the holder piece on your car even if you do not have issues.. it may save you a lot of time and money in the future!

I do not know how much a new resonator would cost from the dealer but I can assure you that it wouldn’t be cheap. By making my own holder I saved myself another huge sum of money.

All in all the new flex pipe has been working fine so far. My total cost to do this was under $80 including the flex pipe and making a new holder. I easily saved myself about $2000 with labor since I would have had to replace both the catalytic conventor and the resonator. The job was certainly worth it but it was a bit of a headache to do. With a bit of (a lot of actually) elbow grease and sweat, you too can accomplish this and save yourself a lot of money.

-Mike