Hello! This is my first post here but I've been a lurker for a while. I have a 2005 R53 with an admittedly large amount of miles (176,000), and I've been able to fix a lot of problems it's had, except for one. I've come to ask a here because I've tried a lot of different things and am stumped at figuring out the issue I'm having.

As the post title says, whenever I accelerate, I have a terrible smell coming from my exhaust. I thankfully can't smell anything when I have the windows up, but if the windows or sunroof are open, the smell wafts in and can get really bad if I accelerate hard. I'm sure people driving behind me don't appreciate it either. I'm sure it's coming from my exhaust too; I tested it by going on some back roads, opening my hatch, and going WOT. It was worse than it ever is with the windows down...

The smell it makes is pretty sulfurous, and it doesn't really smell like burnt oil or coolant. I do notice some white smoke on startup, but that's pretty normal for any car. I also sometimes hear a sort of sputtering noise coming from the engine if I accelerate hard. I've done a lot of things to try to fix it but nothing seems to be changing it at all. Here's what I've done relating to the problem:

• Replaced PCV valve (old one seemed fine, actually)
• Supercharger service (oil and change gaskets) and replaced the plastic intake duct
• Replaced exhaust header and cats (Davico brand. I've heard that sulfur smell can be from bad cats; maybe something is overheating them, or maybe the cats I got are crappy)
• Tried different brands of gasoline

I'm not sure what else to try or what it could be. My car has, at rare times, thrown an O2 sensor code for the downstream sensor, so maybe those could do with a replacing. I also rarely get a CEL and a P1498 "unmetered air after compressor" code, but only if it's really cold out and I'm going slowly like in a drive thru or parking lot.
Does anyone know what it might be, or what I could do to diagnose it better?

Thank you

 

I would suspect your Catalytic Converter. Did it do it before you changed them? Did it stop for a short time after you changed them?

I’ve read that aftermarket Cars are junk. I changed one on a Passat once and it only lasted 5 months.

 

Yeah, a sulfur smell from the exhaust is usually bad cats.

 

Assuming the new cat is OK, I'm thinking there's an intake air leak somewhere.
Does the cat ever get red hot?

 

The car did seem to make that smell before I replaced the cats. It didn't really go away with the new ones either, but I have noticed it getting worse. They could be getting too hot but I would need to possibly get an infrared thermometer and check. Could bad O2 sensors also cause the problem?
As for the intake leak, definitely possible, especially because of that P1498 code I get sometimes. I'm not sure where it would be though. I replaced the gaskets for the supercharger and the inter cooler boots as well. Could there be a leak around the fuel injectors maybe?

Also, I can actually see some yellow deposits in the exhaust pipes, probably from sulfur as well! That's pretty crazy

 

If I do get another new cat/header, would this be a good one to get?
https://www.rockauto.com/en/moreinfo...739&cc=1430887

 

Bosal is one if the more popular brands. I’d fix you leak first.

 

Before you spend $$$ on another cat, I'd get a smoke test done for air leaks.
If the cat is overheating, you won't need a thermometer - you'll see it glowing red!

 

Smoke test sounds like a good idea -- I'll give that a try! I was definitely thinking that I would want to fix any intake/overheating issues before spending a lot on a new cat