I have an '06 6MT Sedan. I am the original owner. I put on the MREV and ISO-THERM spacers when the car was new. I put on the HR y-pipe a couple years ago. I recently put on a stillen Z-tube. We had a lot of flooding here last year, and I had to drive through fairly deep water regularly to get in and out of my development. As a result (I assume) one of the OEM cats brick came loose and was sliding around inside the lower cat housing. The car still ran fine, and the brick didn't start to break apart. A couple of months ago I replaced the OEM cats with Z1 high flow cats (which fit perfectly and look to be well built - I also wrapped them), new OEM gaskets front and back, I also replaced all 4 O2 sensors with NTK sensors. I've put about 3K miles on with the new cats, I like the deeper exhaust note, but I don't like the angry wasps in a can sound, the hissing when the throttle is closed, the rasp or the smell - it smells like the car is running really rich at low speeds, resulting in an aftercat smell of rotten eggs, and I have a stumble that moves around depending on temperature and throttle position between ~2,500 rpm and ~3,200 RPM, if I give it too much gas. Last year, I replaced the OE clutch and the 4 cam position sensors with new Hitachi sensors. The car had been running fine until changing the cats. No codes.

I've watched the real-time consult data, nothing sticks out at me. I've read on the 350Z forums about some people reporting a lot of the same issues, the usual recommendation is that a tune and a larger Y-pipe will fix it, even if the rest of the exhaust system is stock. I'm torn between spending more money on more aftermarket stuff or just buying a new set of OE cats and calling it a day. I'm all for getting a tune, but almost every time I've tried an aftermarket part on this car, it is not as good as the OEM part.

I plan to put the old OEM cats back on to verify that the problems go away - there is a possibility that one of the upstream O2 sensors is defective, but it's unlikely. It's also possible that I touched something with my 4 feet of socket extensions while I was taking the old cats off, but I don't think so.

I figured I would post this here because the G35 sedan is not exactly the same as the 350z, and see if people here could confirm that they had these issues and a tune and exhaust change resolved the wasps in a can noise, the rasp, the hissing, and potentially the hesitation and smell? The smell is not at all like a test pipe - it is more like a poorly tuned 1978 Ford Fairmont with an old feedback carb and catalytic converter. If I temporarily make the car leaner through software, the smell does seem to be less obnoxious.

I appreciate any input.

 

Rotten egg smell is a failed catalytic converter. Once they make that smell they are cooked and need to be replaced. Once you remove them you'll probably find it's badly melted inside on the element.

 

The rotten egg cats are the new ones - they still look fine internally, they are a 100 cell psi metal spiral design as opposed to the OEM, which looks like a ceramic brick front and a denser spiral metal rear. No signs of deformation or of them looking any different than when I put them in. And, they smelled this way pretty much as soon as I put them on. I put the OE cats back on last night have driven about 150 miles so far, watching the short and long-term alphas for both banks, the car runs and smells fine. Both sets of cats are clean and not sooty on the inlet side. The aftermarket cats are surprisingly black and sooty on the outlet side, OE is fairly clean on the outlet side - and they have almost 200K on them.

My gas mileage went down slightly with the new cats, but not a lot, and not enough to be outside the normal margin of error - particularly with me trying to tell if it is stronger/weaker, etc. Around town I didn't see much of a change - if anything, slightly better - but that could be due to me short-shifting to avoid the bees in a can noise. Next month I will be doing two 2,000 mile trips - one with a trailer and one including a few days in and around deal's gap. I want to get this sorted ahead of time with enough time left over to be certain that everything is right. I'm really leaning towards just buying new OE cats. I have no plans to sell this car ever, and they cost about as much as a set of tires. It is a third car that is just for fun, but I still put more miles on it a year than most people do on their primary vehicle. And after all these years, I haven't had any real issues. The first MAF and radiator fans that were covered under warranty have been the only non-wear items that have required attention. I've had to replace more wheel bearings, brakes, and tires than any single car should go through, but my biggest complaint is the dashboard cracking...twice.

I suspect that fuelling is being messed up with the HFCs - there were no leaks before or after the cats. I don't know if it is due to better/worse scavenging at low speeds, or turbulence, some interaction between the intake parts and the exhaust, or maybe this is normal. Everything on the intake and exhaust side is sealed. I replaced my OEM MAF when it failed w/ a new Hitachi MAF at about 140K miles, the original O2 sensors at about 100K, and then again when I put on the new cats. I'm not certain that the new O2 sensors are good, and it is possible that the MAF is bad again, but that seems unlikely. It's also possible that I've had a rich condition for a while and the OEM cats were able to better scrub the exhaust to have almost no odor, and also leave the outlets clean due to the higher heat. The car definitely feels sluggish at low speeds with HFCs, along with the stumbling between 2.5K and 3.2K, maybe it feels stronger at high RPM - but that would be open-loop anyway. I used to have an Innovate LM1 wideband O2 reader, which could be useful now as a sanity check, but I gave it away at some point and am not really interested in spending a lot of time on this.

As an aside, I had an FX45 that I put test pipes on after the OEM cats disintegrated. This induced a flat spot around 3K rpm that couldn't easily be resolved until new OE cats were put back on, so this experience isn't totally unfamiliar to me. With the FX, I didn't have any good options for tuning, but the FX was noticeably stronger everywhere else, so I was able to live with it like that for many miles.

 

If they smelled like rotten eggs when you put them on brand new then you got defective cats, they absolutely 100% guaranteed only make that smell when they're burning up. I've been around quite a few cats that have died.

You should call the vendor you sourced them from and have a nice long chat about this because they sold you a defective part. If your OEM cats made the smell as well then I'd be leaning towards a fueling issue, but they're not, so it's almost definitely a defective set of HFC's.

The engine feels sluggish because there's probably blockage in those HFC's where they're melting, if you can't see it on the outside of the honeycomb then it's on the inside.

 

I appreciate your response.

Yeah, I might call them. The OEM cats were fine until our area started flooding every afternoon. The rear portion came loose on the passenger side and was sliding up and back in the housing, rattling under some conditions - it actually sounded remarkably like a rod knock until I got under the car and realized the deep metallic knock was coming from within the right cat. I put them back on to verify that the car ran properly with them and that I wasn't chasing some other issue. It was fine. I bought new OEM cats and put them on, the car runs perfectly. I've been driving it every day w/ no hint of any symptoms or odd smells. And it feels stronger from idle to redline with the OEM parts, so...

I don't know if the HFCs are actually bad, but they definitely don't seem to be good. I can shine a flashlight through them and not see any obvious blockage, but I probably will call the vendor. All things considered, the price for them was less than two tires, so not worth getting too upset about.