Rough exhaust smooth idle

[djKibblez]

Hoping to get some help from the every friendly community here. I have a 97 M3 and for some time now at idle, the exhaust note is not consistently smooth. It will be smooth but eventually, the exaust will gently rumble which will cause the car to vibrate slightly. The idle is completely fine. I've replaced just about every sensor on this car, including O2s, temp, coolant, as well as spark plugs and coils within the last 6 months. No vacuum leaks or exhaust leaks whatsoever.

Could this be a head gasket issue? I'm not familiar with troubleshooting symptons of a bad gasket. Can't think of anything else.

The car runs great other than the this issue. No faults, pulls strong, passes emissions.yada yada.

There was a blog post I found a while back about how a leak in the EVAP system could cause a rough exhaust note. I've checked this as well with no obvious signs of a leak.

Perhaps I'll post a video or sound clip of what I'm hearing if that would help.

Thanks
Chris

[JitteryJoe]

Vacuum leaks can be tough to spot. Possible your ICV is not operating optimally? Maybe one of you injectors is rich or lean at idle?

Have you run a diagnostic to check the fuel trim levels at bank 1 & 2 at idle and at higher RPM? That can help tell you if there is a vacuum leak or the injectors are wonky. There are some good youtube videos on diagnosing rough idle and fuel trim diagnostics.

Just some things to think about.

[djKibblez]

Hey Joe, all I have is a Schwaben scanner which I don't think can do real time analysis, but I'll look. The idle is just fine however- there is no change in RPMs at idle and power delivery is smooth up to near red line, but that's just my butt dyno talking. And I did run an extensive smoke test and found no leaks in the system. I just recently cleaned out my ICV as well which would cause an idle issue when cold. ICV's been operating fine thankfully. I'll google those vids on fuel trim. Thanks.

[LockDots]

How many miles on the cats?

[djKibblez]

160k+

[LockDots]

Could be the cats clogging up. I've seen them go longer than 160k but everyone's case is different. If it's solely you're exhaust and your idle isn't affected, I'd start there.

[djKibblez]

Could be the cats clogging up. I've seen them go longer than 160k but everyone's case is different. If it's solely you're exhaust and your idle isn't affected, I'd start there.

Is that something that can be visually inspected or should I just replace the cats at this point?

Some of the welds on the shield around the cat are broken and I've noticed water leaking out of it. Ugh it's probably time to replace anyway.

[LockDots]

Is that something that can be visually inspected or should I just replace the cats at this point?

Some of the welds on the shield around the cat are broken and I've noticed water leaking out of it. Ugh it's probably time to replace anyway.

Not really, they're welded in. The water is probably being collected in the heat shield. If the water's actually coming from the cats, then they sound rusted and you might as well replace them. OE is extremely expensive as they're sold as an entire midpipe. Have some Magnaflows welded in

[djKibblez]

Yeah.shit There goes my wife's xmas gift. Sorry honey.

I saw a bolt-in magnflow on ecs and bavauto...cant' believe the price for OE cats from BMW. holy cow.

[scooper]

check idle control valve/vacuum system and motor/trans mounts. Also check plugs and ignition coils.