[Shaninnik]

Hi guys, I am new to M world. I recently purchased lightly used 2021 M4 Comp, 14k kms., and almost immediately installed single midpipe. First thing I noticed was weird hissing noise (or maybe rushing air or even very light high speed rattle, hard to describe) under boost and drone, rechecked the installation and sure enough it was touching mid brace. I had it adjusted and drone is gone but hissing noise was still there. Since it was not installed properly in the first place I went to another shop and had them recheck everything and personally inspected every small detail, all was good, they were unable to find the issue but confirmed that noise is not right. It def. is not turbo spool or intakes, much louder. I don’t remember hearing it with OEM midpipe but I didn’t have enough time with the car to really notice, it is possible that it was there just silinced by secondary cats/resonator but cant be 100% sure. I also started suspecting that car is not making enough power, I know hp gauge in car is calculated value, but I have never seen it go over 420hp mark. And I haven’t really seen boost pressure (I assume this gauge is real) go over 15psi, maybe even less. While trying to figure this out I was testing it and floored when merging on a highway - almost immediately got drivetrain error/limp mode. Errors are 120308 boost pressure control: pressure too low and 120408. I can 100% reliable replicate this every time I floor it. Visually inspecting turbos/ charge pipes/ etc all looks good.Now car is still on warranty and I have service apt. tomorrow, but I still sit here panicking and feeling pretty bad that my new to me car is broken and god forbids they blame it on aftermarket midpipe. Did anyone had anything similar happen to them or have any ideas what can be wrong?

[Info@mad-us.com]

Originally Posted by Shaninnik

Hi guys, I am new to M world. I recently purchased lightly used 2021 M4 Comp, 14k kms., and almost immediately installed single midpipe. First thing I noticed was weird hissing noise (or maybe rushing air or even very light high speed rattle, hard to describe) under boost and drone, rechecked the installation and sure enough it was touching mid brace. I had it adjusted and drone is gone but hissing noise was still there. Since it was not installed properly in the first place I went to another shop and had them recheck everything and personally inspected every small detail, all was good, they were unable to find the issue but confirmed that noise is not right. It def. is not turbo spool or intakes, much louder. I don’t remember hearing it with OEM midpipe but I didn’t have enough time with the car to really notice, it is possible that it was there just silinced by secondary cats/resonator but cant be 100% sure. I also started suspecting that car is not making enough power, I know hp gauge in car is calculated value, but I have never seen it go over 420hp mark. And I haven’t really seen boost pressure (I assume this gauge is real) go over 15psi, maybe even less. While trying to figure this out I was testing it and floored when merging on a highway - almost immediately got drivetrain error/limp mode. Errors are 120308 boost pressure control: pressure too low and 120408. I can 100% reliable replicate this every time I floor it. Visually inspecting turbos/ charge pipes/ etc all looks good.Now car is still on warranty and I have service apt. tomorrow, but I still sit here panicking and feeling pretty bad that my new to me car is broken and god forbids they blame it on aftermarket midpipe. Did anyone had anything similar happen to them or have any ideas what can be wrong?

Sounds like you have a boost leak. If you want avoid the dealer just take it to a indy shop and request a smoke test.

[mike@x-ph.com]

double-check the install with the single midpipe for a boost leak

[SwankPeRFection]

If you didn’t have that leak noise before the midpipe install, then you need to check any boost lines and connections around your intake manifold. Sometimes asshat shops will sabotage a car with something simple like that in an effort to get the person to come back and then they find something else to upsell them on. They’ll usually sabotage it in a way where it can’t cause engine damage or failure, so yanking a vacuum hose will do just that. If it’s not something simple like that, then it’ll be a leak due to a cracked manifold or the boost pipes up front, but the new S58 design isn’t known to be that problematic… unless it sustained damage from something. Open the hood and find where the hiss is coming from.

[Shaninnik]

Yeah, not so easy unfortunately. Second shop who are more experienced with bmw mods tried to open hood and brake boost but nothing. No noise that way. We inspected all connections and everything appear normal. No visible cracks either, but you need to really start disassembling things to be able to see everything. Only thing that was noted is that flange on new midpipe that connects to downpipe was slightly misaligned when welded creating a lip and possibly preventing gasket from fully sealing. I am waiting for a replacement and also will replace gaskets, but I doubt midpipe leaking at downpipe connection would cause boost leak, I really hope that it’s the issue, but doesn’t seem likely, is it?

[SwankPeRFection]

No. Boost leak issues are on the intake side, post turbos.

You need to simply block off one of your turbo intakes and pressurize the other one with a boost leak tester and that’ll tell you if you’ve got a leak somewhere on the intake side. A smoke test will also accomplish the same thing, if the shop has a machine. Your “second more experienced” shop brake boosting to try to find a leak makes me laugh. Where are you that you have such shitty shops that actually charge money for diags like this? 🙄

[Shaninnik]

lol, man, brake boost was just a quick experiment, no one charged anything for it, they were diagnosing exhaust noise at that time, not low boost pressure error, it escalated shortly after. Since the car is on warranty I am trying dealer first before I get deeper into diagnostics with indy shop. Just wanted to get community opinion since it’s a weird coincidence that I started hearing noise and getting error only after midpipe install and but it seems unlikely that exhaust leak can cause boost pressure errors.

[Info@mad-us.com]

Not sure what kind of a boost leak you could manage to get while installing a mid pipe but, hey I guess anything is possible.

A small gap on the midpipe to downpipe flange is not likely to cause a leak with the gasket in place. If it was you should see a little black soot by now. If you have a photo post it.

[Shaninnik]

Yup, exactly, it was all extremely weird, it did not sound like a boost leak yet I was getting low boost pressure error.

Anyway, I canceled my apt with dealer today and installed my OEM stuff back. Full power back, full boost back, no noise.

My fault really, I should have just ordered MAD midpipe as I wanted initially, but it was all on backorder and I was fooled by a website which turned out to be just a dropship from Asia

Enjoy the photo of finest craftsmanship:

[SwankPeRFection]

So really it was exhaust flow restriction causing the condition. I wonder if the sound you were hearing was cavitation happening in the turbo exhaust housing and up to the restriction in the new pipe. Weird…

[Chi58]

Originally Posted by Shaninnik

Yup, exactly, it was all extremely weird, it did not sound like a boost leak yet I was getting low boost pressure error.

Anyway, I canceled my apt with dealer today and installed my OEM stuff back. Full power back, full boost back, no noise.

My fault really, I should have just ordered MAD midpipe as I wanted initially, but it was all on backorder and I was fooled by a website which turned out to be just a dropship from Asia

Enjoy the photo of finest craftsmanship:

I was just going to say inspect the the mid for restrictions good find