Funkwagen

While I was driving this morning, I noticed that the exhaust note has changed and become quite a lot louder past 4k rpm, as well as introducing some noticeable vibration. It sounds to me like there's a leak in the exhaust system somewhere, but wouldn't that be noticeable throughout the whole range of engine rpm?

It changes quite suddenly at ~4k rpm, and sounds completely normal before that. Has anyone had this happen/know what this could be? I've always run 91 in the car, as well.

Carsy

A leak or a baffle loose inside.

Try placing a thick rag over the exhaust & apply pressure. You should hear a hissing sound of gas escaping if there is a bad leak. Don't burn yourself.

Funkwagen

edit: no, no it was not.

Funkwagen

So, the issue turned out to be the intake manifold. What sounds like a funny exhaust note in the cabin turns out to be a faulty intake manifold. This also caused quite a lot of vibration and what felt like a loss of power at high engine RPM.




The drivers side cylinder bank longitudinal switch flap shaft and it's actuator were the issue; it was too coincidental that this noise would occur exactly around the engine RPM they're supposed to open up.





The highlighted area was the problem. On initial inspection, it looked like the actuator arm and the longitudinal switch flap arm(12/3) had become disconnected. The actuator arm wasn't broken, and the switch appeared to be okay, so I reconnected them (the arm and valve connect with a basic ball socket) and figured it was problem solved. The noise went away, and everything was completely fine.

It then happened a second time, so thinking it was a fluke was ruled out. I took an even closer look and prodded around, and found that the switch (12/3) was wobbly and not remaining seated correctly. This made it shift off its rotating axis, and the actuator arm would pop off of it. I then tried rotating it freely, and it seemed to "stick" at a certain point slightly, which means it's highly likely that the flap shaft inside the manifold was broken or malfunctioning. What's interesting is that this problem never once threw an error code or check engine light, leading me to think it was something in the exhaust system that'd come loose. Also, the opposite side flap shaft switch was completely fine.

I've ordered a new intake manifold, they aren't cheap; about $1100 CAN.
I'll see if I can put a decent DIY together when I replace it in the coming week, and I'll also post pictures if there's any visually noticeable damage within the old manifold

Carsy

A good find. Thanks for sharing. John.

edgalang

OP, even though I cannot relate (hopefully never), this post still deserves a

This is a perfect example of proper technical follow through that I enjoy reading on the forum.

vipsa

Ditto. So many threads where the OP never returns to update the thread.

Funkwagen

This forum has been extremely helpful and provided a ton of information for me, I can only repay the favour by adding to it!

The procedure was fairly straightforward, I can post the WIS document if it's allowed; it's better than any DIY I'd be able to make. I was able to complete the work with just the WIS document. The most difficult parts was keeping the gaskets aligned while fitting the new manifold to the engine, and removing the fuel rail from the old manifold - it's in there pretty tight. Patience and a sturdy hand is required!

I expected to see some plastic carnage in the old manifold, none was to be found actually. The only noticeable thing was that the left flaps were indeed requiring more force to actuate, but the reason is still unclear. I'll have to disassemble the whole manifold to find out. I think the actuator may be part of the problem, because it pops when actuated, unlike the other side. I still need to get a vacuum test gauge to fully test the operation, as well as find out of the electronic valves are operating properly

I think this old manifold might be repairable, so if I manage to find the main issue and fix it, I'll keep it as a possible spare (maintaining 3 w204s!)


Here are some images:





Old Manifold, everything looks like it's where it should be.





New manifold for comparison





Looks to me to be a lot of oil? I'm not sure what's normal, here.





The manifold, in it's state when it was removed. Note the LHS actuator arm and longitudinal switch