White Knight II

Hello there, I recently picked up a nice 10th ae 7 and it's been really great but it's giving me this weird problem. I've spent a lot of time searching around the forum for information and I've tried numerous things but I'm having to resort to this because I'm not sure what else to try.

Basically, I'll start up the car from cold and drive it around easy for ~10 minutes until it's warm. Then I'll start boosting in 2nd or 3rd gear fine. The car makes good power; the turbo boosts nicely up to about 8psi or so and there are no strange noises, just the quiet whistle from the turbo and a very nice whoosh from the BOV when I let off the throttle. All is well. Then after doing this a few times, the problem starts. As soon as the car starts to boost, I hear this shrieking noise. I'll let off at 5psi and get a nice whoosh so I don't think it's compressor surge. Or is it? There are no noises other than the BOV whoosh when I let off, the sound only occurs while under boost. And sometimes the noise will fade after ~5 psi. I think load matters too. I can boost up to 5psi and I won't hear it that much if I'm under 3k rpm. But over that it screams.

The car has a stock s4 turbo and ic and a blitz BOV. Other things of interest may include rebuilt motor and turbo with ~20k on them, full RB exhaust, ported wastegate, proper fuel mods, all emissions removed. The BOV is welded onto a 2.25" OD aluminum pipe, which is connected to the turbo and ic using some silicone couplers. I thought the sound may have been coming from leaking rubber couplers so I just replaced them with silicone ones and tightened them down real well with screw clamps. No luck though, I still get the sound after boosting a few times. I've also tried loosening/adjusting the BOV for fun but there was no change. I also just put the stock airbox, tid, etc. back in and there were no cracks or leaks anywhere. Does anyone have any idea what this could be?

j9fd3s

iTrader: (3)

thinking it might be a bad turbo, or maybe a slipping clutch? its also possible that theres a hole in the exhaust somewhere. my 79 has a failing exhaust gasket, and it whistles over 5000rpm...

White Knight II

clutch is new but interesting idea about the exhaust.. although, I can rev the car up to 5-6k without boosting and it doesn't make any noises.. hmm. I just feel like it's turbo related b/c I don't think it was making any noises when the stock charge pipe/BOV was in place.

White Knight II

I was thinking about this some more and is it possible that there could be some sort of "pocket" of pressure due to the different pipe sizes? The turbo outlet is connected to a 2.25" pipe that's about 6"long which is connected to the 1.75" ic inlet. Due to the small ic inlet, could the turbo be charging that section up to boost pressures higher than what my autometer gauge is showing/what the engine's seeing, causing the couplers to leak or worse, forcing air back into the turbo?

White Knight II

ok, this is driving me nuts.. I've driven around some more and noticed that it doesn't matter when or how many times I boost or what the engine temp is.. I think it depends on the RPM and boost level. After driving around for 45min or so I tried boosting again for fun and there was no strange noise. I think I was easy on the throttle and the RPMs were sort of low. When I tried again, nearly flooring it in 2nd gear at 3-3.5k, it started screaming again??

My only other idea is this: The BOV is connected to a small pipe that is welded to the charge pipe. If you look inside the charge pipe at the hole where the small pipe connects, it isn't flush. There's a small 1 cm section protruding out into the charge pipe. As air rushes by and blows past this extension, could it be causing this noise? Sort of like when you blow air over the rim on a beer bottle? It started making sense to me because I don't hear the sound at certain RPM/boost levels and I do at others. And if the amount of air depends on the RPM/boost level, then maybe there's a sweet spot or a certain amount of air that really makes the car scream. Any input?

texFCturboII

iTrader: (17)

Check to see if there is any shaft play in the compressor wheel of the turbo. If there is an excessive amount I would say that that is the culprit of the shrieking sound, the wheel is grinding against the housing.

White Knight II

but why would it only do it sometimes?

Boosting to 5psi in 4th under 3000 rpm = no sound
Boosting to 5psi in 2nd over 3000 rpm = sound

I thought 5psi is 5psi is 5psi as far as the turbo is concerned..

texFCturboII

iTrader: (17)

I wouldn't be boosting under 3000rpms anyways. But that is wierd, I was just throwing it out there. My guess is exactly what j9fd3s said, bad turbo, or an exhaust probelm.

White Knight II

I decided to pursue the idea that it might be an exhaust leak. I was checking the bolts on the turbo manifold and found that the bottom two were loose; I could actually unscrew one of them by hand. After tightening them back down I started the car (from cold) and started feeling around for leaks. I couldn't feel anything but there was a little smoke on start up and I could see it seeping out of the bottom of the manifold. So I guess the bad news is I have a leak (possibly a bad gasket) BUT I drove the car around for half an hour or so and the sound was gone! Other than the fact that it stinks inside the car, what are the adverse effects of having a leak in the turbo manifold? I'm not losing boost or power but are there any major problems that could come about if I don't correct this soon?

texFCturboII

iTrader: (17)

Just that the leak will continue to get worse and worse and the car will eventually sound like it's broken. ask me how I know.