In the process of test driving an S6 today, a noise came from under the hood that seemed odd to me......it sounded like the turbo was surging! This is a rock stock 95' S6 and always has been. If I load up the engine (read: left foot brake) in second gear, I'll start to get a loud jet engine noise from under the hood around 2k RPM and as it passes 3k RPM, it turns into the familiar "flutter" sound of a surging turbo. Then, as you pass 4k RPM, the noise goes away altogether.

So, the question is: Has anyone ever heard of a K24 surging? The turbo seems too small and sized appropriately for a 2.2l, so how could surging be happening? There are no holes in any of the boost hoses. The frequency valve seems to be doing its job (it's clicking with throttle application), but the by-pass valve is definitely the original unit. I would assume the by-pass valve is bad, but would that cause surging or the noise I describe? I have driven cars with bad by-pass valve and I have never heard a sound like this.

It has a SLIGHT hesiatation as you accelerate through the gears, but I attributed this more to bad gas because I put $20 worth of priemiumn and it was much better. Almost to the point of the hesitation being gone, but I could still feel a hint of it.

Any ideas before I make and offer and/or buy this car?

Thanks!

Is the BPV bad?

visial inspection of them doesn't reveal any and normally, you would hear the "Whoooosh" of air being lost under boost. However, as soon as this "Noise" goes away, the motor and everything under the hood is all normal. So, you are right, I don't actually KNOW that there isn't any holes, but there isn't any of the usual signs of holes in them.

The BOV is suspect because it's look original. It may have been done, but it certainly doesn't look like it. Plus, would a bad BPV make a "Flutter" noise like that and ONLY make that noise between 2K ans 4k RPM?

This would not put me off the car. If it's nice and clean, buy it.

Good luck!

You would have a small vacuum leak due to the ruptured diaphram but the valve itself would just remain in the closed position and shouldn't cause any of the problems your describing.