Question about flow noise

superfans · 04-18-2006, 08:38 PM

I recall that the 997 owners' manual mention that "the flow noise heard at full steering lock is designrelated and does not indicate a defect in the steering system". I am wondering what exactly do they mean by "flow noise". What does it sound like and where does it come from?

AtomicDog · 04-18-2006, 08:48 PM

superfans,

The flow noise that you are referring to is caused by the power steering rack reaching the end of its travel. At this point, there is a "dead end" for the hydraulic fluid in the power steering system. The pressure surge that this causes is mitigated by a relief path that prevents overpressure damage to the rack and plumbing (rubber supply hose).

Flow through the relief passage is often accompanied by a squealing or whine due to the flow restriction presented by the relief valve. It is nothing to worry about at full lock. If it happens all the time, it would indicate a failure of either the pump or relief unit. This would be a sign of impending doom for the power steering assist.

Hope that made sense.

Chris

superfans · 04-18-2006, 09:03 PM

Thanks for the explanation. Does the squeal or whine come from somewhere near the air-con control panel or the steering wheel?

AtomicDog · 04-19-2006, 05:42 AM

superfans,

I don't have, nor have I been in a 997, so this is a guess...

The relief is normally incorporated within the steering gear. In P-cars, this is the steering rack. In a non rack & pinion car, it's the steering "box".

The shaft that connects the steering wheel to the steering gear functions as a listening rod (like you would place on a component and put your ear against). It functions like a poor man's mechanic's stethoscope. Where the steering shaft passes through a bearing or support, that noise is transmitted to the surrounding structure. Since this is forward of the dash, I believe this could be as you describe - particularly if it only occurs under full-lock conditions.

Chris