high pitched squealing off throttle when warm.

[Tahiti450]

RS4 B8.5 high pitched squealing / screeching sound when warm and throttle closed. Help!

Hi I am new to the forum and hoping for some expert advice. Problem started about 3 weeks ago - there is a piercing metallic sounding high pitched squealing sound (bit like an idling jet engine) which occurs soon after start up, (I.e not when absolutely cold) and is present only when the throttle is closed or very nearly closed, so at idle, on over-run, engine braking and also when the throttle is cracked open to maintain a high idle or constant no load speed. It is inaudible above 3000rpm. The sound is reminiscent of a dry bearing on a fan motor or idler pulley etc but as above only occurs when the engine is off-load and the only consistent association is with maximum or near-maximum manifold depression. The pitch does not vary with engine speed but does have 2 slightly different pitches about a semi-tone apart. Having listened around the engine bay and underneath with the undertray removed and with a stethoscope (open end) it seems to be concentrated around the rear of the engine but it’s hard to pin point due to the nature of the sound. I am thinking something associated with the vacuum system, combination valve etc.... a leak or a valve acting like a metal reed at high vacuum. Oh ... and no DTCs and the car drives absolutely fine. Driving me nuts at the moment! Any help would be great, thanks. Alan

[Rusty Racer]

It sounds like you know your onions and have checked the obvious. There is a secondary air injection pump that runs off a output shaft under the car (off the timing chain) might be worth a look?

[Tahiti450]

Hi thanks, yes that was one of my suspicions but when I searched through the workshop manual around the secondary air system it looks like there is an electric pump at the front behind the secondary coolant radiator and having poked and listened around there I couldn’t pick anything up. If there’s a pump at the rear of the engine / timing cover as you say then that would fit with what I found, and it certainly sounds metallic.
Update of symptoms which might help... as above..it only happens when the engine is fully or almost fully off load and therefore only at or about maximum manifold vacuum... but additionally I found that if you crack open the oil filler cap with the engine idling, allowing air to be sucked into the crankcase the noise stops immediately and restarts as soon as the seal is reestablished. So it seems related to the crankcase ventilation / purging system .... any takers????

[Rusty Racer]

Interesting that happens when you remove the oil filler cap? I believe their is a crankcase pressure relief valve somewhere!

[Rusty Racer]

Watch the saving salvage guy on you tube, he rebuilt a 4.2 engine! Worth a watch if you have time and he shows the sai pump too

[Tahiti450]

Cheers... that’s interesting and sounds relevant... I’ll see if I can find that video

[Tahiti450]

Had a good look at the (many) videos of the rebuild (B8 4.2 CFSK engine) etc but I’m still struggling with this secondary air pump... on the left side there is the water pump and the air con compressor driven from the aux gearbox in the timing cover and on the Right there is only the starter motor and the alternator (driven from the ancillary belt at the front) ... I didn’t see any engine driven air pump tbh... could this be on the B7 maybe? Fascinating project though, but not one I’d take on lightly! The symptoms on my cra are still essentially the same but actually seem to be diminishing a bit when the engine is sometimes disappears completely for a while... also I’ve noticed if it’s raining / wet road this seems to reduce the symptoms.... I assume water is getting in somewhere and lubricating or dampening some oscillation ???. I’m going to set the car up on the drive and have it idling ... whilst the noise is there and try spraying water mist with the pressure washer around / under the engine bay and see if this kills the sound and helps locate it...

[Rusty Racer]

Well done for watching all the videos!
I could easily have got it wrong tbh - the secondary air pump normal runs at start up for 20 seconds or so (while the Rebs are slightly raised) and as and when required if you have been at idle for a long period.
Otherwise the crankcase pressure valve ... not sure where it is but they have been known to put them in some daft places to.

[johnnydog]

Going back to basics - years ago when cars had fan belts as opposed to auxiliary belts, different humidity used to produce different noises from the fan belt and the alternator pulley when either was worn or glazed. I know this is a very basic suggestion, but is it worth changing any auxiliary belt and checking the pulley bearings it drives? Maybe nothing to do with your issue but it was the first thing that sprung to mind.....

[Tahiti450]

Ok, here’s the answer.... thanks both for your input... much appreciated ... the short version...yes indeed Rusty Racer ...left hand PCV diaphragm and tongue seal perished and split so valve not functioning! Good news the PCVs (2 - one for each bank, are right at the top and easy to access... I didn’t suspect these before because the noise wasn’t coming from there)
Longer version.... pcv not functioning so at idle full manifold vacuum applied to the crankcase (something I didn’t mention and realised wasn’t right was that with the engine idling the oil filler cap was almost impossible to get off ... full vacuum being applied to it which I eventually realised wasn’t normal ... you’d expect a a bit of negative pressure for crankcase purging but not hard vacuum). Further detailed search for the origin of the noise ( stethoscope with tube and section of brake pipe in the end so I could listen to anywhere in the engine bay... again the noise was not localising anywhere until I listened inside the access cover for the bellhousing... WTF! Piercing squealing inside the bell-housing I’d previously noted this and wondered if it was gearbox issue but didnt fit with geabox function (normal). Read some threads on xs crankcase vacuum this immediately pointed to PCV failure... instead of closing at low manifold pressure it remains open and applies full vacuum to the crankcase through the breather hose. So the noise.... now I can’t physically confirm this but the only explanation I can give is that at high vacuum air is actually sucked past the crankshaft rear oil seal, causing the lip seal to vibrate and act like a reed causing the shreaking noise which is then nicely amplified by the bellhousing. Neatly explains why unseating the oil filler cap immediately stopped the noise...
solution ... well replace PCV obviously (I will do both as the other must be near to failure now) ....problem... its Saturday pm and dealer is closed and part # 079 103 245 L is not listed anywhere on the web (suffix J and M only and they physically different and are for different applications).
As I need the car and am very impatient anyway I have spent a couple of hours hand making a replacement diaphragm and tongue seal out of a pair of industrial safety rubber gloves along with the ever useful superglue. The result after much faffing around and trial and error ... not perfect but it now works well enough to have reduced the vacuum in the crankcase to modest levels... the valve doesn’t quite seal fully... but the the noise has gone and the engine runs fine and hopefully it’s good enough until I can get the new ones.
Phew... this has been driving me mad for weeks .

[Rusty Racer]

Glad you diagnosed it!
I guess when the oil filler cap is removed you should get a slight increase in revs if everything is okay.
Do include some photos if you can

[Tahiti450]

Ha, yes so am I ...thanks !! I’ll post some photos of the makeshift repair but basiscally the original diaphragm (left hand pcv) was split in multiple places and the tongue had separated. I have replaced this one with new OEM for £150 ... there is no alternative and although you can get replacement diaphragms (including supposedly indestructible ones) for some of the Audi PCVs on line for pocket money.... not the RS4... oh no... so local stealers it is I’m afraid. Had a look at the right hand one as well and although it was still sort of functional it was failing.... so rather than forking out again I took the one I “repaired” and re shaped the hose so that it would fit on the RHS instead. As I said I don’t think it’s sealing properly but it seems to be adequate at the moment. In due course I’ll fit a new one when I can cope with the resentment of paying through the nose for a piece of injection molded plastic / synthetic rubber that’s probably worth about £5!
Cheers Alan

[Rusty Racer]

Great pictures!
Can you take one with the location on the engine? Thanks