B6 Avant 1.8T 'Squelch' sound behind dash, Boost loss

[sjacklin]

Dear fellow Audi'ists - I have a problem that I know you can help solve.

I've owned my A4 B6 1.8T (Stock) Avant for just over 18 months now, and I can safely say that it is the best car that i've ever owned. Which is saying something, considering I bought it with a blown HG (unknown to me) and a desintigrating cooling system (also unknown to me), but we can agree that it's been a love/hate relationship.

Since acquisition, I have fixed a majority of the problems on my spare time and have really been able to enjoy the car. Unfortunately, something still remains amiss and has continued to elude me and cause great frustration.


I have listed the symptoms that I am experiencing, along with my attempted remedies below. I've bulleted rather than TL:DR to save us all from a headache.

Symptoms
-Intermittent P0171 (System Too Lean), typically happens once every few days
-'Squelch' sound when partial throttle applied/let off, sounds like it is right behind dashboard on drivers side, usually only occurs after car is at operating temperature (see Youtube video: https://www.youtube.com/watch?v=HACmDLcEQ54)
-Solenoid clicking behind dash, seems similarly responsive to 'squelch' sound, eg. click when partial throttle on then click when partial throttle off.
-Audible 'whistling' when WOT accelerating, i quote because it sounds like someone blowing mostly air and getting a small high note in
-Variable idle once warmed up, 820-760 rpm. Sometimes braking brings this down to ~620-640
-Fuel economy is not very economical

Attempted Solutions
-New 710N DV Last week, old DV was not holding vacuum (confirmed by testing it) and there was clear audible turbo fluttering when coming off WOT.
-Replaced PRV (Pancake valve) after noticing significant emissions gunk buildup around the top. I will note that the NEW PRV installed last year is also getting quite dirty/oily on the outside, not sure what the cause of this is or if is relevant
-Replaced multiple check valves that were showing signs of wear/leakage, though post removal testing shows that they were performing their function and not leaking.
-Boost Test: I have performed two seperate boost leak tests (Warm car/cold car) to no avail. Both attempts I managed to get the system to 'push back' a 10 PSI number according to the regulator on the pump. The only sounds I get are air coming from the oil cap and hissing around my TIP, the TIP noise I attribute to the 90 deg pipe elbow that is attached to my air compressor is causing some acoustics.
-New fuel injector seals recently. I see some evidence of leakage around the ports but nothing major, and no apparent airflow on the boost leak test
-New crankcase breather 1 yr ago. Old one disintegrated as I tried to remove
-SJP looks fine

Please, if you have any insight into what may be the culprit, I would really appreciate hearing from you.

Sean

[sjacklin]

*bump* with a revised title.

[sjacklin]

Second bump. I just re-read over my post to make sure I wasn't being too cryptic. Hopefully I am explaining the issue adequately.

[sjacklin]

One last shot. Does anyone have any suggestions or troubleshooting tips that I could perform? If my original post needs clarification, that feedback would be welcome as well.

Sean

[sjacklin]

Hey all,

I have come to a partial solution to my original problem and thought i'd follow up to help anyone stumbling across this post.

After work today I decided to have a go at my PCV (crankcase pressure relief, not PRV Puck) valve which is located underneath the intake manifold.



Bingo! replaced and couldn't wait to take her for a spin. Sadly however, as soon as the car warmed fully I began to hear the squelch sound return. I also noticed that my high pitch 'attempted whistle' sound is still present under WOT.

Given that my attempted boost leak tests would have sent all my pressure out of my oil cap (due to failed PCV), I am going to try to find a leak again as soon as possible.

It appears that almost all of these vacuum lines are original and I am slowly getting around to replacing all of them one by one. Is it possible that there is an obvious offender exists that commonly fails by my current mileage (250K km)?

P.S F&*( theives. This happened over the weekend.