mtnboydl

All-

I've got a (2012 A8 4.2L) high-pitched squeal at idle that goes away under load, which seems to match up with the concept of a vacuum leak. I was on a long road trip when it started and CEL turned on. I was able to swing by a local dealership (Audi of Clarksburg WV) where the service manager Jason was extremely helpful.

They put it on the lift and pulled codes. The mechanic isolated the leak sound (that was like an alarm whistle it was so loud) to the back of the engine. The codes pulled were:
P227900: Intake Air System Leak
P209800: post catalyst fuel trim system bank 2 too lean

His explanation at that time was that the rear main seal was failing, causing the vacuum leak. He went on to say that the 4.2 pulls a vacuum across this seal which is counter to every other engine I've heard of that generates positive crankcase pressures and leaks oil from the rear main.

It appears that if the main is failing, the engine has to be completely dropped for replacement, which is devastating with only 75k miles on odometer.

Does this sound accurate to you all? I will be leaving the car with my home dealer for them to repeat the diagnostic and get started on the repairs.

BTAudi

The primary issue is a faulty PCV valve which will cause increased crankcase pressure which can then lead to a blown rear main seal, especially if you drive with the faulty PCV valve. The pressure needs to go somewhere and eventually blows the rear. Pun intended

mtnboydl

Thanks for weighing in!

I never had a CEL for any errors for the PCV valve, so that's a bummer. If the seal goes out, why does the crankcase pull a vacuum? The rear seal should still be positive pressure???

I'm terrified to guess how much this will cost.

BTAudi

When my PVC valve went I had the P227900: Intake Air System Leak code. I don't why the rear main goes, my mechanic just told me not to drive on the bad PVC valve because he's had experience with the rear main blowing from it.

MP4.2+6.0

The PCV explanation sounds plausible to me too. Thus a seal failure unfortunately can be an expansive symptom of an underlying PCV problem. PCV failures typically don't throw direct codes--more indirect, like vacuum codes and such. The valve itself it typically not electrically driven, but rather vacuum and spring pressure stuff.

mtnboydl

The mechanics conclusion about the rear main seal was supposition based on where the sound was coming from, and not from visualization. Is it possible that those codes and the loud high pitched whistle is from a vacuum line and not the rear main seal? I still don't grasp how a positive pressure crank case will draw a vacuum across a bad rear main seal.

It just seems strange to me to have the code for the air intake system and near simultaneous rear main failure.

Like I mentioned in the first post, my experience with rear main seal failure is an oil leak, which I do not have. If I don't have to have the engine dropped, that would certainly be better....

The car will be re-diagnosed by the dealer near my house on Monday.

MP4.2+6.0

PCV in it old simple style is basically just a little plastic ball that regulated vacuum pull from the crankcase at and near idle. Audi turned theirs into the typical Rube Goldberg overdone Swiss watch stereotype for big bucks, but still same principle. If it fails open, it can pull a boat load of vacuum. Pre PVC valves, oil vapor/blowby used to go out through what was called a road tube or road draft tube. Similar principle of setting up a vacuum assist to pull it out of crankcase. PCV valves were literally the first smog device put into cars back in mid 60s to eat this nasty oil vapor stuff by feeding back through combustion chamber. Thus, the positive pressure you are thinking of is more than overcome by the engine vacuum regulated with PCV valve. See fourth paragraph of linked site for more info: https://auto.howstuffworks.com/posit...ion-system.htm

Not sure if it still exists on D4's by the way, but on both my 2000 C5 A6 4.2 and my 2006 D3 A8 W12 they had another device in the vacuum line on the way to the power brake booster called variously a suction pump, a suck pump or various other slang. Basically a regulated orifice to assist in vacuum boost for power brakes. When those failed on some older mid 80's Audis that even then had similar design, they could pull so much oil that it could dump a smoke cloud right out the exhaust. That's gotta be a lot of vacuum, and probably wasn't good for seals either.

mtnboydl

That's a very helpful explanation. So the PCV fails, allowing a huge vacuum to pull through the crankcase, more than offsetting the positive pressure generated by the engine function.

Bummer.

Any insight what a job like this will run me?

BTAudi

PVC replacement will be $1200-$1400. My dealer was cheaper than the Indy shops

mtnboydl

Seems like if mine has the high pitched whistle, it might be the rear main too? Or maybe not?