Underboost Cause?

[VAG_4ever]

Hey guys, recently my CEL came back on, giving me a P0299 code, underboost. I'm only boosting around 15lbs at peak when my gauge usually reads 20-21 at peak. I messed around with the car a bit and found out my PCV was cracked, so that got deleted, plugged the tip and nipple hose and am running a line to the ground til i get a catch can. Since I'm still under warranty, I decided to take it to the dealer, dropped it off this morning and they called me this afternoon saying they need to replace the entire crankcase breather system, which oh by the way, isn't under warranty and costs $1,800. Screw that.

So now I am here, because that doesn't really make sense to me?? How would the breather system cause an underboost code?? Isn't that a vacuum related issue, not a boost related issue? The only things I can think of that would cause the code would be my wastegate stuck, n75 (but I don't think its that?? because I unplugged it and boosted even less, is there anyway to test those before buying a new one?) or my DV, which I checked and looks fine. Any other suggestions? I'm pretty sure my turbo isn't gone cause my car drives fine, besides boosting low. Any help would be appreciated on these two issues! Thanks guys.

<3 Meredith

[A4Dit]

When I was having underboost issues on my GTRS it ended up being that my wastegate actuator arm was loose. The wastegate was not opening up.

[dubgli]

i think you mean it was not closing tight.. i think if the waste gate is not opening up, you'd keep boosting...

[blacka4]

oh snap, I have not had that problem but, things I would check:

1 look for a boost leak (I don't think you have one but its worth a look)
2 check wastegate actuator, or the wastgate itself (however my experience with a bad wastegate is it will actually over-boost.) if its stuck open you will not produce any boost and will cause this problem
3Do a pressure check on the Crankcase breather. I don 't know why its so much...

Still I would seek out an independent opinion...why isn't it covered under warranty?

[VAG_4ever]

oh snap, I have not had that problem but, things I would check:

1 look for a boost leak (I don't think you have one but its worth a look)
2 check wastegate actuator, or the wastgate itself (however my experience with a bad wastegate is it will actually over-boost.) if its stuck open you will not produce any boost and will cause this problem
3Do a pressure check on the Crankcase breather. I don 't know why its so much...

Still I would seek out an independent opinion...why isn't it covered under warranty?

I don't know why its not covered.. I did a pressure test and the only 2 leaks were from the PCV and IAT sensor and those got fixed. Did the pressure test again and nothing was leaking. I'll talk a look at the wastegate when they give me my car back. I still don't understand why the crankcase breather would cause underboost.

[A4 TSCHUSS]

First off, Meredith list exactly what your mods are. Do you have a stock or aftermarket diverter valve? Stock or aftermarket intercooler? Obviously you are chipped if you are boosting that high but what else performance wise is done? I doubt it is the wastegate on your stock K03.

An underboost code is most likey due to a boost leak from a stock diverter valve having a tear in it leaking by a little bit of boost and the computer senses that it isn't reaching the desired boost using the map sensor and then throws the code. This could also be due to a tear in a hose somewhere or even a hose that has came loose. If you are all stock in the intercooler department then it is most likely not a loose connection but a hose somewhere that is connected to the intake manifold in some way. The first thing I would check though is the diverter valve if yours is stock. Swap it out with a friends or inspect it.

Edit: you said you did a boost test, were you certain there was a good amount of pressure in the plumbing? I don't know how people do boost tests but I know when I have done them I would set the regulator on the compressor at like 20psi and then plug the hose in, this would result in the pressure going down to zero and showing no pressure or maybe 1psi in my intake track according to my boost gauge in the car. Everytime I would do the testing I would need to set the pressure up at like 50psi to get a descent amount of pressure in the system (according to the gauge in the car). What I am getting at is make sure you are actually getting more than 1-2psi of pressure in your piping by reading your gauge while doing the test then search for leaks.

[VAG_4ever]

First off, Meredith list exactly what your mods are. Do you have a stock or aftermarket diverter valve? Stock or aftermarket intercooler? Obviously you are chipped if you are boosting that high but what else performance wise is done? I doubt it is the wastegate on your stock K03.

An underboost code is most likey due to a boost leak from a stock diverter valve having a tear in it leaking by a little bit of boost and the computer senses that it isn't reaching the desired boost using the map sensor and then throws the code. This could also be due to a tear in a hose somewhere or even a hose that has came loose. If you are all stock in the intercooler department then it is most likely not a loose connection but a hose somewhere that is connected to the intake manifold in some way. The first thing I would check though is the diverter valve if yours is stock. Swap it out with a friends or inspect it.

DV is aftermarket, forge. Intercooler is stock. Its chipped, k&n drop in, exhaust, and that is pretty much it.

[A4 TSCHUSS]

Edited and added more about the pressure testing. So after you found the 2 leaks you fixed you then had the code still come back? I would test again with good pressure and if you still find nothing then swap out the diverter valve still to rule it out.

[NvrFllw]

I had the same problem last year ( you know when I didnt make it to Michigan) and they replaced every single vacuum line and check valve and every rubber hose under the hood under warranty. I think the dealership is screwing you big time. Oh yea and I think I have another vacuum leak.

[VAG_4ever]

Edited and added more about the pressure testing. So after you found the 2 leaks you fixed you then had the code still come back? I would test again with good pressure and if you still find nothing then swap out the diverter valve still to rule it out.

Its not the DV, I already ruled that out.

[VAG_4ever]

I had the same problem last year ( you know when I didnt make it to Michigan) and they replaced every single vacuum line and check valve and every rubber hose under the hood under warranty. I think the dealership is screwing you big time. Oh yea and I think I have another vacuum leak.

Did you throw this code? Or just had the same breather system problem?

[Drew-B6]

I am throwing this code too, it seems to only happen when I am really racing the car around and give it some hard boost. What chip are you using? I am using APR 1+, think it may have something to do with the programing? I am at a lost with this too!!!

[VAG_4ever]

I am throwing this code too, it seems to only happen when I am really racing the car around and give it some hard boost. What chip are you using? I am using APR 1+, think it may have something to do with the programing? I am at a lost with this too!!!

APR as well.. Mine is on all the time though, no matter how I drive it.

Swaping n75's, checking out the wastegate, and doing another pressure test and maybe boost test tonight. ill let ya guys know what happens. If there are any other suggestions as to what I should do tonight when I get my car back, let me know! Thanks!

[03AudiA41.8T]

Did you ever solve this issue? It seems like i have the same thing going on with my car.

[A4SoftWalker]

Me too with a KO4 on APR chip

Tried all the troubleshooting in this thread and had 034 look at the car for a day not inspiring.

[VAG_4ever]

I have no solved the issue yet. I put back on the new pcv, replaced dv, n75, and then my bleeder valve of the breather system cracked, which is under the intake mani. ordered a new gasket and will be taking that all apart hopefully sometime this week when I have time.

[VAG_4ever]

Update:

The entire breather system is fixed. DV has been replaced. N75 has been replaced. Turbo been checked. No leaks that I can find in the system. And I'm still getting this underboost code and STILL in limp mode (5lbs).

I don't know what else it could possibly be?? I guess my last options are wastegate (which i highly highly doubt) or something is wrong with the tune. Guess ill call my apr dealer?? Gahhhhhh. frustrating.

[Drew-B6]

Thanks for the update! Let us know what the APR dealer says!

[VAG_4ever]

Well, apparently my cat is clogged and that is why I'm throwing this code and in limp mode.

[A4SoftWalker]

Haaahaa good one.

That is what the folks at 034 Motorsport told me. Guess what??? Bought their cat and denied them the glory of me paying them to install by installing myself.
Same difference.
Changed tunes from APR K04 to K03... guess what? Boost came back
Drove car for over a week at K03 21psi boost with wimpy K03 boost curve.
Went back for K04 tune - problem back max boost is 15psi
Tried old guy's tip of the intake manifold leaking and bolted the DV direct to the intake manifold bypassing my boost guage and the "T" connection - same difference.
Now I'm mad as hell and gonna call APR today.
All I have left is check valves and then
"F***" it just gonna go low boost Elim.
Tired of being SLOW... I know EXACTLY how u feel.

[VAG_4ever]

Yea, I know its frustrating, and I'm very worried this isn't gonna fix the problem.
However, I did go through an outside source that knew I wouldn't be using their product or labor, just a diagnosis and I somewhat trust them, so maybe they are right and 034 was just telling you that to get you to buy their product?

I don't know, but I'm gonna go ahead and get a test pipe, and if that doesn't fix it, I'll be grrr'd. >:-(

[dougyfresh]

You want an OEM cat? I have one I can just send you to test if this is the issue.

[VAG_4ever]

Well they said they unplugged the o2 sensor and it was boosting 10lbs instead of 5lbs. Does that mean its the problem? Does that make sense?

[VAG_4ever]

You want an OEM cat? I have one I can just send you to test if this is the issue.

I will let you know. Thank you very much.

[dougyfresh]

I will let you know. Thank you very much.

No problem.

[[email protected]]

I've never heard of removing the o2 sensor to see how much boost changes, but it does make sense. If the cat is plugged you are creating a lot of excessive back-pressure that will restrict the amount of boost you can run. Opening up a small hole will at least allow SOME air to flow.

They need to check the pressure levels pre and post cat to be 100% certain. Or just take it off and look. But even if it doesn't solve your problem, a clogged cat is a clogged cat...get rid of it.

[audibbs]

I've never heard of removing the o2 sensor to see how much boost changes, but it does make sense. If the cat is plugged you are creating a lot of excessive back-pressure that will restrict the amount of boost you can run. Opening up a small hole will at least allow SOME air to flow.

They need to check the pressure levels pre and post cat to be 100% certain. Or just take it off and look. But even if it doesn't solve your problem, a clogged cat is a clogged cat...get rid of it.

hi i also have the same problem and was told it might be a clogged cat let us know if thats the fix thanks

[fR3ZNO]

Super necro bump. Don't care.



I've been having the same problem with limp mode (I also have APR Stage 1+), and after doing some reading on NefMoto, the P0234 code I've been getting is actually due to an underboost and not an overboost.

I have tried everything I can think of to fix this except for replacing the cat. FWIW, my car has some crappy aftermarket OE style cat that was installed by the PO. So, I am not sure if it is clogged again (assuming the original cat was clogged and replaced) or what. I'm thinking of just YOLO'ing it and getting a Manzo test pipe from eBay to see if the problem still occurs.

Anyone care to chime in?