B7 A4 Boost Issue (view logs)

[audi8844]

Hi all,

I have been having an issue lately with my boost fluctuating. I am sometimes able to get good runs in, but most of the time it is fluctuating, as the logs show. I conducted a leak test without any clear leaks found (maybe I missed something here somehow). Is such fluctuation indicative of a boost leak? It definitely appears that the ECU is requesting the proper boost, but something is causing the boost to drop off. Do the N75 duty cycle values seem acceptable? They hold fairly steady through the run. However, you can see that the run where I checked N75 duty cycle, the boost seems to be acceptable without the fluctuations. Maybe I will have to try again and log N75 and boost until I get a run that acts in the manner of the first boost log. I also threw in a log for fuel pressure just to check that. For side notes, I have the rev D piston DV, and I just installed the JHM test pipe (I was having this issue prior to the TP). I believe all other mods are in signature.

First Boost Log
http://i574.photobucket.com/albums/ss183/audi8844/Boost%20Log%204.4.2015_zpstrskmtm2.png (http://s574.photobucket.com/user/audi8844/media/Boost%20Log%204.4.2015_zpstrskmtm2.png.html)

Second Boost Log with N75 duty cycle log
http://i574.photobucket.com/albums/ss183/audi8844/N75%20and%20Boost%20Log%204.4.2015_zps3fvd6k9x.png (http://s574.photobucket.com/user/audi8844/media/N75%20and%20Boost%20Log%204.4.2015_zps3fvd6k9x.png .html)

Fuel Pressure Log
http://i574.photobucket.com/albums/ss183/audi8844/Fuel%20Rail%20Pressure%20Log%204.4.2015_zpsxp2ynei v.png (http://s574.photobucket.com/user/audi8844/media/Fuel%20Rail%20Pressure%20Log%204.4.2015_zpsxp2ynei v.png.html)

[aluthman]

I'd lean towards a faulty N75 or possibly a sticking WG actuator.

[jimrobbington]

That's weird as hell.

[audi8844]

Aluth, I was leaning toward that as well. I even had an N75 show up today, in case I need to replace it.

Jim, I know... you're telling me. I originally thought it was just a boost leak. But then I get runs (shown above) where I am right around the requested boost, other than the initial spike.

Hopefully, I can get a good (or technically a bad) pull, showing N75 and boost to see if they are related.

[audi8844]

On a side note, does anyone know what the typical N75 duty cycle should be? Anyone have logs of this for me to review?

[B7OrangeA4]

Do you have a boost gauge? Are you having weird vacuum readings?

[aluthman]

Another thing to possibly look at is your MAP sensor if your boost gauge doesn't match the logged boost.

[jimrobbington]

I don't understand what would make it fluctuate like that. Almost like the wastegate is fluttering. But you would hear that if that were the case. And that could be due to a mechanical problem or am electrical one. I am swaying towards the electrcal side of things because it's completely fine sometimes. If it were mechanical, it would likely do it all the time.

[aluthman]

I'm inclined to believe that it's electrical also. That looks like the PID controller is having a hard time correcting the error and is overcorrecting until it eventually gets it right.

[audi8844]

Do you have a boost gauge? Are you having weird vacuum readings?
Yes, I have a boost gauge. I am always right around 25-30 inHg at idle. Nothing unusual there. Are you looking for possible low readings or high (I would assume high)? I seem to be closer to 30 inHg since installing test pipe. But I attribute that to better scavenging with reduced exhaust pressure.


I don't understand what would make it fluctuate like that. Almost like the wastegate is fluttering. But you would hear that if that were the case. And that could be due to a mechanical problem or am electrical one. I am swaying towards the electrcal side of things because it's completely fine sometimes. If it were mechanical, it would likely do it all the time.
That's what I'm thinking too.


Another thing to possibly look at is your MAP sensor if your boost gauge doesn't match the logged boost.
Boost gauge is matching the logged boost. When the boost is fluctuating like shown, you can certainly feel the difference in power when it drops off and comes back up.

[jimrobbington]

No cel?

[B7OrangeA4]

Yes, I have a boost gauge. I am always right around 25-30 inHg at idle. Nothing unusual there. Are you looking for possible low readings or high (I would assume high)? I seem to be closer to 30 inHg since installing test pipe. But I attribute that to better scavenging with reduced exhaust pressure.




Im in a similar situation with my car where the leak tests came back with no leaks and checked everything that i could think of. I feel a slight loss of power but when ran logs the actual vs requested was fine. My vacuum reading is different every time i start and idle no matter the weather or shut the car off and come back a few minutes later. It sits anywhere between -10 and -18 with little to no correlation to load and rpm dropping to 800

[audi8844]

No cel?

No CEL. The only related code I have had in the past few months related to this is a slight vacuum leak code P0456-001, which my gas cap warning came on one morning. But that happens like once per year anyway, with no additional effects.

[audi8844]

Update:
I just took a few additional logs today, so that I could hopefully see the N75 duty cycle along with the boost logs, when the boost was fluctuating. My results are shown below. I overlaid the N75 and the boost logs to see the interaction between the two. It does appear that the N75 duty cycle is changing and the boost is reacting as such. Do the values for the N75 seem correct here? It appears that the N75 is having trouble keeping up with the boost spike and is eventually over-correcting. I know the initial boost spike is somewhat typical, but my specified boost seems quite low for the unitronic 1+ tune, as well. But, I really have no "acceptable" data to compare to. Any thoughts on this data? Anyone have comparable data? TIA

http://i574.photobucket.com/albums/ss183/audi8844/3rd%20Gear%20Boost%20and%20N75_zpsnc0zeljn.png (http://s574.photobucket.com/user/audi8844/media/3rd%20Gear%20Boost%20and%20N75_zpsnc0zeljn.png.htm l)

http://i574.photobucket.com/albums/ss183/audi8844/2nd%20Gear%20Boost%20and%20N75_zpss3kzsbts.png (http://s574.photobucket.com/user/audi8844/media/2nd%20Gear%20Boost%20and%20N75_zpss3kzsbts.png.htm l)

[yeoj112689]

I am not sure if this has anything to do with it but when I replaced my DV to the new style, I broke the tab that holds the sensor on :( and it came off while driving with my temp fix of zip ties and was doing the same thing.

Maybe different from what you are having but it if helps check your DV again. Easy check.

[audi8844]

I will take a look as soon as I can

[baldy]

Did you adjust your wastegate at all by chance? I only ask since I was having crazy boost control like this and it turned out to be my wastegate was cranked a few turns too tight by someone....

See post 43 here: http://www.audizine.com/forum/showthread.php/537870-S3-K04-built-in-canada?p=8788169&viewfull=1#post8788169

[rongeur]

I am in not way a master st this sort of stuff but observationally you are only fluttering above 13-14 psig in every example log above before the boost curve adjusts to the specified. This seems to me to be mechanical rather than electronic as there is a stable threshold which the fluctuations occur.

you have a new n75 already, pop it in and check, cant hurt since already paid for.
I like the idea of the waste gate being the problem and would have it checked out.

[audi8844]

I am in not way a master st this sort of stuff but observationally you are only fluttering above 13-14 psig in every example log above before the boost curve adjusts to the specified. This seems to me to be mechanical rather than electronic as there is a stable threshold which the fluctuations occur.

you have a new n75 already, pop it in and check, cant hurt since already paid for.
I like the idea of the waste gate being the problem and would have it checked out.
Agreed. Hopefully I can find time in the next couple of days to swap the N75. If that doesn't fix it, I will get the wastegate further inspected.


Did you adjust your wastegate at all by chance? I only ask since I was having crazy boost control like this and it turned out to be my wastegate was cranked a few turns too tight by someone....

See post 43 here: http://www.audizine.com/forum/showthread.php/537870-S3-K04-built-in-canada?p=8788169&viewfull=1#post8788169
Wastegate has never been adjusted. But I will verify nothing is loose here for whatever reason.

[CVTwhatadumidea]

Agreed. Hopefully I can find time in the next couple of days to swap the N75. If that doesn't fix it, I will get the wastegate further inspected.


Wastegate has never been adjusted. But I will verify nothing is loose here for whatever reason.

Any update on this? Did the N75 fix the issue?

[audi8844]

Sorry, I had an update in another thread, as I made a few changes. I never had to replace the N75. Ray @ RAI asked if the oil smelled like fuel. I always thought it did slightly, but he suggested the the internal seal on my stock HPFP may be bad, because my fuel trims were sporadic at idle. I decided to take that opportunity to upgrade my HPFP with the JHM fuel pump. After replacement and an oil change, this issue is gone. I can't say this was 100% for sure the problem, but the problem is gone, so I am happy. I also got the JHM 93 oct tune, which is awesome compared to my Uni tune I had previously. Pulls MUCH harder (with same hardware).

[Jake@JHM]

Sorry, I had an update in another thread, as I made a few changes. I never had to replace the N75. Ray @ RAI asked if the oil smelled like fuel. I always thought it did slightly, but he suggested the the internal seal on my stock HPFP may be bad, because my fuel trims were sporadic at idle. I decided to take that opportunity to upgrade my HPFP with the JHM fuel pump. After replacement and an oil change, this issue is gone. I can't say this was 100% for sure the problem, but the problem is gone, so I am happy. I also got the JHM 93 oct tune, which is awesome compared to my Uni tune I had previously. Pulls MUCH harder (with same hardware).

Glad our parts helped [up]