Boost issues. I bet this will stump you...

[jubatusjames]

Over the course of the last 6 weeks I've done a fair bit of forum searching, part replacement, and even phone calls to GIAC tech support with no success as of yet, and I'm running out of ideas... Now it's your turn.
2003 A4 Quattro with 170k miles, Manual tranny, HFC, upgraded diverter valve, new N75, new plugs, new MAF, and GIAC stage 1 tune. I tuned the car 6 weeks ago and shortly after noticed it would oscillate boost at mid throttle, especially in 4th and 5th gear. The boost would quickly hit 15 psi, then drop quickly to 5, then back up to 15, back to 5, for as long as I held the throttle at the same spot (about halfway down). It did NOT do this at full throttle - I would run about 19-20 psi no problem. I called GIAC, they recommended running a MBC in parallel with N75 valve, which was probably causing the ECU to read overboost and open wastegate, then close it again, open it, etc. I set MBC at 18 psi (via handpump), ran it in parallel; no change (which didn't surprise me because it was dumping boost at 15 psi, which would never be enough for the MBC to open.. duh). So I bypassed my N75 valve altogether and ran just the MBC at 19 psi (with the N75 still connected electronically of course). Voila, boost fluctuations at mid throttle gone. So right, I have the wrong N75 valve. I don't feel like spending 200 on the race edition, so I decided I'd just keep the MBC, set at 19.

BUT NOW:
At 1/3 throttle in 1st and 2nd gear the car spools up quickly to 15 psi, then dumps it all to 0 and stays there (unless I adjust throttle). This creates an annoying and jerky calm/city driving experience. If I give it more throttle it will keep building boost, but then I'll be ramming into the person in front of me. So yea, annoying, but I can live with it. Here's the part that really drives me nuts:

At highway speeds in 3rd, 4th, and 5th gear if I slowly increase throttle from 0-full the turbo spools up but the boost max's out at ~14/15 psi and a CEL AND the traction control icon will come on, but when I take my foot off the gas the CEL and traction control icon DISAPPEAR. HOWEVER, if I floor it from zero throttle, the boost hits 19 psi (the MBC setting) and holds it all the way till 6k RPM no problem. I've ran block 003 on the vag com during these highway sprints and monitored MAF flow and throttle valve position. During the slow acceleration when the boost only reaches 14, the throttle position only gets to 21% and the MAF reads low at about 110 g/ms (or whatever the units are). When I floor it quickly though the throttle hits 100% open and the MAF reads 170.

So far I've tried disconnecting the MAF - no change, and resetting the throttle body max positions (key to on with pedal held down for 5 secs, then key to off) - no change.
So... it can't be the N75 cause thats still bypassed. Could it be a bad MAF messing with the throttle opening fully (but then why didn't disconnecting it change anything?), or could it be a faulty throttle body valve that is getting "stuck" under slow opening? Or something else entirely? OR could it just be that GIAC makes shit tunes and they are pretending like my issue isn't commonplace?!

I'm sick of being cornfused! Thanks in advance for any tips or advice anyone out there might have!

[MoparFreak69]

Shitty tune. No way around it. When I got my Gti tuning all out of whack it would do very much like you described. Boost bounce, crazy random throttle snapping shut for no apparent reason,etc..

[texasboy21]

I doubt its the GIAC tune, its the same tune everyone else is running (OTS).

Have you done a boost leak test?

[F16HTON]

Over the course of the last 6 weeks I've done a fair bit of forum searching, part replacement, and even phone calls to GIAC tech support with no success as of yet, and I'm running out of ideas... Now it's your turn.
2003 A4 Quattro with 170k miles, Manual tranny, HFC, upgraded diverter valve, new N75, new plugs, new MAF, and GIAC stage 1 tune. I tuned the car 6 weeks ago and shortly after noticed it would oscillate boost at mid throttle, especially in 4th and 5th gear. The boost would quickly hit 15 psi, then drop quickly to 5, then back up to 15, back to 5, for as long as I held the throttle at the same spot (about halfway down). It did NOT do this at full throttle - I would run about 19-20 psi no problem. I called GIAC, they recommended running a MBC in parallel with N75 valve, which was probably causing the ECU to read overboost and open wastegate, then close it again, open it, etc. I set MBC at 18 psi (via handpump), ran it in parallel; no change (which didn't surprise me because it was dumping boost at 15 psi, which would never be enough for the MBC to open.. duh). So I bypassed my N75 valve altogether and ran just the MBC at 19 psi (with the N75 still connected electronically of course). Voila, boost fluctuations at mid throttle gone. So right, I have the wrong N75 valve. I don't feel like spending 200 on the race edition, so I decided I'd just keep the MBC, set at 19.

BUT NOW:
At 1/3 throttle in 1st and 2nd gear the car spools up quickly to 15 psi, then dumps it all to 0 and stays there (unless I adjust throttle). This creates an annoying and jerky calm/city driving experience. If I give it more throttle it will keep building boost, but then I'll be ramming into the person in front of me. So yea, annoying, but I can live with it. Here's the part that really drives me nuts:

At highway speeds in 3rd, 4th, and 5th gear if I slowly increase throttle from 0-full the turbo spools up but the boost max's out at ~14/15 psi and a CEL AND the traction control icon will come on, but when I take my foot off the gas the CEL and traction control icon DISAPPEAR. HOWEVER, if I floor it from zero throttle, the boost hits 19 psi (the MBC setting) and holds it all the way till 6k RPM no problem. I've ran block 003 on the vag com during these highway sprints and monitored MAF flow and throttle valve position. During the slow acceleration when the boost only reaches 14, the throttle position only gets to 21% and the MAF reads low at about 110 g/ms (or whatever the units are). When I floor it quickly though the throttle hits 100% open and the MAF reads 170.

So far I've tried disconnecting the MAF - no change, and resetting the throttle body max positions (key to on with pedal held down for 5 secs, then key to off) - no change.
So... it can't be the N75 cause thats still bypassed. Could it be a bad MAF messing with the throttle opening fully (but then why didn't disconnecting it change anything?), or could it be a faulty throttle body valve that is getting "stuck" under slow opening? Or something else entirely? OR could it just be that GIAC makes shit tunes and they are pretending like my issue isn't commonplace?!

I'm sick of being cornfused! Thanks in advance for any tips or advice anyone out there might have!

Smoke test the car, more than likley there is a leak somewhere in the system.