Another Boost Issue Thread

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I have a 2000 S4 with a GIAC X chip, turbo-back exhaust, N249 bypass, upgraded TBB, F-hose and 710N diverter valves. Normally, I would see a ~20 PSI spike and then boost would hold at 18 PSI, but now boost slowly builds up to 15 PSI and then tapers off to about 12 PSI at redline. I pressure tested the intake system and everything holds just fine. I unplugged the MAF and floored it - same issue.

I've searched around, but it seems that people typically experience boost spikes or very little boost when their N75 goes out on them, not just a 5-7 PSI drop. I cannot think of anything else that would cause this problem with my car, but the N75 valve. Does this sound like it could be an N75 issue?

Thanks in advance for your help.

I should also mention that I am NOT getting a CEL and I have even checked for hidden codes and came up empty handed.

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Bueler? Bueler?

[AudiSportB5S4]

Not having a CEL yet and boost issues after a pressure test would lead you to your N75... The simplest way to do this is to find a car with a well working N75 and swap it in. If you see a difference, then there is your answer. If you are having the same issues then you know you can move past that and onto something else.

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Awesome. Thanks for the reply. I'll try to find someone local, but if I can't then I'll just go buy a new one. I mean, it's more than likely that it would eventually go out, right?

[damunky]

Have you tried logging Requested vs Actual boost (block 115) ?

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Unfortunately, I don't have access to any logging software (only have an OBD2 scantool). That said, I don't see why the car would request 12 or 15 PSI. It seems like it would only request full boost or, if in limp mode, 5 PSI. Am I correct in that logic?

[damunky]

Unfortunately, I don't have access to any logging software (only have an OBD2 scantool). That said, I don't see why the car would request 12 or 15 PSI. It seems like it would only request full boost or, if in limp mode, 5 PSI. Am I correct in that logic?

Well, if you are starting to run lean, the ECU will bring down the boost as a safety precaution.. By logging requested vs. actual you could see if the ECU is requesting 21 psi and the actual is lower ( therefore not a problem w/ what the ECU requests and possibly the n75) or if the ECU is requested less boost due to another problem (high EGTs, o2 sensor values, etc etc).

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Well, if you are starting to run lean, the ECU will bring down the boost as a safety precaution.. By logging requested vs. actual you could see if the ECU is requesting 21 psi and the actual is lower ( therefore not a problem w/ what the ECU requests and possibly the n75) or if the ECU is requested less boost due to another problem (high EGTs, o2 sensor values, etc etc).

So the ECU won't automatically kick down to limp mode boost when it detects that it's running too lean?

It just seems odd that it wouldn't throw a fault code for a defective sensor or a "too lean" reading. That's why I'm leaning toward N75.

[S4-Richmond]

Second that logic. If you are showing zero codes and no history of any intermittent or implausible codes for O2 or EGTs then you are looking at boost leak or N75. Try to find someone to log actual vs requested first or pressure test again. What pressure are you using to check for leaks ? Sometimes if you use 10-15 psi then it won't crack open some leaks. Is it always now 15 psi or intermittent ? If intermittent then check BPV operation or stickiness of pistons.

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It's always about 15 PSI at WOT. I tested for leaks at 10 and 20 PSI. There aren't any leaks. The BPVs are brand new 710Ns.

[damunky]

It's always about 15 PSI at WOT. I tested for leaks at 10 and 20 PSI. There aren't any leaks. The BPVs are brand new 710Ns.

Is there any way you can swap N75s with someone local to see if that's the problem? If not I guess it doesn't hurt to replace it anyways.

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I'm going to see if there is anyone. Like you said, if no can then I'll just replace it since the chances are A) that it's the problem and B) if it's not the problem, it would be eventually.

[lbs4]

i have 3 n75's that are all good working order. dont buy a new one incase that isnt your problem its a waste of money. i had a boost issue that pointed towards an n75 but i tried changing it, well 3 times, and no help. i can send you one

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Thanks. I'm just not sure about using a used N75. How many miles are on it? I'm not questioning your integrity at all, but I've bought used parts in the past to replace a broken parts, experiencing no change and hunting for another solution, when in reality the problem was that the used part was damaged to begin with.

[damunky]

Thanks. I'm just not sure about using a used N75. How many miles are on it? I'm not questioning your integrity at all, but I've bought used parts in the past to replace a broken parts, experiencing no change and hunting for another solution, when in reality the problem was that the used part was damaged to begin with.

Strangely enough, my car also just started to drop max boost down to 15 ish max. Let me know if changing the N75 fixes your problem, looks like we are in the same boat lol.

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Will do. I just saw your thread about it. Looks like others are experiencing the same, damn issue. Haha. Out of curiosity, what tune are you running? GIAC?

[damunky]

Will do. I just saw your thread about it. Looks like others are experiencing the same, damn issue. Haha. Out of curiosity, what tune are you running? GIAC?

I'm running an ABT tune haha. I'm going to go look at my car now and try to swap n75s.

[Quattro_2.7tt]

Cant you unplug the N75 to have the turbos run at wastegate pressure?

[Trigger Happy]

If your N75 is bad and causing boost spikes on a apr 93 program, would it also cause spikes on the stock program?

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Cant you unplug the N75 to have the turbos run at wastegate pressure?

Yes, and they do.

[Quattro_2.7tt]

Yes, and they do.

Oh whoops I'm retarded didnt read you have a chip!

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Ugh. Anyone in the area want to log my car? I've replaced the N75 valve and put in new spark plugs. The car now spikes up to 18 psi and then slowly tapers to 15.

Out of curiosity, I tested the green check-valves that are under the Y-pipe (I believe they're part of the PCV system) and both of them failed. Could that cause my boost issue?

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Someone? Anyone? Could those green check-valves being bad cause boost issues?

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Bump for an answer.

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Ok. I'm still having this issue. Any help would be greatly appreciated!

Just to recap:

- Car is Stage 2+ with GIAC XR chip and used to hit and hold 19 PSI all day
- Now, only hitting and holding ~15 PSI (sometimes 17, but tapers down).
- Replaced N75
- No boost leaks
- No CEL or stored codes
- Found faulty check valves (green ones on top of intake manifold), but not sure if that would cause the problem.

WTF is going on?!