Can't boost more than 5psi. Car runs fine otherwise.

[s4oak]

So I finally got around to installing a boost gauge and found that my car is only boosting to 5 psi. No CEL. Needle on the boost gauge moves smoothly to 5, and it idles at 22 inHG, so I don't think it's a boost/vac leak. Car runs and drives smoothly. I bought it used and found a GIAC sticker on the ECU, so I'm in the process of finding out what programs it has. It smells like it's running slightly rich, and even on all highway, 20 mpg for a tank is a ceiling. My first guess is a bad N75 which is causing the wastegate to pop open at 5. Is there a way to test a N75 without a vagcom? Also, I haven't been able to find whether there are 1 or 2 N75s on the car. One per turbo or just the one under the air box?

Anyone have any other ideas?

[ECS Tuning-Audi]

I'd say its either related to the N75 or MAF. Usually people test the N75 by pulling the line that leads to the waste gate. This will hold the waste gate flapper closed allowing the turbos to build more boost. Be aware that boost will build quickly so I would not advise letting the boost get over 10psi for safety measures.

Jason

[AudiA4_20T]

have you shut off the car, turned it back on and tried again?

[s4oak]

have you shut off the car, turned it back on and tried again?

Lol yes. I have reset the ECU as well.

[s4oak]

I'd say its either related to the N75 or MAF. Usually people test the N75 by pulling the line that leads to the waste gate. This will hold the waste gate flapper closed allowing the turbos to build more boost. Be aware that boost will build quickly so I would not advise letting the boost get over 10psi for safety measures.

Jason

Thanks, I'll give that a shot.
I've read it's possible to try cleaning the MAF before completely replacing it. Will some rubbing alcohol do the trick?

[ECS Tuning-Audi]

Thanks, I'll give that a shot.
I've read it's possible to try cleaning the MAF before completely replacing it. Will some rubbing alcohol do the trick?

Its possible cleaning the MAF may help, but might be only a temporary fix. I would avoid rubbing alcohol. I'd suggest some electronic parts cleaner. It's widely available at electronics parts stores like Radio Shack or the like.

Jason

[airjawed]

You only have 1 N75... the way to check it without vag com won't help you since you are not making boost. If you were making too much boost then checking the wastegate line would mean something. Also blowing into the long end of the n75 should result in air blowing throw(again a check for overboost).

My guess is you are in limp mode or the tune is set for stock at this time.

[DeleriousZ]

5psi kind of sounds like limp mode to me too. you're not using the valet key are you? lol

[s4oak]

You only have 1 N75... the way to check it without vag com won't help you since you are not making boost. If you were making too much boost then checking the wastegate line would mean something. Also blowing into the long end of the n75 should result in air blowing throw(again a check for overboost).

My guess is you are in limp mode or the tune is set for stock at this time.

5psi kind of sounds like limp mode to me too. you're not using the valet key are you? lol

Yeah I was thinking limp mode too but there's no CEL or signs it's otherwise in trouble. Idle is smooth, accellerates smoothly, no weird noises. Everything seems fine except the boost pressure. I'm going to check which mode the chip is in next week when I have a cable.

[alm001]

I don't think you will always get a CEL in limp mode. Have you scanned for codes?

My egts were bad and it would limp, and not always give a CEL

[s4oak]

I don't think you will always get a CEL in limp mode. Have you scanned for codes?

My egts were bad and it would limp, and not always give a CEL

I scanned at O'Reilly's and had nothing. My friend has a generic OBDII-USB cable so I'm going to scan with vagcom-lite and see if anything comes up. I'm also going to check the diverter valves and make sure they aren't leaky.

I think I'm going to start by replacing the N75 and cleaning the MAF, and then if that doesn't solve it and the DVs are good I'll look into the egts and o2 sensors.

[DanS4]

So I finally got around to installing a boost gauge and found that my car is only boosting to 5 psi. No CEL. Needle on the boost gauge moves smoothly to 5, and it idles at 22 inHG, so I don't think it's a boost/vac leak. Car runs and drives smoothly. I bought it used and found a GIAC sticker on the ECU, so I'm in the process of finding out what programs it has. It smells like it's running slightly rich, and even on all highway, 20 mpg for a tank is a ceiling. My first guess is a bad N75 which is causing the wastegate to pop open at 5. Is there a way to test a N75 without a vagcom? Also, I haven't been able to find whether there are 1 or 2 N75s on the car. One per turbo or just the one under the air box?

Anyone have any other ideas?

Scan for codes, with VAG. No CEL doesn't mean anything.

[airjawed]

I scanned at O'Reilly's and had nothing. My friend has a generic OBDII-USB cable so I'm going to scan with vagcom-lite and see if anything comes up. I'm also going to check the diverter valves and make sure they aren't leaky.

I think I'm going to start by replacing the N75 and cleaning the MAF, and then if that doesn't solve it and the DVs are good I'll look into the egts and o2 sensors.

Another thing to check for is that both turbos are actually spooling. If one turbo is sitting still then you lose all boost. I would start with the vagcom codes and also do some logs for actual vs requested boost.

[Tech/Sales@JHM]

I scanned at O'Reilly's and had nothing. My friend has a generic OBDII-USB cable so I'm going to scan with vagcom-lite and see if anything comes up. I'm also going to check the diverter valves and make sure they aren't leaky.

I think I'm going to start by replacing the N75 and cleaning the MAF, and then if that doesn't solve it and the DVs are good I'll look into the egts and o2 sensors.

s4oak-

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[s4oak]

I was looking around online some more and saw people claming 30mpg at 65mph on a GIAC tune. If that's really the average then I have more wrong than a bad N75. At that speed I'd be lucky to get 22mpg on the dash computer.

[S4 S4 S4]

Does your boost go above 5psi at all? Have you checked your WG lines?

[s4oak]

Boost holds firm at 5psi but never goes above. I plugged in a vagcom today and had no codes. I also tried swapping a working N75 from a 1995 S6 and still had the same issue. The car is definitely running rich though. My next thought is either EGT sensors or 02 sensors. I haven't checked the WG lines yet but I'll do that next time I pop the hood.

[s4oak]

Well after disconnecting the line from the N75 to the WG, I was able to build more boost, so I do have a bad N75. The S6's must have been incompatible. Now I just need to figure out why it's running rich.

[TipsyTurtle]

Didn't see your post before I posted.

[airjawed]

Well after disconnecting the line from the N75 to the WG, I was able to build more boost, so I do have a bad N75. The S6's must have been incompatible. Now I just need to figure out why it's running rich.

How much boost is it making now?

[s4oak]

Well it builds indefinitely since the wastegate is permenantly closed. I got it up to 15 before easing off the throttle.

One question I still have: Does disconnecting the N75 hose mean it's definitely a bad N75 or could the car be in limp mode over bad EGTSs and disconnecting the N75 is just bypassing the ECU's decision to run limp mode?

[airjawed]

Well it builds indefinitely since the wastegate is permenantly closed. I got it up to 15 before easing off the throttle.

One question I still have: Does disconnecting the N75 hose mean it's definitely a bad N75 or could the car be in limp mode over bad EGTSs and disconnecting the N75 is just bypassing the ECU's decision to run limp mode?

Your intuition is correct. You may have either a bad N75 or the ECU is in limp mode. Unplugging it does bypass things but does not tell you anything for sure. If you drive while easing into the throttle does it run well? If so that leans towards a bad N75 in my mind.

Have you tried to blow into the long connector at the N75 while it is unplugged from the electrical connection? It should not allow air to blow through if memory serves me correct.

[B5in]

I had the same problem and it ended up being a WG line, it's easy to check, just pull the line going to the WG's from the N75 and either suck on it with your mouth or get a vac line tester and see if it holds suction. Over time it eventually got worse and then started giving me codes but at 1st I had nothing.

[S4 S4 S4]

I had a huge leak in my WG line that would spike to 15psi then hold at 5psi. Took me 2 months to finally track down the leak. I pulled off the line to the n75 and blew smoke in it. Never would of found the leak if I hadnt of used the smoke.

[SRSslpr]

To the OP: Did you ever figure out exactly what the issue was? I'm having the same problem currently where my car will only boost to 5psi. I'm not having any misfire issues and I can't find any boost leaks anywhere.

[TipsyTurtle]

had this happen to me right after i posted in the thread, replaced my N75 with a bnib one and it runs great. when i vag'ed the car i got overboost pressure or somethin similar, no codes now and it pulls smooth to redline.

[s4oak]

Update: I replaced the N75 with a new one from the dealer and nothing changed. Still 5psi. Went back and actually logged requested vs. actual boost levels with vag com since last time I just checked codes. Requested boost is significantly higher than actual, so I think it must be a leak in the wastegate line or a problem with the wg actuator. Since it holds boost if I disconnect the line, I know it's not a boost leak.

SRSslpr: disconnect the hose coming off the bottom of your N75 and see if it boosts higher than 5 psi. Be careful and watch your boost gauge or you will overboost and very expensive things can happen.

S4 S4 S4: How did you blow smoke into the line? Just with your mouth? Not sure if I'm missing something.

Also, I logged EGTs and both sensors in block 112 were at 945 the entire time, even after running the car for about half an hour and running at WOT during the log. Am I right in believing this indicates they need to be replaced?
Edit: O2 sensors were at 0 the whole time too which I believe is weird. Most people see something like +/- 1% during a normal log, correct? I'm going to try re-logging O2 and EGT since I am using an ebay cable and I wonder if they just weren't recorded properly.

[airjawed]

Maybe the vacuum lines are plugged in incorrectly from the previous owner? If you are staying at 5psi it sounds like you are operating on wastegate pressure. In order for the pressure to only stay at 5 the turbo line has to be feeding into the wastegate line either because the N75 is stuck open(not the case since you used a new one) and opening the wastegate.... or the lines got crossed by accident or deliberately(tried to put a manual boost controller on).

You are not in limp mode-requested boost is high
N75 is new
You make high boost when N75 is disconnected

Must be something 'after' the N75

Does the long end of the N75 go to the turbocharger or does it go to the valve cover(short hose)?

[s4oak]

Maybe the vacuum lines are plugged in incorrectly from the previous owner? If you are staying at 5psi it sounds like you are operating on wastegate pressure. In order for the pressure to only stay at 5 the turbo line has to be feeding into the wastegate line either because the N75 is stuck open(not the case since you used a new one) and opening the wastegate.... or the lines got crossed by accident or deliberately(tried to put a manual boost controller on).

You are not in limp mode-requested boost is high
N75 is new
You make high boost when N75 is disconnected

Must be something 'after' the N75

Does the long end of the N75 go to the turbocharger or does it go to the valve cover(short hose)?

I'll double check the hoses later today. I logged requested/actual boost again today side-by-side with wastegate duty cycle, and I saw the same results with requested/actual. Requested a max value of 2230 mbar but actual was 1340. Wastegate duty cycle surprised me. This value was 0 the entire time. It's my understanding this means the N75 is doing nothing to hold the wastegate shut. Now I'm wondering if the dealership gave me a bum N75. Would the ECU be smart enough to report 0 if it's due to a torn WG hose and the N75 is good?

[Ribbie]

945 for the EGT I think means they are toast. Someone correct me if I am wrong. The ECU sees a reading of 945 and dumps extra fuel to "cool" the exhaust gas temps.

The O2s are misleading the ECU again.

[airjawed]

945 for the EGT I think means they are toast. Someone correct me if I am wrong. The ECU sees a reading of 945 and dumps extra fuel to "cool" the exhaust gas temps.

The O2s are misleading the ECU again.

EGT's are fine... they will not read below 945 when running. Once it gets over 945 then it triggers a value and dumps fuel.

[airjawed]

Do you have any codes? Do an output test... see(actually listen) if the N75 clicks. Maybe your wiring/connector is bad?

Could still be limp mode... maybe ecu is asking for 0 duty cycle because of some other issue.

[s4oak]

Do you have any codes? Do an output test... see(actually listen) if the N75 clicks. Maybe your wiring/connector is bad?

Could still be limp mode... maybe ecu is asking for 0 duty cycle because of some other issue.

No codes. I'll check the N75 later today. I don't think it's limp mode; why would the ECU request 17 psi but 0 duty cycle?

[airjawed]

No codes. I'll check the N75 later today. I don't think it's limp mode; why would the ECU request 17 psi but 0 duty cycle?

I honestly don't know how the ECU behaves in limp mode. I don't know if it would shut down the N75 or if it would request less boost. I've had limp mode before but have not seen logs of limp mode.

No codes may be a good thing. But with aftermarket tunes it sometimes just means that the ECU isn't looking for things... many times things are 'coded' out. When my stock turbo blew I didn't get any codes... I made no boost and the ECU requested much more...still no code.

Do the output test and check hoses for proper routing/connections.

[s4oak]

I just checked and the N75 is connected properly. I pulled the WG line from the N75 and blew into it, and there was almost no resistance and a hissing from the driver side, so it looks like I need new WG lines. I'm a little surprised because I expected WG lines to cause overboost. I'll see if I can get these replaced this weekend and follow up.

[airjawed]

I just checked and the N75 is connected properly. I pulled the WG line from the N75 and blew into it, and there was almost no resistance and a hissing from the driver side, so it looks like I need new WG lines. I'm a little surprised because I expected WG lines to cause overboost. I'll see if I can get these replaced this weekend and follow up.

There are 2 lines going to the turbos... the wastegate line should hold pressure... the housing line will blow with no resistance. Check both lines.

[s4oak]

There are 2 lines going to the turbos... the wastegate line should hold pressure... the housing line will blow with no resistance. Check both lines.

I pulled the one coming off the bottom of the N75. After the N75 it splits and the right side goes under the Y-pipe to a metal line I believe. I'll check the other hose as well but the way the hoses are plugged in it would be tough to swap them even if you wanted to. I'm pretty sure it was the WG line. I'll try replacing them with a re-route rather than replacing the stock lines since it's so tight without pulling the engine.

[S4 S4 S4]

Pull off the bottom line to the n75 and blow smoke into and put your finger over the end to block it. It will pinpoint the leak.

[s4oak]

Update: I had the wrong hose. I assumed the WG hose was on the bottom of the N75. Checked the one coming from the short horizontal nub and it held as much as I could blow. Unplugged the MAF and saw no change. Checked all the fuses and they are all good. Now I'm officially stumped. Something is causing the ECU to go into limp mode without a code. No leaks of any kind.

Also, possibly related, possibly not: when I was plugging my MAF back in, I found these dangling down between the airbox and firewall. What are these plugs for???

[s4oak]

So I found out they are my rear O2 sensor connectors. Whoever did the aftermarket exhaust must have just deleted them. Would the lack of sensors cause limp mode without a CEL?