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Discussion Starter · #1 ·
A while ago, I disconnected the kickdown switch on my AEB because it was seriously pissing me off (this was before I knew about the VAG-COM programming that could be done). Since it works well enough for my purposes, I left it that way.

As I was driving home this evening, I was going up a long hill in top gear, and whereas I would normally downshift, I decided to keep the gearbox in 5th and peg the gas so I could check how much boost I'm getting. Because I'd disconnected the kickdown switch, I was actually able to push the gas all the way to the floor without the car shifting on me (hence the sidestory.)

I've never seen the boost gauge go above 7.5psi or so, but that was under conditions where the engine could actually rev up and shift into the next gear. This time, the boost jumped to 5.5-6psi as I expected, and then climbed slowly towards 7. Then 8. Then 9. It finally topped out around 9.5psi.

The only thing I've done to my turbo system is fix various vacuum leaks, including replacing the diverter with a Hyperboost model. None of this controls the boost that the turbo can produce, only how much the system can maintain as the air moves towards the intake manifold. In other words, I haven't done anything that should have increased the boost capacity of my car.

So why am I getting 2psi more boost than I should? Is something else failing?
 

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What you did is create the highest possible load request as a combination of gear and rpm. The ecu pegged the N75 duty cycle and it would never catch up due to the grade so the boost kept building.
 

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Discussion Starter · #3 ·
Isn't the N75 supposed to stop blocking the air pressure trying to open the wastegate when the boost reaches a maximum of ~7psi, though? I thought the electrical component of the N75 serves to delay the opening of the wastegate, not to trigger it. Do I have it backwards?
 

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i know my car does that, and the n75 does not block boost or work like a boost controller. it adjusts how the boost curve looks under certain situations, you happned to find one of those conditions

I know when i installed my manual boost controller and set it to 10 psi, then i later took the n75 out of the loop and still had the same 10 psi *althought it did kick in sooner. Unless my n75 was dead, the n75 does not act like a boost controller
 

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Isn't the N75 supposed to stop blocking the air pressure trying to open the wastegate when the boost reaches a maximum of ~7psi, though? I thought the electrical component of the N75 serves to delay the opening of the wastegate, not to trigger it. Do I have it backwards?
Then75 has no brain, it only does what the ecu tells it. It doesn't measure boost. The B5's are load controlled, boost is just a means of getting to the load target. If the load target doesn't get reached, boost will continue. Same reason boost levels change depending on ambient temps.
 

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Can a faulty PCV system cause overboost? I have overboost problem on 98 AEB (it happened suddenly several months ago as if something failed)
I've replaced diverter valve and vacuum hoses and had turbo out to check operation of actuator.
I guess I'll have to replace N75 at some stage to see if that is it but I was also thinking about replacing PCV system as it could well need doing anyway I'd imagine at 150K miles. I was wondering if the one (or more ) of the one way valves in this system could be causing my problem if it had failed?

I can hear a noise building as boost builds and this is linked into my issue, though it's not quite how I would expect air escaping under pressure to sound.
Sounds faintly metallic and faintly like something spinning faster and faster. I guess it might be air being sucked in somewhere rather than blown out somewhere? Dunno if this is feasible. Annoying though. Been unable to track it down for months
 

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Can a faulty PCV system cause overboost? I have overboost problem on 98 AEB (it happened suddenly several months ago as if something failed)
I've replaced diverter valve and vacuum hoses and had turbo out to check operation of actuator.
I guess I'll have to replace N75 at some stage to see if that is it but I was also thinking about replacing PCV system as it could well need doing anyway I'd imagine at 150K miles. I was wondering if the one (or more ) of the one way valves in this system could be causing my problem if it had failed?

I can hear a noise building as boost builds and this is linked into my issue, though it's not quite how I would expect air escaping under pressure to sound.
Sounds faintly metallic and faintly like something spinning faster and faster. I guess it might be air being sucked in somewhere rather than blown out somewhere? Dunno if this is feasible. Annoying though. Been unable to track it down for months
Not sure if you're chipped or not, but check the turbo inlet hose between the airbox and turbo. Make sure it is secured at the turbo and that it is not so soft that it would collapse under boost. That sound you describe sounds like the turbo working extra hard.
 

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Thanks 180

I have always thought it sounds like the turbo working, but when you tell people of the noise they tend to say it must be a boost leak. Hard to diagnose without hearing tbf :)
I thought maybe there was a gap somewhere which was letting me hear the turbo but no gaps. Hoses have been on and off a few times and I know every pipe is well secured.

The inlet hose is a little soft. It's hard to know if it is too soft as I don't know how stiff they should be to start with.

Would a collapsing hose give an overboost error then?

oh, car is bog standard btw

I have been running with the N75 disconnected for a few months now to try and minimise boost but it still pulls much harder than it should, even at 10% throttle!! Can't be good for the old girl
 

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Discussion Starter · #9 ·
i know my car does that, and the n75 does not block boost or work like a boost controller. it adjusts how the boost curve looks under certain situations, you happned to find one of those conditions

I know when i installed my manual boost controller and set it to 10 psi, then i later took the n75 out of the loop and still had the same 10 psi *althought it did kick in sooner. Unless my n75 was dead, the n75 does not act like a boost controller
If the N75 isn't a boost controller, why is it connected to the wastegate, and why does it get replaced with manual boost controllers?

As far as I understand (and I'll admit that all I know, I got from some website or another), the N75 valve allows boost pressure to feed back into the wastegate actuator and limit the amount of exhaust used to spool the turbo, and the electronic portion is just a way for the ECU to influence the operation of the valve to a certain extent.

That said, I still get the point that my engine was overboosting because the ECU was trying to keep up with what my foot was telling it to do.
 

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Sorry for the hijack deusexaethera.

Figured as it was related we might both learn stuff :)

One more thing with my overboost issue is that whilst I'm accelerating (fairly gently usually due to this issue) then if I back off the throttle the tiniest fraction whilst the boost is building the car will hesitate a little. This has only started happening since the other symptoms appeared so I think it must be related.
 

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If the N75 isn't a boost controller, why is it connected to the wastegate, and why does it get replaced with manual boost controllers?

As far as I understand (and I'll admit that all I know, I got from some website or another), the N75 valve allows boost pressure to feed back into the wastegate actuator and limit the amount of exhaust used to spool the turbo, and the electronic portion is just a way for the ECU to influence the operation of the valve to a certain extent.

That said, I still get the point that my engine was overboosting because the ECU was trying to keep up with what my foot was telling it to do.
The n75 is a boost controller but it does nothing without direction from the ecu. Disconnect the wire from the ecu and it's dead. A manual boost controller takes no feedback from the ecu, it's based only on pressure. The ecu doesn't just influence the n75, it gives it 100% of it's directions.
 

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Discussion Starter · #12 ·
Sorry for the hijack deusexaethera.

Figured as it was related we might both learn stuff :)

One more thing with my overboost issue is that whilst I'm accelerating (fairly gently usually due to this issue) then if I back off the throttle the tiniest fraction whilst the boost is building the car will hesitate a little. This has only started happening since the other symptoms appeared so I think it must be related.
We cool. Yeah, I've got the same impossible-to-diagnose hesitation/driveline shock/god knows what going on in my car. As best I can tell, it's kind of like arthritis for the B5; there's no one thing to blame, but a bunch of stuff that's not quite as good as it used to be.
 

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Discussion Starter · #13 ·
The n75 is a boost controller but it does nothing without direction from the ecu. Disconnect the wire from the ecu and it's dead. A manual boost controller takes no feedback from the ecu, it's based only on pressure. The ecu doesn't just influence the n75, it gives it 100% of it's directions.
So, if I disconnect the N75 harness, what happens? Can I overboost to the point that I blow my engine, or will the boost stop once I hit 0psi?
 

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Yeah, I've got the same impossible-to-diagnose hesitation/driveline shock/god knows what going on in my car.
Heh, when I first got my problem I saw posts from people saying they'd replaced loads of stuff trying to track down boost faults.

I thought to myself. 'Ah I'll soon have mine licked. Must just be that frayed vacuum line' ;)

Four months later having pulled all hose and checked multiple times for cracks etc., replaced vac lines, diverter valve and pulled turbo out for checking I'm still no better off and now putting in an order for a new PCV system and figuring I might need new N75 and airbox to turbo hose. Oh and I reckon I'm going to have to pay out for my own VAGCOM setup too. I've only been delaying it in case the car expires. Ah well. Might get a holiday next year instead :D

And I'll be pleased if any of those cure it!!!,
or I'll be gutted if none of them fix it as it'll mean I needn't have paid out for them


I disconnected my N75 harness several months ago and I'm still boosting too much, though it's not as sensitive as it was thankfully
 

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So, if I disconnect the N75 harness, what happens? Can I overboost to the point that I blow my engine, or will the boost stop once I hit 0psi?
You'll get a few psi 3-5 until boost is enough to pop the WG spring.
 

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Discussion Starter · #17 ·
You'll get a few psi 3-5 until boost is enough to pop the WG spring.
Interesting. So if I were to disconnect the N75 harness, and my boost continues to exceed the spring rate of the wastegate, then I know there's something wrong with my turbo. Then I get to spend a thousand dollars on a K04! :rolleyes:
 

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So if I were to disconnect the N75 harness, and my boost continues to exceed the spring rate of the wastegate, then I know there's something wrong with my turbo
Is this right?
Mine is disconnected and still boosting too much
 
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