N75 Woes / Boost Surging/Spiking (AEB/AJL)

[Matt-Matt]

As some of you may or may not know, I have been having issues with my car "Surging" or overboosting and spiking back for some time. I was always careful and managed to avoid driving it hard until I could figure out what was going on.

Basically what would happen is that the turbo would overboost to ~25PSI and then surge back to 5-10PSI and then back to 25PSI and sometimes depending on the throttle applied it would hold over 20 for extended periods, it'd also retard timing (from my logs/research) at times and dump fuel to compensate.. Not a good situation to be in.

Anyway, I initially thought that it was the wastegate on my k03-005 as it wasn't an issue before my First turbo swap, as when I got the car it was pretty much stuffed.. (Long story short, please replace your thermostat asap if the car is not getting to temp). I always just assumed and never did any research past that which was a big mistake.

After it died (assumably from over-boosting) my train of thought was still on the wastegate being the culprit, so I managed to get a k03-029 off the forums as well as some other bits and pieces to go with (ECU, Forge DV, Silicone TIP). I knew that the wastegate on this was set okay and meant to run at around 16PSI as thats what the previous owner advised me he was getting.

Well after a long wait, I was bummed to find that it was still experiencing the same behaviour (with the old chipped ECU) and thought that it might be related to the tune as it appeared to be slightly less agressive, so put in the QMS Stage 1 ECU and same behaviour.

Came back to research, and tested the wastegate on my now "new" k03-029 to find that it was set to 5-6PSI by unplugging the N75.

At this point I was either looking at a MAF issue, the N75 or a boost leak. MAF was ruled out as I'd cleaned that fairly recently and also ran logs and the values it was reporting over a 45 minute drive were fine. So got to a mates place tonight and tried his N75 from his 1.8t and the same behaviour! :swear:

Now as I'd researched a few nights ago, people with similar issues to me (from what they described) said that there were three solutions to their issues, either a bad N75, a MBC to limit boost and possibly an "upgraded" N75, usually the "N75J" being the main one used. As many run MBC's in with the N75 disconnected. I.E MBC connected to vaccum to control boost, with the N75 plugged in to the loom I tried this and my issue was resolved, slowly working my way up to 15PSI with no issues at all. :excited: Apart from the jerkiness associated with this setup and really early spool, but I am aware of it for now and plan to get a more elegant solution for the long run potentially.

TL;DR - My N75 (or another known working one) could not hold the boost of the chip tune I have it appears, using a MBC to limit boost fixed this for me.

Now my question to OzAudi, what N75 valve/MBC/EBC do you use and why? Looking at options that feel closer to a stock kind of feel/sporty kind of agressiveness as opposed to "bam, boost" that I get with the Turbosmart MBC I have right now.

 

[Falco]

Im not that familiar with the 1.8t platform but its pretty similar to the 2.7t which I am. The N75 is about as good as you get with these cars for controlling boost. The duty cycle and how much it bleeds off to control your boost pressure is determined by how its tuned.
You mention a stage 1 off the shelf tune. This wont work with upgraded turbo and a upgraded downpipe. Its too free flowing for the dutycycle programmed for the N75.
I suspect you are seeing your boost spikes around the boost threshold where it will be holding the wastegate closed when it should be opening it. This then causing it to go into limp until the overboost condition clears. Time for a custom tune for your setup
Stay clear of a MBC. People use these as a bandaid to try to fix a bad tune(tune not right for hardware). It just further complicates the problem.

 

[IchBautAuto]

Im not that familiar with the 1.8t platform but its pretty similar to the 2.7t which I am. The N75 is about as good as you get with these cars for controlling boost. The duty cycle and how much it bleeds off to control your boost pressure is determined by how its tuned.
You mention a stage 1 off the shelf tune. This wont work with upgraded turbo and a upgraded downpipe. Its too free flowing for the dutycycle programmed for the N75.
I suspect you are seeing your boost spikes around the boost threshold where it will be holding the wastegate closed when it should be opening it. This then causing it to go into limp until the overboost condition clears. Time for a custom tune for your setup
Stay clear of a MBC. People use these as a bandaid to try to fix a bad tune(tune not right for hardware). It just further complicates the problem.

Yep, all of that. Is the Forge OK? Some experience issues with piston DV's. 5 - 6 psi on the wastegate sounds right for a stock turbo. You didn't mention any results from a Boost and Vacuum test. Lots have had problems with the small hoses that connect the N75.

 

[Matt-Matt]

Im not that familiar with the 1.8t platform but its pretty similar to the 2.7t which I am. The N75 is about as good as you get with these cars for controlling boost. The duty cycle and how much it bleeds off to control your boost pressure is determined by how its tuned.
You mention a stage 1 off the shelf tune. This wont work with upgraded turbo and a upgraded downpipe. Its too free flowing for the dutycycle programmed for the N75.
I suspect you are seeing your boost spikes around the boost threshold where it will be holding the wastegate closed when it should be opening it. This then causing it to go into limp until the overboost condition clears. Time for a custom tune for your setup
Stay clear of a MBC. People use these as a bandaid to try to fix a bad tune(tune not right for hardware). It just further complicates the problem.

This was happening with a different ECU with a 3' Catless Exhaust (how I got the car), changing from the stock DV to a 710n did not change it and adding the silicone pipes didn't change it either. It was behaving marginally different to the k03-005 I had, as opposed to the k03-029 i have now.

Surely silicone hoses wouldn't change the behavior that much? Keeping in mind, I purchased these off a car that had the same turbo and tune that had no issues.

Yep, all of that. Is the Forge OK? Some experience issues with piston DV's. 5 - 6 psi on the wastegate sounds right for a stock turbo. You didn't mention any results from a Boost and Vacuum test. Lots have had problems with the small hoses that connect the N75.

The forge is fine, I plan to get a service kit for it as I got it used as mentioned. But will combine it when I order parts next.

I didn't do a boost or vaccum test, as I do not have the gear for it. That is where I was going to look next for troubleshooting and I am about to actually run a parallel setup today and try it out.

This way I get the benefits of the duty cycle of the N75, with the MBC controlling the top end "spike" and limiting it to around 16PSI. I will obviously have to do some logging with VCDS to ensure that I am not running too rich or too lean.

For reference; this is what I plan to do..

https://s4wiki.com/wiki/Manual_boost_controller

 

[IchBautAuto]

Thats a solution but as Falco said, if you need that, then you have an issue somewhere. I'm from the school that says I'd need to find out what the problem is. Unfortunately there is every chance that this unfound issue may impact on most engine tuning plans you have for the future. Understand the desire to fix the problem and get out and drive but this is one of the quirks of these cars. Been down this road many years ago when electronics was in it's infancy. Resistor pots everywhere to sort things out. Yep, I got it running Ok but I never solved the underlying problem.

 

[Matt-Matt]

Thats a solution but as Falco said, if you need that, then you have an issue somewhere. I'm from the school that says I'd need to find out what the problem is. Unfortunately there is every chance that this unfound issue may impact on most engine tuning plans you have for the future. Understand the desire to fix the problem and get out and drive but this is one of the quirks of these cars. Been down this road many years ago when electronics was in it's infancy. Resistor pots everywhere to sort things out. Yep, I got it running Ok but I never solved the underlying problem.

Of course there's a better way to solve the issue. But everything that I have researched leads to people running different N75 valves or a MBC or MBC in parallel with the stock N75. I know that at least for me that the MBC will at least not overspool my turbo and not damage it? Is it ideal, probably not. But it's what I have right now.

Some have speculated that the N75 just cannot handle the flow of the added boost from some chip..tunes, i have no answer as to why this would be as wouldn't it be on every car?

The only thing left that I have not tested is for boost leaks.. Which I honestly don't think is the issue at this point.

 

[quattro5v]

Now my question to OzAudi, what N75 valve/MBC/EBC do you use and why? Looking at options that feel closer to a stock kind of feel/sporty kind of agressiveness as opposed to "bam, boost" that I get with the Turbosmart MBC I have right now.

Stupid question: Have you gone back to Ed/the tuner with these issues? How long has the motor been overboosting? Since the tune, well after, not long after, etc etc?

Surge in boost is normally attributed to a vacuum leak of sorts. N80 valve which sits directly underneath the intake manifold on the B6 was synonymous with failure and would also send the ECU to overload requesting additional PSI to compensate for losses it sees.

Not an easy work around until you can confirm 100% that there are a) zero vacuum leaks, and b) zero pressure loss in the intake plumbing pre and post SMIC.

 

[Falco]

Some have speculated that the N75 just cannot handle the flow of the added boost from some chip..tunes, i have no answer as to why this would be as wouldn't it be on every car?

If you are having problems with the n75 and its causing overboost. It means it is bleeding off too much charge pressure which would open the waste gate. It means it is not programmed for the current setup. Off the shelf stage 1 tunes do not work with catless down pipes because The turbo has no back pressure preventing it from spooling as quickly as possible. You need to reduce how much charge pressure is bled off to account for the turbos increased ability to produce more boost and earlier. You want to do this with a manual controller. The N75 will do this the second the code in the ecu is changed to allow it to. When the N75 is unpowered. The charge pressure acts directly on the waste gate spring. Once charge pressure is exceeds the wastegate spring pressure, It cracks the wastegate and limits boost. You can see from this, It is your tune that is the issue.
If you want a cheap fix, buy a waste gate spring equal to the boost you want to run and plug your boost reference line on the turbo directly to the waste gate supply nipple, you wont have a limp mode again but you won't chase your tail with a manual boost controller which is clearly not working for you.

 

[rockersteady]

couple of things Matt. before 1st turbo swap you were running a chipped ecu? and that ran how much boost?
it seems you have 2 issues right now (maybe) 1st one is the maybe. your n75 is messed up (or someone messed with it) BUT if it ran ist turbo on first tune then it is likely ok. if first turbo wasnt chipped , what boost did it run?

Second, as falco is barking up the right tree with dump pipe idea but main culprit is the wastegate is too tight for "off the shelf" tune. factory wastegate for b5/b6 is about 3-4 psi. by setting to 5 psi or more, the file should be custom. (if it was run with a stock cat it might be ok)

can you email me with the numbers on the ecu (version number/date) as this ecu was written for somebody else right?

Oh, and the N75 has to be original part number as came with car. the n75's on the B5's are really old now and im starting to see quite a few failures. (and boost control issues)

 

[Matt-Matt]

couple of things Matt. before 1st turbo swap you were running a chipped ecu? and that ran how much boost?
it seems you have 2 issues right now (maybe) 1st one is the maybe. your n75 is messed up (or someone messed with it) BUT if it ran ist turbo on first tune then it is likely ok. if first turbo wasnt chipped , what boost did it run?

Second, as falco is barking up the right tree with dump pipe idea but main culprit is the wastegate is too tight for "off the shelf" tune. factory wastegate for b5/b6 is about 3-4 psi. by setting to 5 psi or more, the file should be custom. (if it was run with a stock cat it might be ok)

can you email me with the numbers on the ecu (version number/date) as this ecu was written for somebody else right?

Oh, and the N75 has to be original part number as came with car. the n75's on the B5's are really old now and im starting to see quite a few failures. (and boost control issues)

Okay so when I got the car it came with a k03-005 (that was stuffed), a Powerchip Group Tune and a "3 inch exhuast" which appears to be catless, I do need to get a race cat for it at some point.

The surging started sometime after changing to a brand new k03-005 and while I had that I thought that it was the wastegate, but it seems like it is not?

I'll email you Ed with more information and the ECU numbers, etc.

N75 is a N75C, I'm 99% certain that this is the stock one for an AJL from '99.

It was overboosting before changing to Eds tune, so it's an underlying issue somewhere I would imagine.

So I'll go back to the beginning.

Purchased the car, was boosting fine (or so i thought) and turns out it had a bad themostat.
The car came with a chip tune from "Powerchip Group" which was always guessable, it was definetly chipped however and a 3' Turbo back, catless exhaust which was boosting fine. Albeit running cold.

Changed that and it started burning oil/blowing smoke.

Changed to a new K03-005 and changed the gaskets then.

The car was fine for a short while, and then had the surging then and only then.

Did some investigation and thought that it was the wastegate, do wish that I'd checked it by disconnecting the N75.. I didn't.

The rear exhaust on that turbo blew recently, so got a k03-029 off someone on here, with the chip tune from Ed, the Forge Diverter/Splitter and silicone intake piping, admittedly I am yet to install some of the silicone intake piping, but have the TIP installed which is the hard/important one.

A MBC with the N75 only electronically plugged in allows the car to boost normally (albeit too early) and jerkiness, having the MBC in parallel (limited testing) allows me to get the charatcteristics of the N75 while having the MBC limit the boost to a nice 15-16PSI.

At this point that is all that I have, I have messaged Ed to find out what the tune is actually for and moving forward I think my next option is to look at different N75's potentially as well as a boost leak test maybe.. Kind of hoping that the tune calls for another N75.. But it won't be as it was happening on the other tune, which was fine at one point.

To add to all of this I have stock coils and using NGK BKR7E's gapped at stock (0.28 or 0.30 I believe), was happening on brand new plugs and have changed them about 3 times over 20,000k's.

 

[rockersteady]

So if your n75 is fine (as it seemed to be from beginning) Id be looking at the hose from turbo to n75 or n75 to wastegate is leaking off air/ or is kinked not letting the wastegate open/ allow n75 to modulate it correctly (it needs boost to open it). it then overboosts and and the extra boost now forces the wastegate to open and it starts all over again. this hose might have been messed with when you put the replacement turbo on.

 

[Falco]

So if your n75 is fine (as it seemed to be from beginning) Id be looking at the hose from turbo to n75 or n75 to wastegate is leaking off air/ or is kinked not letting the wastegate open/ allow n75 to modulate it correctly (it needs boost to open it). it then overboosts and and the extra boost now forces the wastegate to open and it starts all over again. this hose might have been messed with when you put the replacement turbo on.

Disconnecting the power to the n75 limits boost to 5psi because the n75 stops bleeding the boost reference off to the supply of the wastegate from the the n75. Because its limiting it to 5psi tells me that the lines to and from the n75 are fine.

 

[Matt-Matt]

Disconnecting the power to the n75 limits boost to 5psi because the n75 stops bleeding the boost reference off to the supply of the wastegate from the the n75. Because its limiting it to 5psi tells me that the lines to and from the n75 are fine.

What is it meant to be? I was getting a pretty solid 5PSI and got towards 6psi as I moved to the end of 5k+ revs. I'd imagine that this is within margin or error/discrepancy of my boost gauge however.

So if your n75 is fine (as it seemed to be from beginning) Id be looking at the hose from turbo to n75 or n75 to wastegate is leaking off air/ or is kinked not letting the wastegate open/ allow n75 to modulate it correctly (it needs boost to open it). it then overboosts and and the extra boost now forces the wastegate to open and it starts all over again. this hose might have been messed with when you put the replacement turbo on.

Ideally I should try another N75, or even just bite the bullet and order a new one? As for hoses, I know that they're a bit funky with their sizing. I will pull of all the hoses later this week and inspect and see what I can find. I do have around half a meter of new rubber hose that I can use to replace as well as some silicone 5mm hose that I can re-use while I've got the MBC out.

I'm kind of suspicious of the N75 still, considering that effectively disabling it resolves the issue? I know that it might not be that simple, but would it be worth ordering a genuine/brand name part?

I've also noticed that ECS (again, not sure how accurate this is) states that 06A906283E (N75E) fits a B5 A4 1.8t, listing that the N75C was an older part?

I'm honestly getting more and more confused, think I will start out with Ed's suggestion firsts by checking all of the hoses visually and using the N75 that I have right now. I'll also install the silicone intercooler hoses that I have and potentially look at cleaning the intercooler? Have read about people doing that as well.. It does not explain why a MBC "fixes" the issue however.

I think the next logical step then is to try and find a "N75E" or even just another valve and see if it changes the behaviour (before buying).. If I had to buy I'd grab a N75E as per the ECS websites parts.. I could also test the N75C that I have as well (I need to buy a multi-meter really desperately)

 

[Falco]

The wastegate has a spring that keeps it closed. The line of boost supply on the turbo acts on the waste gate spring. Once boost in the turbo exceeds the spring pressure, the wastegate opens. The n75 sits inbetween this and is connected to a vacuum supply. The n75 allows higher boost by apply vacuum to the wastegate line to bleed of the boost supply going to it. Effectively keeping it below wastegate spring pressure which keeps the wastegate closed. Because the n75 is electric, you can program how much it bleeds off and how long it keeps the wastegates closed or open. The n75 works on pwm to allow bleeding of boost to wastegate. Disconnect power to it(n75) and the system runs as if the n75 is not there. You are going into a low boost mode of 5psi. Means that as soon as the turbo reaches this pressure the waste gate starts opening and opens as much as is needed to keep pressure at 5psi(5psi spring). In the higher rpm you could reach 6 psi because the wastegate is fully open and can no longer limit boost building(wastegate too small).

 

[Matt-Matt]

The wastegate has a spring that keeps it closed. The line of boost supply on the turbo acts on the waste gate spring. Once boost in the turbo exceeds the spring pressure, the wastegate opens. The n75 sits inbetween this and is connected to a vacuum supply. The n75 allows higher boost by apply vacuum to the wastegate line to bleed of the boost supply going to it. Effectively keeping it below wastegate spring pressure which keeps the wastegate closed. Because the n75 is electric, you can program how much it bleeds off and how long it keeps the wastegates closed or open. The n75 works on pwm to allow bleeding of boost to wastegate. Disconnect power to it(n75) and the system runs as if the n75 is not there. You are going into a low boost mode of 5psi. Means that as soon as the turbo reaches this pressure the waste gate starts opening and opens as much as is needed to keep pressure at 5psi(5psi spring). In the higher rpm you could reach 6 psi because the wastegate is fully open and can no longer limit boost building(wastegate too small).

That makes a lot of sense, thankyou for explaining that in depth. The MBC is so jerky because it's like a computer ironically. Either on or off and struggles with partial throttle as it does not handle being halfway between on or off, unlike the N75 which can do PWM and modulate/manipulate the wastegate into having a smooth operation.

 

[kermac]

I think the next logical step then is to try and find a "N75E" or even just another valve and see if it changes the behaviour (before buying).. If I had to buy I'd grab a N75E as per the ECS websites parts.. I could also test the N75C that I have as well (I need to buy a multi-meter really desperately)

Just a suggestion about sourcing parts from the US. ecs and other US suppliers have a fantastic range of parts for our older Audis, but in recent years they all seem to have increased their shipping prices to the point where they are totally prohibitive. They generally have a minimum shipping charge of at least $70-US which may be alright if you can throw a few items together and spread the shipping cost that way. But if you want to buy a single washer or something cheap which weighs almost nothing - it's still $70 for shipping.

It can pay to look at other sources like eBay (you've just got to make sure you're buying genuine if that's important) or even (gulp!) your local stealer. I don't buy too much from the big US suppliers any more. If anyone has a way around this, I'd love to hear it.

Sorry - this is not meant to be a hijack!

 

[IchBautAuto]

Just a suggestion about sourcing parts from the US. ecs and other US suppliers have a fantastic range of parts for our older Audis, but in recent years they all seem to have increased their shipping prices to the point where they are totally prohibitive. They generally have a minimum shipping charge of at least $70-US which may be alright if you can throw a few items together and spread the shipping cost that way. But if you want to buy a single washer or something cheap which weighs almost nothing - it's still $70 for shipping.

It can pay to look at other sources like eBay (you've just got to make sure you're buying genuine if that's important) or even (gulp!) your local stealer. I don't buy too much from the big US suppliers any more. If anyone has a way around this, I'd love to hear it.

Sorry - this is not meant to be a hijack!

Spot on. Shipping has got out of control. I posted a few years ago about ab item that weighed 2 ounces (US Terms) and upped the shipping some $40. You can usually negotiate better rates from smaller dealers. I was an ECS fan about 4 years ago but they have s^&$ in their own bed with shipping of late.Plenty of examples of buying local being the same or less than the landed price from the US of late.

 

[Korny]

Spot on. Shipping has got out of control. I posted a few years ago about ab item that weighed 2 ounces (US Terms) and upped the shipping some $40. You can usually negotiate better rates from smaller dealers. I was an ECS fan about 4 years ago but they have s^&$ in their own bed with shipping of late.Plenty of examples of buying local being the same or less than the landed price from the US of late.

I just ordered their (ECS) catch can and asked for their best price and negotiated a lower freight price. It still might be higher than some would pay but they are willing to “talk turkey”.


AUDI S3 8V SEDAN (MY15)
- Misano Red w/Black Optics + Perf Pack + Pano Roof + US grille
- ABT ECU tune & DP | VWR CAI | TMD | HiFlo bend | Intake hose | GFB DV+
- H&R Sports + spacers + RSB.
- ECS Tuning 2 piece rotors + EBC Red Stuff pads (fr & rear)
- ECS Tuning oil catch can kit
- 034 Motorsport - rear subframe mounts / dog bone insert
- Cobra resonated catback s/s exhaust
- Opticoat Pro treatment exterior & interior
- Wagner FMIC + Spulen charge pipes
- P3 Cars gauge + Dragy GPS

 

[IchBautAuto]

I just ordered their (ECS) catch can and asked for their best price and negotiated a lower freight price. It still might be higher than some would pay but they are willing to “talk turkey”.

True, but then again I'm starting to wonder about the prospect of doing business with someone who is prepared to overcharge for shipping and then cut back if you moan about it. ECS use an algorithm for shipping that is truly flawed. I've had boxes that are half full and packed with inflato bags. Happy that shipping is both volumetric and mass related but when you cannot reconcile your purchases vs actual shipping costs it gets a bit flakey. "FREE" shipping in the US is paid for by someone, the customer? It would be a reasonable gesture if OS customers were given a shipping discount to reflect the "FREE" shipping costs that have already been paid. I've largely moved on to smaller firms who just charge you the actual cost of shipping, which can be a pleasant surprise.

I've even found that local suppliers can be competitive or better with door to door costs. Buying from UK / Europe can also be OK if you get VAT / Mwst removed.

 

[kermac]

I buy most of my parts off eBay these days and a lot of them have free shipping yet still arrive within a week. Don't know how they do it.

Of course, there are times when oem is essential and if it's not available on eBay, like Tony I would look at smaller suppliers or even the local dealer. He can often get you a part from Singapore, no shipping charge. And it only takes three weeks!!

It never crossed my mind that ecs might be negotiable on shipping charges. They must have lost a lot of overseas business in the last few years