S4 B5 boost issue

[RichLV]

Hi guys.

I thought I might ask about this here, since I've seen many of you owning a B5 S4 or RS4. So one of my best mates has a chipped S4 and it has come to that we swapped cars for the moment for various reasons.

Anyway, he's fighting a lingering boost issue on his car and we've been going at it for more than a month. Before that it was all ok. We went over all vacuum related things on this car (Bypasses, N75, boost hoses, vacuum hoses, even did a pressure test and nothing boost leak related), and it yielded no results. The only thing left now we have are the turbo wastegates. We have a feeling that they are sticking randomly for some reason. I was driving it back home today and filmed what the boost gauge is telling me.

https://www.youtube.com/watch?v=RONjw2iGSjw

So basically, when it goes, it goes like stink. Literally, I've had my feet cold when I floored this thing after my stock UrS4. The problem is, it doesn't go all the time. As you can see on the video, in the beginning, when I'm flooring it, it is very hesitant to build boost and the gauge needle is fluctuating like crazy. Just before it jumps on building boost it jolts and starts to stutter a bit everytime the needle fluctuates, but it doesn't feel like a misfire, besides misfires usually occur everytime you go for boost, had enough experience with that on mine. At the end of the video you can see that it goes like it should. Boost gets built smoothly and fairly fast.

Other problem is that the car needs a remap. It was chipped for stock turbos, but those died some time ago. Turbo shop guys decided to build something beefier, so at times he gets a Boost Pressure Exceeded code, because these fans are a bit more capable. But I guess that wouldn't explain the sporadic hesitation and fluctuation.

Would really appreciate some advice, as we've been going at it for quite some time. Should we pull the engine to check the wastegates?

[newsh]

S4 B5 boost issue

Very unlikely both wastegates would fail in the same way. If one WG was stuck open the ECU would overspin the other turbo trying to make requested boost.
It looks to me like the N75 is not opening properly so you're getting spring pressure most of the time then it works and gives you boost.
Can you log with vagcom?
You need to log RPM, load, requested boost, actual boost, TPS.
You need to find out if the ECU is stopping the boost or it's a mechanical issue with a boost control component.

When you checked vac pipes, did you replace the one way valve in the breather system?

[RichLV]

Found the culprit. It's the boost cut. When going as it should, the freaking thing boosts to 2+ bar so fast, I can't even get the foot off the gas quickly enough to not incur an overboost cutout. Then it goes into sort of a limp mode only allowing 0.5-0.8 bar for a while. After some calm driving it eventually allows to build boost again. I was driving back home yesterday and was manually cutting the boost off at 1.2-1.4 bar to switch gear and it was building it fast and smooth every time.

So, we need a proper remap. But these turbos are just savage... I'm not even sure what are their power capabilities, if they boost 2+ effortlessly, worst part is that we don't have compressor map for them... I also advised him to stay off boost, exceeding 1.2, because I presume that he's going lean in the top end.

Another project on the way then. Thanks for the reply newsh!

[newsh]

For the time being maybe disconnect the N75 and fit a boost controller so you can set max boost to about 1.5bar

[kymi]

Please, forget the mbc. It's not rocket science for a proper tuner to tune the boost control on these ecus.

[RichLV]

We want to properly prepare the car for it and then just get it to Tallinn, for PRJ to work on it.