Surging 9n3 GTi Under wot

**FLOSPOLOGTI** · 12-09-2015, 09:51 AM

most posts on the topic now seem years old and people proposing a number of things but no clarity on what part is causing the issue - What is the consensus on actual cause of the GTI surging post a reflash? - and what to fix it? (Ie 3rd, 4th, 5th gear surging under wide open throttle) tried a N75j - not fixed, a new divertor valve - not fixed. Very annoying...

**HAR80N** · 12-09-2015, 10:15 AM

i was getting real bad surging after my APR tune the boost would go up and down crazy was over boosting to 22psi then dropping to 15psi it was the N75 valve trying to keep up and cut boost most of it when away once i fittest my FMIC then i put a MBC inline with my N75 valve to slow the air to the N75 valve and it runs very smooth now and comes on boost faster.

**h100vw** · 12-09-2015, 11:11 AM

Op, is you car tuned? Give us all the facts for a better chance of a fix. Chucking parts on without a diagnosis can get expensive and not fix your problem.

**FLOSPOLOGTI** · 12-09-2015, 12:59 PM

APR V2 is installed if that helps with responses. What is a MBC?

**sambb** · 12-09-2015, 01:09 PM

are we talking actual boost surge as seen on the gauge or the power is fluctuating? Does it do it on a hot day but not on a cold night? Is it all the way through a third gear pull or only on the boost ramp up or only at WOT at high rpms as the boost is trailing off?
An MBC is a manual boost controller - usually a little mechanical/pneumatic device that is a straight air bleed or an airbleed coupled with a pressure relief. They can be used stand alone in place of the boost solenoid (N75), in series with your N75 to raise boost (not sure about a smoothing effect - didn't work that way for me) or in parrallel with it to give you scope to lower boost. Straight air bleeds also work very well when placed in series with the N75 leg which bleeds into the TIP as another way to limit max boost too.
I don't think an MBC is your answer though. If your tune matches your hardware then you have a problem you need to sort - an MBC will just add another variable and make it harder to find.

**h100vw** · 12-09-2015, 03:20 PM

Do you have a boost gauge or logging capabilities with vcds?

**PoloGIT** · 12-09-2015, 11:07 PM

Guys, not looking to nick the OP's thread but sambb's post prompts me to ask a question...so I had a fantastic 2 hour drive the other day with a lot of awesome twisty stuff going through gorges, up and down hills etc...anyways, it's not that easy to pass through some of this stuff, so I blast through a bend onto a long straight, and steep, uphill section with a passing lane. In the distance I see a truck struggling up the hill and I really want to get past it before it gets twisty again. So I drop from 5th to 3rd and am thinking there's not a hell of a lot of boost here...I guess I was pulling close to 6k before slipping it into 4th and we are back pulling real hard. Is this to be expected? Is that just the wee T3 running out of lungs at higher revs? Running CC phase 2, which I'm very happy with, but most of my driving is short distance round town etc, so maybe haven't experienced that type of driving on this setup much so far. I guess the midrange is where it's at and if you want top end, it's big turbo time?

**h100vw** · 13-09-2015, 06:41 AM

Originally Posted by PoloGIT

Guys, not looking to nick the OP's thread but sambb's post prompts me to ask a question...so I had a fantastic 2 hour drive the other day with a lot of awesome twisty stuff going through gorges, up and down hills etc...anyways, it's not that easy to pass through some of this stuff, so I blast through a bend onto a long straight, and steep, uphill section with a passing lane. In the distance I see a truck struggling up the hill and I really want to get past it before it gets twisty again. So I drop from 5th to 3rd and am thinking there's not a hell of a lot of boost here...I guess I was pulling close to 6k before slipping it into 4th and we are back pulling real hard. Is this to be expected? Is that just the wee T3 running out of lungs at higher revs? Running CC phase 2, which I'm very happy with, but most of my driving is short distance round town etc, so maybe haven't experienced that type of driving on this setup much so far. I guess the midrange is where it's at and if you want top end, it's big turbo time?

Boost pressure falls away as the turbo can not keep up with the volume of air going through the motor. If you were to log that particular run you would see the MAF remains constant/rising slowly. It's normal.

Not much point revving one to the red line, I rarely do that in mine even with a bigger turbo. It pulls just as well by changing earlier. Yesterday I did a couple of laps of Lakeside in 5th.

**PoloGIT** · 13-09-2015, 03:24 PM

Originally Posted by h100vw

Boost pressure falls away as the turbo can not keep up with the volume of air going through the motor. If you were to log that particular run you would see the MAF remains constant/rising slowly. It's normal.

Not much point revving one to the red line, I rarely do that in mine even with a bigger turbo. It pulls just as well by changing earlier. Yesterday I did a couple of laps of Lakeside in 5th.

Cool, thanks Gavin. Pretty much what I was thinking but thought I best check as still running the standard DV and wondered if that might be a factor.