Surging 9n3 GTi Under wot

[FLOSPOLOGTI]

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]

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]

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]

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

[sambb]

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]

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

[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?

[h100vw]

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]

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.

[noone]

Original diverter on a tuned Polo GTI? Get an aftermarket one.

[PoloGIT]

Original diverter on a tuned Polo GTI? Get an aftermarket one.

There's a shiny new 008 in the shed...just taking my time putting it on. Think I read a comment from Gavin in these pages suggesting he'd changed his opinion on the original (presumably meaning they weren't as bad as first thought), hence I've been lazy.

[louis19]

There's a shiny new 008 in the shed...just taking my time putting it on. Think I read a comment from Gavin in these pages suggesting he'd changed his opinion on the original (presumably meaning they weren't as bad as first thought), hence I've been lazy.

I'm currently running the standard DV on Phase 3. Plan to upgrade it but not in a hurry as it's not causing any issues. I don't think they are as bad as some have made out to be previously.

[sambb]

Nup there isn't anything wrong with them. Do you think porsche would have stuck these things on 911's if they were as utterly crap as people say. They might have longevity problems but just replace one when its shredded. They hardly cost anything. They actually have two big advantages over aftermarket piston style DV's too.
One is that they react far quicker. My Turbosmart piston style plumback on the supplied spring was causing compressor surge. You could hear the ch ch ch ch when you got off the throttle. That sound isn't a DV venting sound, its the compressor wheel of your turbo jerking because pressure waves are smashing into it that could have been avoided if the DV had opened quicker. Went to the next softer spring and it was better but still doing it a bit so back to stock and problem solved. Through all this there was not one iota of difference to boost curve or peak between the DV's.
Also the stock DV will unseat and float beyond about 10in/Hg. Piston style springing won't allow that till much later. If you've ever wondered why the stock DV is so much smoother to drive off boost in traffic, its because its giving a portion of your inlet air the ability to short circuit/bypass the turbo and head straight around to the engine where it probably arrives cooler and freer of restriction and is less susceptable to unintentional boosting from accidental throttle twitches. A piston style DV will for the most part force all your inlt air through the turbo all the time. So the stock DV can flow in this opposite direction too which is better for fuel economy Watch how quickly your car will go into closed loop and the fuel consumption will fall into the 5's when you are below about 12-15in/hg with a stock DV. Piston style DV's have a very hard time 'floating' at mild vaccum pressures because they still 'leak down' around the sides of the bores with sustained vacuum (eg mine has a seating seal but not a bore ring seal) The piston cannot stay up when you suck on the vac tube like a diaphramtaic DV will.
Having said all that though and going back to the original issue this guy had, a leaking DV could cause the boost fluctuations. A piston style DV's base seal/seat could be turd or a stock DV's diaphram could be leaking. Or an over reactive N249 could be pulling it open at strange times. My N249 was doing that - you could hear/see the DV release at bizarre times so I deleted it. There's a good youtube video of a guy with a mk4 golf who rigged a LED up to the N249 and you should see how often it was acting on the DV.

[Angus]

I don't have a tune but an aftermarket dv. And the only time I can feel surge is first thing in the morning when I'm taking off a little hard.

[DeanB]

sammbb, a question about DVs. The Forge, Turbosmart, etc are all piston style, the APR is a diaphragm. I've forgotten but what is the original, is that diaphragm as well? It's been a long time since I changed mine, I've forgotten a lot about the negatives of the piston type until I read your reply