Had this problem for a while now but can't seem to trace it. It's driving me round the bend, hope someone can help.

When I am in traffic or pulling away slowly but not really putting my foot down more holding the revs between 2k & 3K, something really strange happens. The Turbo builds boost rapidly (even thought i'm holding revs) and then dumps it just as quick losing all power/forward momentum, it's almost like stamping on the brakes as hard as possible for a split second. It's caught me out a couple of times and my head nearly hits the steering wheel hence the title. The RS has been chipped BTW.

Des

I've heard of that before, but can't quite remember where. I'll let you know if I remember.

Dodgy map? Leak somewhere?

I've tried lots of different maps but no change. :(

Just to remind , this is not the lounge.

Sticky wastegate? N75 playing up?

What causes boost to be dumped normally?

Release of accelerator.

Hitting boost limiter.

Anything else?

I'm not sure really how the N75 works...something about the control of boost. So quite possibly could be something to do with this.

No error codes? Have you also tried to cap a trace using VAGCOM?

I some loose boost when I hit a pot hole or bump in the road and get the same symptoms but I reckon it's because the bump is causing me to lift me foot off the accelerator....not 100% sure though.

What i can't understand is why or even how it makes so much boost at such low revs. The only thing I can come up with is for some reason it's to do with excess fuel. For the turbo to spin up it needs exhaust gases but at low revs and half throttle it's not producing it. :confused:

If a hose was off would the turbo/wastgate/DV open at all. :confused:

What causes boost to be dumped normally?

Release of accelerator.

Hitting boost limiter.

Anything else?

I'm not sure really how the N75 works...something about the control of boost. So quite possibly could be something to do with this.

No error codes? Have you also tried to cap a trace using VAGCOM?

I some loose boost when I hit a pot hole or bump in the road and get the same symptoms but I reckon it's because the bump is causing me to lift me foot off the accelerator....not 100% sure though.

Ive had that happen to me (loose power when hitting a large bump in the road). Its happened about 5 times now, thought it was me being para :confused:

Had this problem for a while now but can't seem to trace it. It's driving me round the bend' date=' hope someone can help.

When I am in traffic or pulling away slowly but not really putting my foot down more holding the revs between 2k & 3K, something really strange happens. The Turbo builds boost rapidly (even thought i'm holding revs) and then dumps it just as quick losing all power/forward momentum, it's almost like stamping on the brakes as hard as possible for a split second. It's caught me out a couple of times and my head nearly hits the steering wheel hence the title. The RS has been chipped BTW.

Des[/quote']

Mines standard and has been doing this since I got it. Dealer doesn't know what it is, so i've kinda given up and am just living with it. I've had the dodgy intercooler pipe fix, and had it checked for boost leaks because I permanently have the infamous 17705 code. No help really, but just thought i'd let you know you are definately not alone on this one. Oh and if you fix it, please let me know how... :confused:

Is the MAF ever playing any part in the workings of the turbo? Usually I think a dodgy MAF causes the opposite of these symptoms, cutting out - not unusual boosting up and loss.

[sees some straws in the corner of the room... Clutches them]

Mines standard and has been doing this since I got it. Dealer doesn't know what it is, so i've kinda given up and am just living with it. I've had the dodgy intercooler pipe fix, and had it checked for boost leaks because I permanently have the infamous 17705 code. No help really, but just thought i'd let you know you are definately not alone on this one. Oh and if you fix it, please let me know how... :confused:

That's interesting as yours is standard.

I've learnt to drive through it if that makes sense. No fault codes at the mo apart from the decat light. How the hell the turbo spins up with out me putting my foot down is beyond me. Yours isn't the only standard one with this fault so I don't think it's anything to do with the chip. :confused:

Thing is with mine, i dont think the turbo's is actually spinning up much. I was thinking about this the other day and it feels almost as if there's boost/inlet pressure thats built up while i've been stopped and it pours in when i set off again if that makes sense?? Then it gets dumped as boost does start to build because its now too high? Probably not the cause but thats what it feels like.

EDIT: I also get limp mode about once every other week and i've heard that this can be caused by exceeding max boost allowed by the ECU

How the hell the turbo spins up with out me putting my foot down is beyond me. :confused:

That is consistent with a sticky wastegate. AIUI, the turbo is capable of providing a lot more boost than is actually required (even at low engine revs?) but the wastegate normally opens at relatively low boost pressure. The ECU increases the boost as required by "bleeding off" some of the control pressure via the N75 valve.

Now, imagine the wastegate is a bit sticky. It stays shut at low revs, allowing the turbo to spin up, until the boost pressure "pops" it open. The turbo stalls and you lose all the boost pressure for a moment, until the ECU takes control and starts modulating the N75 to bring the boost back under control.

It's a plausible theory, even if it's completely wrong

What i can't understand is why or even how it makes so much boost at such low revs. The only thing I can come up with is for some reason it's to do with excess fuel. For the turbo to spin up it needs exhaust gases but at low revs and half throttle it's not producing it. :confused:

Was thinking about this on the way nito work this morning........your thoughts were something to do with the fuelling...perhaps your getting some kind of surge of fuel?

Exploring the possibilities of what could cause a surge....

....fuel filter needs changing...blocked/unblocked kind of thing....

...dodgy fuel sender/sensor..

...fuel pump on it's way out...

...dodgy FPR....

...I know you said you didn't think it was the map but perhaps under certain conditions it's momentarily fooled into thinking more fuel is required?

Just tossing around ideas for those more in the know to work off from.

I know I know nothing about cars....but from a laymans point of view there is a relative constant, what about the throttle rheostat? If the rheostat has a dead spot/ciruit then would that not cut the throttle and be noticed when the throttle is being held at that point rather than the usual movement of driving?

Just an idea...

Problem solved, it's the N75 opening the waste gate early. New one fitted.

Thank you everyone for your help.

Problem solved, it's the N75 opening the waste gate early. New one fitted.

How was it eventually diagnosed and by whom, Des? I ask because my car is out of warranty in 2 days time and I can't afford the 'change a component and let's see if it fixes it' method of fault diagnosis if used by a dealer. :(

Process of elimination i'm afraid m8 and by yours truly. Mind you it does help if you have a black vRS as a parts donor car, you won't tell Dave will you.

I would have thought that between myself, Jon and Paul we've just about broken everything on the Octy so should be able to help.

Well at least it's a lot lighter now.