Boost problem on part throttle...

[SideshowBob]

Hi all

My vRS has been having this boost issue for a while now, which I keep forgetting to sort because it's not a major prob - it just shows up under certain circumstances. But I'd like to get it sorted and start tracking down why it's doing it.

It idles almost perfectly, hardly any fluctuation in revs at all, and has probably never had a throttle body clean in 95k (I have a new gasket ready and will be doing this on saturday). So no issue there.

On heavy or full throttle again it's fine, it pulls smooth and strong in any gear.

However if I use say 50% throttle, I can feel the boost level fluctuating. It's nothing severe but I don't think it should be doing this.

It was remapped back in January and I noticed the uneveness on the drive home, but I think it's slightly more noticeable now than it was back then. I don't think it's a boost leak, since it's absolutely fine on full throttle, so I'm suspecting it's the N75 valve? I guess at 95k it would be well past it's best anyway, and they seem to be known for causing probs?

The last time it was checked for fault codes was in May, and there was nothing relating to engine or drivetrain, just a fault because the aircon needed regassing.

Any ideas or suggestions wilkommen. :yes: :yes: :yes:

[pauldazzle]

Clean the throttle body, & report back.

[VRS Mike]

n75 faulty?

[SideshowBob]

Ok, have cleaned throttle body today (it was actually really clean already, as was the inlet hose) but it's made no difference. I hooked it up to freebie vag-com and checked for engine fault codes but there weren't any.

Anyone have any ideas what I can try from here?

[Raf_]

What dump valve you got? rk696 had a similer problem but he says it's sorted since getting a new DV, we both got THIS ONE Well impressed with it, seems well made and over £30 cheaper than a forge one

[pauldazzle]

Ok, have cleaned throttle body today (it was actually really clean already, as was the inlet hose) but it's made no difference. I hooked it up to freebie vag-com and checked for engine fault codes but there weren't any.

Anyone have any ideas what I can try from here?

Did you remove the throttle body? because it doesn't get gunked up on the pipe side of the butterfly flap. It's the manifold side of the butterfly flap that needs to be checked & cleaned.

[SideshowBob]

Yep I removed the TB from the manifold, and just as you say that's where all the gunk was, although not much of it.

I have a Forge 007p, I believe with a yellow spring.

[pauldazzle]

Could be the remap. I had a similar problem on a generic map on a Golf Mkiv. Full throttle was smooth, but partial throttle was really jerky, & unpredictable at cruising speed. I tried loads of things to resolve it including 007 green & yellow springs, N249 bypass, new throttle pedal, throttle body clean etc. Gave up in the end & had Revo stage 1 put in, & it was much better after that. If you still have the stock DV try sticking that back in as I spoke to Revo tech support once about this, & they weren't that keen on aftermarket DV's, & advised using the stock DV, which I gotta say seemed to be smoother than the 007p when I switched back.

[SideshowBob]

Is there any way I can check for obvious problems with freebie vag-com, before I start swapping parts out? The forge was fitted at the same time as the remap so I'm not sure what hoses connected to what, with the original DV. To be fair also, it's a performance torque map and they are reputed for their smoothness.

[pauldazzle]

It's always a good idea to check with vagcom before you start throwing parts at a problem. The forge DV plumbs in the same as the stock DV, however you can actually plumb the Forge in on it's side too. I've tried both ways & there was a slight improvement of requested boost against actual boost when logged with vagcom with it sideways!

[jmf]

Ah the age-old problem. Most of the people with 1.8Ts I know have it, to a larger or smaller degree. Unfortunately I haven't heard of a "cure" yet, so I guess it all depends. Let us know if you solve it

[Hofmiester]

Yep, mine is the same although much reduced now.

When I bought it, the Bailey DV30 was worn and so was leaking vacume pressure and not opening properly, I didnt know this at first so changed all my vac lines to silicone...that improved it as more vacume from less leaks helped the worn DV to open more. Then another Brisky member lent me a standard DV to test with and the problem was totally gone.......I then "upgraded" too the 007P with the Yellow spring and it is much better than the DV30 but not as smooth as the OE DV.

To do it again I would have just bought 2x OE DV's and kept one in the boot should the other break.

Ron

[chicken_eyebrow]

Interesting point there Hofmeister, I've had 3 DVs in the past 6 months and out of the three, the OEM DV is the only one that was smooth, with the other two (Forge Split-R and Forge 007P) I often get a slight judder if I snap the throttle closed at high boost pressures.

Edited 10 August, 2010 by chicken_eyebrow

[SideshowBob]

So surely a brand new 007p with a yeller spring, should be fine?

I'm trying to decide whether it's a case of 'they all do that sir' or if there's a problem somewhere? There's no jerkiness as such, just uneven boost on medium throttle openings.

Is it possible to log requested vs actual boost without the full vag-com?

[Raf_]

I've got a brakecomp DV on mine and so far so good, came with 2 springs and i fitted the stiffer one even tho my car is standard. It's smooth and seems to work fine, saved me over £30 on a forge one.

[Bogwoppit]

If you running just a stage 1 remap shouldn't you be using a green spring?

It might be worth emailing Forge and see what they recommend.

Edited 10 August, 2010 by Bogwoppit

[SideshowBob]

According to forge:

Valve Spring color coding

Green - 5-15 PSI

Yellow - 15-23 PSI

Blue - 23-30 PSI

Red - 30 + PSI

My stage 1 map is running 18psi peak, and 14.5-15psi held (according to Will at P-Torque) so I would guess that yellow is ok? However could the spring be too strong at lower boost levels (ie part throttle) and be causing the lumpiness?

[Bodge]

I'm running 19psi peak, and have the Forge FMCL007P, with what I assume to be a green spring.

It does dump very quickly if you even slightly back off the throttle while accelating. I've learnt to be gentle on the throttle now if backing off, but I guess it could benefit from a higher spring rate. I might give the yellow spring a bash....

Anyone know the actual spring rates (N/mm), rather than just the colour codes?

[chicken_eyebrow]

My stage 1 map is running 18psi peak, and 14.5-15psi held (according to Will at P-Torque) so I would guess that yellow is ok? However could the spring be too strong at lower boost levels (ie part throttle) and be causing the lumpiness?

That's only going to be a problem if you are snapping the throttle shut at part throttle - in which case if the spring is too strong then most of your boost will be trying to go back through the turbo instead of out through the DV. So when you get back on the throttle it takes a second for it to get back to speed again. But from your original post it sounded more like a problem when the throttle is constantly open i.e. you are holding the throttle at a certain point but can feel the boost coming and going. That to me sounds more like a boost leak or a map/N75/wastegate problem.

Edited 11 August, 2010 by chicken_eyebrow

[SideshowBob]

That's only going to be a problem if you are snapping the throttle shut at part throttle - in which case if the spring is too strong then most of your boost will be trying to go back through the turbo instead of out through the DV. So when you get back on the throttle it takes a second for it to get back to speed again. But from your original post it sounded more like a problem when the throttle is constantly open i.e. you are holding the throttle at a certain point but can feel the boost coming and going. That to me sounds more like a boost leak or a map/N75/wastegate problem.

Fair point... I can hear it 'dumping' even when I shut off on a light throttle so I think on that side the DV seems ok.

[Hofmiester]

I think that is normal, for example you are on the gas...building up boost pressure......then the car in front slows so you back off a little.....the DV will vent a as there is a vacume in the inlet manifold from the reduction.

My car does this and my 2 previous 1.8Ts did it but witht he OEM DV is is smooth and non intrusive but with the Forge it is very obvious.

[VRS Mike]

yeah the second you lift at all it dumps the boost, its certainly more noticeable on a remapped car because there is more boost. I must be lucky because mine is pretty smooth, I think alot of its down to how you drive, sometimes you come slightly off the throttle without really realising it?

[SideshowBob]

Mine sounds similar to both yours ( madmike01uk / Hofmeister ) on a slight throttle. I quite like that characteristic and as you say, the more boost you're running, the more you'll notice the transition from boost to off-boost. The only situation during which I'm unsure whether I have a problem or not, is on a medium throttle. Let's say you're joining a motorway and heading up an uphill sliproad (so there's a bit of load), and you're perhaps giving it 40-50% throttle in 3rd/4th gear to get up to speed. If I do that, I can feel the boost level fluctuating. I never hear any dumpvalve 'venting' noise, so I was guessing that it was the N75 aiming for a certain boost, missing it, reigning it in, missing the target, allowing more boost through etc etc you get the picture I'm sure.

Is that how an N75 behaves when it's on its way out?

I doubt I have boost leak because on full throttle it seems fine.

[VRS Mike]

I have just changed my N75 as R-Tech suggested it wasn't controlling boost correctly when they remapped it, it has made it much smoother. I managed to pick up a new unit off of here for £30 but new I think they are £60?

[chicken_eyebrow]

Yeah they're about that brand new, possibly a bit more if you have to buy one from a public counter at a dealer instead of someone like TPS.

Sideshowbob, if you're thinking of replacing the N75 it might be worth your while to find a local member, or a mate with a 1.8T car like a Leon or Audi that can swap their N75 valve over for an hour or two just so you can see if that's the problem. There's lots of members on here that have bought N75s and that hasn't cured the problem and they've had to ebay them.

There are all sorts of things that can control the boost and that might cause a fluctuation

1. N75

2. Wastegate actuator

3. Wastegate itself

4. Divertor valve

5. N249 / N112 valves

6. ECU, going on signals it is recieving from various sensors around the engine

That's not even counting things like split pipes etc.

Edited 13 August, 2010 by chicken_eyebrow