What's controlling boost on my car?

[AndyP]

Ever since I bought my car, it’s boosted to 1bar, I always assumed it had a chip or a spring. Some investigation a while ago revealed it had neither, so I fitted some chips designed for 1bar boost and the car ran fine. Further tests have revealed that despite having a stock spring, it still boosted to 1bar with the WGFV unplugged, it also gets almost full boost on part throttle. The wastegate cap still has the tamperproof seal in and there are no spacers on the spring to preload it.

Last week I fitted another ECU with an ABY MAP and chips for 1.4bar boost, but it wouldn’t boost over 1bar. So basically I can’t get it to go above or below 1bar. I’ve since proved the turbo will do it by fitting a stiffer spring to get it up to 1.2bar.

It's been leak tested, has a new WGFV, new divertor valve and has no codes on VAG-COM.

Any ideas what could be causing this?

[Gazjen]

Non standard WG spring?

[twoqu]

Andy,

I have some data that might help:
From left to right:
20V 82mm free length, 38.5mm OD, 2.8mm wire thickness.
10V 75mm free length, 37.0mm OD, 3.0mm wire thickness.
? 65mm free length, 40.3mm OD, 3.6mm wire thickness.
I also measured 25mm from spring perch down to wastegate cap.
The 10V wastegate perch is sort of a cone. It is in the middle of the pic. As they are interchangable there is no guarantee that somebody hasn't mixed and matched over the years perhaps to get rid of an adjustable one!

[AndyP]

The spring I removed yesterday ( gave 1bar with WGFV unplugged ) measures exactly as per your 20vt specs and my wastegate cap looks like the one on the far right. I didn't take any measurements off the spring perch, but as it still had the tamperproof cap in and the screw was full of resin, I assumed it was std.
This is the 2nd std spring I've tried, both of which have been tested in other cars and gave 0.4bar mechanical boost.

[twoqu]

Well the pic of the wastegate cap on the right is of an adjustable one! The 20V is on the left!

[Pirvu Eugen]

Maibe the vent hole from the cap is blocked or the wg is not closing good and lets some gases blow by. Just a thought

[twoqu]

Here's a better shot of a 20V cap!
I guess somebody has fitted an adjustable perch and then resealed the top. Pop the cap off and measure the dimension of the perch to wastegate top to see if it differs from what I measured.

[UrS4boy]

Are you sure the moisture trap hose hasn't blown off so the ECU is not getting a boost signal? When this happens on C4 AAN 20vt's (virtually the same as an ABY) all you get is the wastegate spring pressure boost because the ECU thinks there isn't any boost to control.

Here is an example of what I am talking about:

http://forums.quattroworld.com/s4s6/msgs/20401.phtml

[Pirvu Eugen]

This will not trigger the fuel cut?

[Jenks]

Any ideas what could be causing this?

Sorry Andy but I couldn't resist.......Is it 'The Force'

[Acidburn]

andy's car is a 3b

[UrS4boy]

andy's car is a 3b

So are you saying 3B's don't have moisture traps between the intake manifold and the ECU?

(I don't know whether they do or not)

[Acidburn]

So are you saying 3B's don't have moisture traps between the intake manifold and the ECU?

(I don't know whether they do or not)

lol same here, afaik the only thing I remember seeing is i think a one way valve.

Only reason I commented that his car's a 3b was incase that some of the advice given was more aby/aan rather than 3b.

Be interesting to find out what the cause of this problem is

[AndyP]

Well it's not the moisture trap, if it has one, as the boost gauge is T'd into the ECU line and that works fine. I've never come across an S2 that a wastegate cap like the ones Robin posted up, they have all been like mine.
When I get chance I'll take the top off and measure where the spring perch is.

[Rusty]

Have a look for Newsh's posts on WG size, I think he had the wrong WG on there.

If the spring is proper and the length is too then the WG diameter is about all I can think of.

The OE spring on the OE ABY WG should lead to a gap (with the WG cap undone) of approx 40mm.

Best to get it running correctly as a completely stock setup first, then tune it to your desired setup, that might show something that's a little awry.

Also, check that the boost line that goes to the WG isn't blocked, leaking or damaged, if no boost is getting there then the WG will stay closed a lot longer and won't modulate properly.