complacentsee

Depends on the controler method for the boost system. Differant tuners use different methods.

All controllers have different responce times to things like this. I think most of these boost controllers use PID control methods, and they are relativly adjustable and have tuneable performance in responce times ect. It would be concieveable that a controller would have a bit of a hard time controlling the boost with a boost leak. Boost leak should increase with the pressure applied... I doubt a turbo waste gate is a very linear method of control as well.

Short story: Boost leaks do cause surging.

CTAudiDriver

2nd, do you have a boost guage or vag-com? a boost log with vag com or looking at your guage would tell you if inconsistant boost is your problem.

I had a leak and it would go up to full boost, lose about 4 psi and than build right back up again, now without it it just gets to full boost and holds.

Like dks said you would think that a leak would cause a lower boost level throughout but I think the fact that the pressure rises fairly fast under initial spool up that the air doesnt have time to leak out (depending on how large the leak is) and once you are at your max boost level and the ECU is trying to hold it there it will lose some boost with the settings it is trying to follow, then once the pressure drops, it notices it and compensates by closing the wastegates a bit and forces more air in to keep its requested level of boost.

DksMester

I have just purchaged a VAG-COM cable and will recieve it one of these days. So I can have a look at it :P
But I had the car to a mechanic, this other day, to look at the car comp. It had two errors. One O2 sensor fault and one overboost error. Unfortunately they got deleted before I got the chance to read spec on them (if that is possible).
After deleting those errors, the car drives better. Not total flawless I think, but better. So the O2 sensor must be defect. However the car is running slower acc. now. Maybe it is the overboost there is gone
I ran the car around for 15 min and then we checked the computer for errors again, but none had come up.
But im looking forward to getting the VAG-cable so I can take a drive and have a look at how the boost is running (if that is possible).

-Jacob

Indyxtype

The overboost code makes it sound like you are looking at a boost leak which also could match your description. Symptoms of a leak often only hit at a high boost levels (WOT) because it is usually not like someone drilled a hole in one of the pipes, but rather there is a tear which leaves a flap over the hole. At low pressures the stiffness of the rubber can hold the flap closed and thus the system remains closed. However, when you increase the pressure in the system by adding more boost, the flap is forced open and purges off some of the air. The turbo initially sees inadequate boost and increases its output to try to compensate, however when the ECU sees the turbo spinning faster than normal it freaks out and cuts the throttle. Thus, power build up, shut down, power build up, shut down. I had a tear in the infamous TBB which gave me these symptoms along with an embarrassing run against a Z3 2.5.

DksMester

Thanks for the reply Indyxtype. Sounds logical what you are saying, maybe I should have a look at my TBB too However the car did run better when the errors were deleted, but I feel it is slowly coming back, these last days. Wouldn't a bad O2 sensor cause this? Is that running at WOT?

complacentsee

The car should be in open loop mode. Meaning the 02 sensors are not used for fueling feedback. With the vag-com you'll be able to graph: requested boost, actual boost, throttle plate posision. Using this, it really helps narrow down the range of problems from software, sensors, or issues with the control methods.

Indyxtype

That may actually be another indicator of a boost leak. If the code is cleared out, the ECU would have no record of a boost leak and thus no reason to cut throttle; until the next time it sees the turbo overshooting. If the leak was small, under the right conditions, the turbo would be able to keep up with the leak and compensate for the loss of pressure without triggering an ECU threshold. However, under different conditions i.e. temperature, ambient pressure, humidity, etc, the leak might become bigger and thus really work out the turbo enough to trigger a code and cut throttle. Keep in mind, if the ECU was not involved a boost leak it would just seem like your car was a little slower as it would be only getting hypothetically 0.8 bar as opposed to 1.0 bar of boost (0.2 vented out the leak) or in most cases no difference at all as the turbo is able to make up the difference completely. The ECU is what would cause the surging and clearing the code would temporarily alleviate this until the next time the “flap” opens up. This is not to say however that clearing the code is fixing the problem. If you have a boost leak, you need to track it down and fix it ASAP and drive at low boost in the mean time! Not only would it create that annoying surging feeling, but it would also be causing your turbos so spin out of spec as they try to make up the difference in boost demand. Needless to say, this condition isn't very turbo friendly. Get a pressure tester for AWE or if you're feeling crafty, build one yourself and see what you can find. I picked one up when I had the problem and never even used it because I found the tear in the TBB when I went to hook it up, however I'm keeping mine just because it's a good thing to have around, especially for $20. The MAF is another possibility as they can causes some screwy stuff, however my money is on a boost leak and it makes sense to start with the simple and cheep diagnostic, as opposed to having to shell out the several hundred on a new MAF when the old one is only in question. I highly doubt that it is a O2 sensor or EGT sensor as those would most likely put up a code with their name on it if they were causing such an issue. In the mean time, for confirmation one way or another, log the channels mentioned by complacentsee as they will be able to tell you IF there is a leak or not. If there is, you will still need the pressure tester to track down WHERE it is.