Irregular Boost

[Sp@cE418]

Here I am with another boost question :)

I'm Stage 3-.

Sometimes, my car feel strong and push a "clean" 18lbs.
Today, nothing particular (cold weather), the boost was very strange.
I looked at the gauge and notice that the boost wasn't constant.
Still on full throttle and trying to look at the gauge, the needle was going up to 20lbs and down to 14lbs ...
I was ... WTF !
Any idea ? N75 ?
Where to check first ?
If it was a boost leak, would't be always under 18lbs ?

BTW, I VAG-COM my car and have no code ...

[AudiSportB5S4]

no, a boost leak will cause erratic boost just as you described.... when you have a boost leak somewhere along the system, what happens is the car tries to meet the requested boost so it asks the turbos to work even harder to meet the req... so usually it will quickly spike then come down fast then go back up a bit, it will not be stable at all...

the cheapest and easiest way to see if your N75 is broken is either swap one out with a friend or someone with another S4

[Sp@cE418]

no, a boost leak will cause erratic boost just as you described.... when you have a boost leak somewhere along the system, what happens is the car tries to meet the requested boost so it asks the turbos to work even harder to meet the req... so usually it will quickly spike then come down fast then go back up a bit, it will not be stable at all...

the cheapest and easiest way to see if your N75 is broken is either swap one out with a friend or someone with another S4

Thanks man !

I'm gonna pressure test my car then ...
I'll post the solution when I find it !

[Sp@cE418]

Thanks man !

I'm gonna pressure test my car then ...
I'll post the solution when I find it !

There's something I don't understand ...
If I have a boost leak, why sometimes the boost is very constant and sometimes not ...

And yeah I don't have bigger intercoolers but the runs are in the same conditions ...

Thanks !

[AudiSportB5S4]

if the problem is intermittent you may want to look at swapping out N75s with someones car where you know the boost control is normal... A big boost leak is really noticeable, you can hear air swooshing and it'll sound like an STI or EVO. A minor boost leak could be anything from a torn vacuum line to a hose that only comes off at certain pressures etc... thats why a pressure test is great. It can really help. Getting access to a compressor isnt always easy if you dont have one but it really is the best test for boost leaks.... you could also want to look at your diverter valves... what are you using ? stock, or did you upgrade? if stock, could be a torn diaphram..

[Sp@cE418]

if the problem is intermittent you may want to look at swapping out N75s with someones car where you know the boost control is normal... A big boost leak is really noticeable, you can hear air swooshing and it'll sound like an STI or EVO. A minor boost leak could be anything from a torn vacuum line to a hose that only comes off at certain pressures etc... thats why a pressure test is great. It can really help. Getting access to a compressor isnt always easy if you dont have one but it really is the best test for boost leaks.... you could also want to look at your diverter valves... what are you using ? stock, or did you upgrade? if stock, could be a torn diaphram..

I know it'a not a big boost leak ... for sure because the car still pulls very hard...

My DV's have been upgraded with Forge Motorsport for a while ...
I understand what you mean by a minor boost leak could happen just at high boost ...but sometimes my car feel hard at 18lbs on WOT and sometimes again at WOT, same speed, same gear, same weather even same road the boost is erratic ... Sound strange isn't ? Maybe i'll take a look at my N75 before to push to hard on trying to find a boost leak that is not ...

Thanks man !

[gmann]

Think of putting a pin in a balloon. You won't notice it at first, but once you get a lot of pressure inside the balloon it will be more noticeable. Same with your car, you may have a small hole that isnt noticeable with normal driving. But when you push more boost there's more air trying to force its way out causing a boost leak. It's going on with my car right now, but I'm too lazy to work on it. :D Good luck.

[Sp@cE418]

Think of putting a pin in a balloon. You won't notice it at first, but once you get a lot of pressure inside the balloon it will be more noticeable. Same with your car, you may have a small hole that isnt noticeable with normal driving. But when you push more boost there's more air trying to force its way out causing a boost leak. It's going on with my car right now, but I'm too lazy to work on it. :D Good luck.

Yeah I know that ... the problem is that it's intermittent ...

[rageman2]

Sounds more like throttle cut to me - particularly if it is related to colder weather (colder air=more dense).
Log the boost and throttle position when this happens - that will tell you if you are getting throttle cut (which I have had without codes in the past).