lose boost with diode mod
 

I have an 01 A4 that I just did the diode mod to. I have been playing with the mbc the past few days and I have it to where its spiking around 12-13psi but it steady falls off to 9 then slowly falls from there. Im not sure if this is just a boost leak, or thats how the mod works but from the DIY thread I took it as it held at boost. So I guess Im just looking for some insight on what could be the possible problem and what I need to do to fix it.

Could very well be a boost leak. But first, you need to make sure you understand the function of a MBC: chances are your MBC is a bleed type, where once a certain pressure level is reached (as set by the preload on the spring) it will vent the excess pressure. So if your MBC is set at 13psi, it essentially won't allow and boost pressure past 13psi.

On the other hand, it will allow any boost pressure up to 13psi without venting. In your case if there is only 9psi present, then your MBC isn't doing anything. You're losing boost elsewhere... if you're running a stock K03 it's typical for boost to begin tapering after ~5000 RPM. You can build a boost leak tester and see how effective that is. If you don't have (major) boost leaks it's possible your turbo is the culprit.

Thanks for the reply, I do plan on boost leak testing. And yes im running the turboxs bleeder mbc, i have it run inside the cab of the car. So i do know that its venting cause i can hear it like crazy. But for me to get a better under standing, i have the mbc running in paralle with the n75, between it and the wastegate. Now when i go WOT and the k03 spools i should in theory hit 13psi and hold right... I just never have own'd a euro, i had a dodge neon that was in the process of being swapped to the srt and those are a total different ball game than this so i just wanna make sure i have it.

It's a small ass turbo, it doesn't hold much boost above 5k. The more you spike, the more it appears to hold

ok cincy, so i read somewhere that your running the same mbc that i am. If thats true, how many "full turns" did you do to get to the 14psi cause i honestly think im at 7 full and its still falling drastically to 9psi. i did a half turn up today to run an s2000 and i can tell it pulls harder but im just not seeing it on the gauge.

I run the Hp not the standard turboxs mbc.

ok well im hopfully getting an AEM digital boost gauge so i will know exactly what im at

A more expensive boost gauge isn't going to change anything

No it will just give me a more clear reading on what im actually boosting then a needle thaat is a guessing game, its much more convenient to look down and see a number in psi the to be like I think I hit 14 let me try again

To each their own