Well i have been having boost fade for almost since i remember upping my boost from 5 psi to 8 psi intercooled, with a mbc. I had always thought it was my MBC, but i took that out today, adjusted my actuator rod as far in as it would go, then used one of those brass "t" vac fittings to use as a bleeder in the vac line at the turbo to wastegate, and that worked great, hitting a solid 10 psi. But after about 5000-5500 rpm's, the boost goes from 10 psi to about 8.5 psi, every gear it seems. It only does it when i get to higher rpms.Its done this with the MBC hooked up, or not hooked up, BUT it never does it when i just run the wastegate spring only at about 4-5 psi. I did a boost leakdown test, and fixed everything but the TB throttle shaft leak, its not terrible, but it leaks some. Could that do it? I cant think of anything else. My exaust is a gutted cat, 2.5" resonator, and 3" resontator back to a borla 3" inlet oval muffler, so that cant be it...Will a fairly small boost leak cause this? If so, then why only does it do it in high rpm's? Any suggestions?

Ok, i have an idea. I really think its my TB plate shaft thats leaking boost, but i may try this to see what happens. Since i took out the PCV valve, i have that nipple capped off on the intake. I will try and hook up a vac line to this, run that to my MBC, then from the MBC to my wastgate actuator. The fitting on the compressor will be capped off. The way i look at it, when the boost pressure starts to drop off, the MBC will then close back up to its setting of 10 psi, and therfore close the wastegate some to build back lost boost pressure. I'm thinking this should work. I thought of this by another post of the proper placement for the MBC vac boost lines, and some have said to null the psi drop from an IC, is to run a vac line from the intake to the MBC, instead of using the outlet on the compressor housing. I will try that and see what happens, until i can fix my leak at the TB.

If I were you I'd run it from one of those nipples on the back of the manifold. I had a boost leak at the BOV since the day I was first turboed up until like a week ago. I never noticed because my MBC would compensate and still get me 11psi because I had the signal running from the manifold. I took the boost controller off and could only get 8psi until I fixed the leak. Now I get 10psi w/ no boost controller.

*edit* Ok I guess that's what you said. I thought you meant run it from the valve cover side of the PVC but now I know what you mean. If it is because of a boost leak then that should definately take care of it.

yeah my pcv was leaking under boost, so i took that out, then ran a hose from the pcv valve cover nipple and teed into my breather/catch can hose. This should work, as i will find out tomorrow. I need to get this fixed asap, as i'm going to a import drag race up here on friday :) Hopefully i can do better than the last track outing hehe..

I'm pretty sure I know what your talking about...but could you possibly post a pic of what you did to the PCV valve? My boost does the same god damn thing.....spikes up to 12-13psi, holds it for about 2K rpms, then falls down to 10psi around 5500rpm. I need to fix this problem.....

Ok, i'm in the process of doing this, its all hooked up now, just some more adjusting of the boost controller. I'll reply here tonight with pics and the final word on how it all works...

PRESTO!!!!!! Works excellent! Boost does spike to about 11 psi, but almost instantly drops to 10 psi, and stays there MUCH better than before. Much, much better :) Def pulls quite a bit harder up top now. Here are the pics of the current setup now.

Here is a pic of the intake and valvecover. I completely took out the PCV valve, and then on the left hose nipple on the valvecover, i ran a vac line from that and teed into the breather line, all going to my custom catch can/breather. Here you can see the line coming off of the intake, right to a T, with one end capped off, and the other going to the input of the boost controller. I had to use a T, i couldnt find any other way to use 2 diff size hoses to hook together.




Here is a pic of the T connection for the once-pcv valve. One hose connects to the left valvcover nipple, one hose connects the the breather nipple, then it goes to my catchcan/breather.



Here you can see the new vac line going along the oil feed line, this vac line is going to the boost controller.



Here you can see the capped off turbo housing nipple.



Pic of my home-made catch can, right infront of the battery. Its a vitamin C bottle, with small breather filter in the bottle cap, filter bought at autozone for about 8 bucks.



Just an overview of my engine bay...


I still yet have to fix my TB boost leak, hopefully i can get around to it soon.. Boost fade= FIXED :)

Thanks Matt.....I'll try that myself as soon as I get the supplies :)

so this mod is sopose to stop boost fade such as i hit 9psi then at certain rpm or shifts (auto) it drops to say 7psi? this is sopose to hold it at 9psi?... also would this effect smog in anyway?
thanks


Michael 99gs
HRC Stage 2
Stage 3 process begins anyone got a 420a block thats not spun/head for sale?

http://www.dreamwater.net/purplebeast

Why don't you guys pressure test your intake, find your leak, and fix it instead of tricking the MBC into making the turbo work harder and running a hotter charge through your engine???