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3.4 pulley top swap and only seeing small boost

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bfox507

New member
i just recently top swapped my 04 impala ls with a gen 3 blower. i am using stock l67 injectors and i also installed a xp cam with all the necassary mods such as 140# valve springs, DR chain, modded retainers, etc but i also am running a 3.4 MPS. I have all the gauges i need connected and hooked up right but i am not seeing anymore than 4 to 5 psi of boost only when i actually get on the throttle. I'm not 100% sure if those number are right for the pulley size i am running but from my research them seem to be a lil lower than what i was expecting to get. i got my cpm programmed by zzp for the colder t stat and 3.4 pulley with the xp cam and top swap. any help and knowledge on this issue would be greatly appreciated.
 


Vacuum leak, or bad boost bypass solenoid.

I had a bad boost bypass solenoid and my car would only build 3.3psi max of boost on a 3.4" pulley with a gen 3. Now with a new bypass solenoid I see roughly 8 - 9psi. At around 80 ish I saw a peak at about 10psi
 
I would think boost bypass solenoid/valve as well. Try pulling off the vacuum line on the BBV that is NOT directly connected to the vacuum port on top of the blower. That usually fixes the issue for most and no adverse affects from doing so
 
97AutoXVette said:
I would think boost bypass solenoid/valve as well. Try pulling off the vacuum line on the BBV that is NOT directly connected to the vacuum port on top of the blower. That usually fixes the issue for most and no adverse affects from doing so
Click to expand...

You also have to plug that line.
 
97AutoXVette said:
I would think boost bypass solenoid/valve as well. Try pulling off the vacuum line on the BBV that is NOT directly connected to the vacuum port on top of the blower. That usually fixes the issue for most and no adverse affects from doing so
Click to expand...
damy5yme.jpg

Like right there in the blue? That can be removed from the bypass solenoid and plugged off?
 


Yep, the first pic. Unplug that hose and stick a bolt in it to plug it. If your solenoid is bad, you'll get %100 of your boost back but the car can't disable the boost for TCS or reverse.

EDIT: I honestly can't figure out what would happen if the smaller hose was off. Even a fully stuck open Bypass solenoid, still plugged into that small hose doesn't stop boost as long as the line from the "T" is plugged.
 
industrialfish said:
Yep, the first pic. Unplug that hose and stick a bolt in it to plug it. If your solenoid is bad, you'll get %100 of your boost back but the car can't disable the boost for TCS or reverse.

EDIT: I honestly can't figure out what would happen if the smaller hose was off. Even a fully stuck open Bypass solenoid, still plugged into that small hose doesn't stop boost as long as the line from the "T" is plugged.
Click to expand...
I kinda want to say it ends up causing a lean condition. I can't say that for sure but on my old engine, I had a bad boost bypass solenoid and couldn't figure out what was causing the lack of boost. My dad and I plugged off the line coming left off of the bypass solenoid and I ended up with a lean condition but was able to build full boost.

eryty5a6.jpg


This is that engine now though o.O

I was noticing the spark plugs were getting eaten left and right and found out that engine was toaste.

Again, not sure whether or not the lean condition was from plugging off that line
 
You can in fact just remove the line going from between the "T" coming off the fuel pressure regulator, all the way to the boost bypass valve. I did this and removed the boost bypass solenoid all together with no adverse effects at all. I'll send a picture this coming weekend when I am back by the car (work trip this week). Just plug off the right side of the T and leave the non blower side of the boost bypass valve open to atmosphere so the diaphragm in the valve can still function correctly. No fueling issues from this at all. It will not affect your AFR tables, only prevents the solenoid from being effective under boost cut conditions commanded by the ECU
 


To be clear, you can remove both hoses from the two pictures in post #8. That's what I have done to mine and I have no problems and full boost
 
I tried plugging that vacuum line and I did get to 10 psi so I went and bought a new solenoid but then I hooked it all up but I still only see 3 psi...is there any chance my PCM could be messing up the boost cause I checked all the lines and no leaks
 
The solenoid is an open/closed valve only. I'd assume if the PCM was messing up and forcing it open, you wouldn't have any boost at all.
 


Just wanted to show my setup for reference. You can see what I have removed as far as the BBS goes and the vacuum lines. It is running great now with this setup and I see full boost with no issues

you can see I just use a L-fitting coming off of the fuel pressure regulator to the vacuum line that runs down to the intake manifold under the blower snoute
 
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i can get 3 lbs of boost with the solenoid open, i had an incorrectly tuned pcm that didnt have the gtp setup take like it should, so i had no boost display on the dash and the solenoid was open full time, but with the 3.4 pulley i still made 3 lbs of boost
 
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