Thread: 3.4 pulley top swap and only seeing small boost

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

At least thats my best guess for you

Originally Posted by 97AutoXVette

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

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

This line in blue?

What can happen if the line here is disconnected?

Originally Posted by industrialfish

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.

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.



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

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

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