Need help - Vacuum loss and can't resolve

[68GMCCustom]

When my truck was running this new genIV LSx motor with a carb'd setup...it ran with 15-17" of vacuum. Truck ran well but a bit rich. I also installed brake booster along with a vacuum storage tank along with other brake upgrades during the build. Brakes were awesome...could throw a passenger threw the windshield.

Then came the change to efi. Removed the carb'd setup and on went plastic efi manifold and ported throttle body, running a custom harness w/e38 ecm (t42? tcm) combo, w/4L80e trans, running a custom tune by well known tuner. That's when things went south. Vacuum has been in the 10-12" range since then and through 4-5 different tunes. Idle seems similar to when carb'd...just not making the same vacuum any more. This motor is not old, and is/was a "new" 0-hr. motor. Less then 1000mi now.

Items checked off -
Changed pcv setup - no change
Closed off any/all ports including one for brakes to check vacuum level - no change
Obtained new OEM efi intake manifold and gasket set - replaced w/o any change
Throttle body - swapped from my TBSS - no change
Also -
Pulled the injectors and setup a test bed. All injectors fired and sprayed similar strong streams
Pulled each injector connector one by one while running - each cylinder appeared to pull engine down when injector disconnected and picked back up when reconnected

I need to resolve this. While the motor is running and making good power it obviously isn't right at 10-12". I have HPTuners and have tried some things there w/o any improvement. With the vacuum at this level as you might expect my brakes absolutely suck now...not a problem a carb'd to efi swap should create!

Ideas?

[68GMCCustom]

Ah come on guys......anything?

[truckdude239]

unplug and block off your brake booster and check your readings again

[homemade87]

Was the cam you had in it for a carb set up. If it was then when you put the efi on it may not of liked it. I do know there is different grinds for the two. Just a thought.

[68GMCCustom]

Thanks guys.

Actually, already unplugged everything and capped them all off - no change
Cam is a Comp LSx cam... .223/.231@.050 .610/.617 lift and 112 lsa. Don't remember it specifying carb or efi. You may be on to something, but its not something I would have thought make a difference. I suppose it certainly could tho. Wish I had a way to know for sure?

[BR3W CITY]

Aside from the common stuff or maybe a crank in the plastic manifold?
HAve you tried spraying the stuff underhood with carb cleaner, then watching your fuel trims in HPT to see if the car is making up for it.

This is a stupid one....got a loose valve cover gasket?

[68GMCCustom]

I'm almost ashamed to say, but I bought a new one....believing that it HAD to be the intake, or a gasket...and so -


no change...but I have a spare L92/LY6 intake!

hadn't checked valve covers...but I will!

[homemade87]

Originally Posted by 68GMCCustom

Thanks guys.

Actually, already unplugged everything and capped them all off - no change
Cam is a Comp LSx cam... .223/.231@.050 .610/.617 lift and 112 lsa. Don't remember it specifying carb or efi. You may be on to something, but its not something I would have thought make a difference. I suppose it certainly could tho. Wish I had a way to know for sure?

Would not hurt to call comp and give them your grind # and talk to them about whats happening. I am sure they will have some insight on the difference you are having. Be interesting to here what they have to say.