ok ive been trying to figure this out for at least 4 months now troubleshooting everything under the sun and im spent and just literally want to burn it to the ground

backround info....

stock ls motor w/ a pnp head running on a p75 ecu

now the motor runs great up top and gets decent gas mileage....yet soon as you get off the trottle it dies....

put the vac/boost guage on it and at idle it barely pulls (10in hg) and i loose all brake vacuum...

ive tried multpile ecus, iac's, double checked wiring etc...and im totally stumped

any help is greatly appreciated!