check engine light on codes p0171& 0174

[mttsn42z]

mttsn42z,Hello to all. My 96 mark viii check engine light is on with codes p0171& p0174. I change all 4 o2 sensors. light is still on . Need your help. trying to pass emmision test...
any suggestions... thanks for all of your help

 

[chris2523]

a vacuum leak is most probable.
there is a rubber T fitting in the back next to the throttle body.
check it for dry rot.

 

[unity]

Ditto on that T fitting. Biggest cause. Second is the intake tube gasket at the end of the tube. Two problems. First, its actually deceiving when installing the tube. It may seem like its on when its not seated well. Second is it fails in a way. I think its glued on, but that comes off. So the gasket can move when installing and it may fold causing a leak.