the code is p0171, running too lean bank one

i have read a shit ton of stuff on this and many say its the mass air sensor, but there is no mass air sensor on a 96 accord, only mapp and tp sensors on the intake

ok ya i should have put all of this:
i have a 2.2l non vtec

it has an air intake, and catback (but the cat is stock)
i have replaced the:
fuel filter
the o2 sensor in the header
i have used many injector cleaners
changed the t.b. inorder to change the tp & map sensors
i have had the vac lines checked and they were fine
the fuel pump has good presure


the shop i took it to said they have no idea what it could be, but he did say that if it was the exhaust manifold then it would have shown when the vac lines were tested
he also said that there could be an internal leak in the intake manifold, and the only way to find out is to take it all apart.

ok so i think that is all i know.... i will add anything if i remember