EXCESIVE FUEL CONSUPTION BAD MILLEAGE

Those earlier models w/o OBD2 computers are usually harder to figure out. You could spend weeks replacing parts to find out it's nothing more than an air hose is unplugged. (This was me not too long ago) Your best bet is to ask some friends where an honest mechanic is, and get it checked there. Yeah he might charge $60 labor to replace a simple part, but it's better than $600 on self diagnosis.

But please post when you do find the issue