I have a 94 gmc sonoma with the 2.2 4 cyl.I just but in a rebuilt 4 cyl new. Oil pump everything on it is basically new. When I start it it takes a couple seconds then oil pressure comes up to about 18 psi if I dont let it warm up it will drops instantly when I start drivein down the street. It never really comes about that it suppose to read about 56 psi at 3000rpm when warm and it does even move when I get into it even when I tack it out. Any ideas why this might be happening like this?