Possible causes include:
Excessive main and rod bearing clearances (someone may have installed the wrong-sized bearings or standard-sized bearings on a reground crankshaft). Only 0.001" of extra clearance in the main bearings can reduce oil pressure by up to 20%!
Excessive camshaft bearing clearances (a bearing may have slipped out of place when the cam was installed).
Excessive clearances inside the oil pump.
Leaks between the oil pump and engine.
Oil pump relief valve stuck open or installed backward.
A loose or mispositioned oil pickup tube (too high in the oil pan).
A defective oil pressure sending unit or oil pressure gauge.
Low oil level in the oil pan.
Cracks or leaks in the oil galleries or gallery plugs.
Cracked oil pump housing (improper installation).
Oil viscosity too thin for hot weather.
Oil viscosity too thick for cold weather.
Saturday, February 14th, 2009 AT 9:29 PM