1997 Chevy lumina 3.1 and it makes knocking noise coming from the engine when I start it It goes away after about 2 minutes what could this be?
The condition may possibly be piston slap.
If you have lower end knocking (rods or mains) an oil pump won't do much good. Plan on an engine rebuild with rod reconditioning and have the crank turned. Also, check the cylinders for taper and while it's down have the heads reconditioned, replacing any valves that look too far gone. Replace cam and lifters if there is any pitting or flattening on either.