What is the idler pulley?
Oh look, it's the dumbass Crown Vic troll once again. Oil pumps rarely, if ever fail. An oil pressure problem is most likely due to worn engine bearings, but one should always check the gauge, the pressure sensor and the pressure relief valve before deciding to rebuild or replace the engine. And WTF did you ask about the idler pulley? It's completely unrelated. It simply puts tension on the serpentine belt.
Odd lead in question....
By now the crankshaft and camshaft bearings are wearing. When you have worn bearings the oil pressure drops more because the clearances are bigger and oil blows out between the bearings and the part riding in the bearing. Just replacing the oil pump is a waste of time and money.
On your engine it is the one(s) without the spring loaded serpentine main drive belt tension, on cars with a OHC drive belt there is also a idler pulley on the OHC timing gear drive belt.