oil psi drop

[biscuit]

looking for a little quick input.

2 separate stock engines, 1 complete rebuild (1500 miles), other a junkyard engine (130k).

i put new melling oil pumps in each. the rebuild has 1500 miles on the new pump, the jy has about 6k on the new pump.

both are running 5w30. the new engine ran 20-30 psi (idle to rev), my rebuild held 50-60 idle to rev.

as of recent, both engines are ranging from 12-18 psi. all readings are from dash, i will be confirming with mechanical gauge.

my question is. what can cause a sudden drop in pressure ranges. i know the psi is in spec. but what can cause the change in psi ranges.

these changes happend from one day to another, not a gradual drop.

any input would be greatly appreciated.

[jsawduste]

Resistance values of the sender/wiring and/or gages.

[gradon]

I've seen fram filters drop pressure. 10w-30 is spec, so the 5w could be part of the drop. Sounds like you think the melling pumps are the factors.

[Cheromaniac]

Firstly verify the oil pressure readings with a mechanical gauge. If the oil pressures with the engine hot are still low, you could have one of the following scenarios:

1. Oil pressure less than 15psi at idle but 40+psi at 2000rpm = Good oil pump, excessive bearing clearances. Thicker oil a temporary band aid.
2. Oil pressure normal at idle but less than 40psi at 2000rpm = Weak oil pump, normal bearing clearances. Replace oil pump.
3, Oil pressure low across the board = Weak oil pump, excessive bearing clearances, and possibly missing oil galley plug.

[biscuit]

on the junkyard engine. had a brand new oil sender, it failed. its needle range was fairly correct but values were bad

bad showed about 12-19
new one shows 43-63

once i saw oil on the wrong side of the switch, i swapped it out. so i never verified actual oil pressure with mechanical. no noise, good flow over rockers at all times. i'll be checking the other engine/switch.

thanks for the input!!!