Oil pressure driving me crazy

[Mr_Rich]

Originally Posted by 68c10airstream

In my post above (# 20) i was remembering customers coming in with a dash oil pressure gauge reading incorrectly and changing the silver tall can near the distributor. I,ve swapped a few of these tbi motors and agree about some switch?? down by the oil filter boss. When i looked in alldata circuit #31 looked to be shown for 2 different circuits, so i assumed the base cheap truck would use a switch for a bulb on the dash, and a upscale deluxe truck with full gauge package (oil pressure gauge) would use a sender, not realizing that these trucks used both if i understand correctly. I'm not here to upset anyone just trying to help, and through the years using alldata has been hit and miss, especially with wiring.

I think you're rattling some of my geezer brain cells loose as well. That "silver tall can" your referring to can be found on LS engines and referred to as OPI's and they break easily. Certainly hope no upsets in this discussion.
Although I don't recall seeing any since all the trucks I'm familiar with have gauges I can assume that some had idiot lights and would have a plug under the left exhaust manifold instead of a sender. I had to re-think that idea after one of my previous posts.

[Mr_Rich]

Originally Posted by speedygonzales

Correct me if I'm wrong but didn't you say a mechanical gauge has already confirmed your dash gauge was working correctly? So why are chasing down a way to make the dash gauge read differently?

I had the exact same problem you had and the lower end was wiped out. Guess what I noticed. The rods were "tight" but the bearings and crank were wiped out.

I have never heard anyone say their gauge was doing like yours and it turned out to be either the sensor or the gauge. Really low pressure when the idle speed decreases is a bottom end problem. Period.

You bought an engine on Craigslist? SMH

Pull a main cap.

I think you nailed it on this one. I now recall I helped a co-worker swap out a 318 in a Dodge van in the 80's. (Was that fun!) The oil pressure gauge showed like ~5 psi at idle. When we got the engine out I removed a main cap and it looked like the crank had spun against the edge of a file.
I think cutting open the oil filter and checking for debris will probably verify this situation.

[xs-style]

Oddly enough cutting open the filter didn't show much. There's no noticeable particles in the filter element itself. There are some metal shavings around the side of the canister but they aren't covered in oil,they're fresh. that's consistent with cutting it open.

[ironroad9c1]

Dont be so concerned with Pressure when the important thing is FLOW, if the lifters are not clattering its got plenty of flow,when you get high oil pressure, you are seeing high resistance to oil flow, you really dont want that. and yes the oil sender doubles as a backup to the fuel pump relay, and not a safety device for shutting the fuel pump off as low oil pressure.

[billnorman]

There is a little goodie in the oil pump called a Pressure Relief valve. It's supposed to hold a ball tight to the oil pump outlet unless the pressure (in a cold motor) exceeds a pre-set value usually 30 pounds. At normal temperatures it still vents this pressure at 30 PSI but a hot oil develops much less pressure so it bleeds less oil off. If this check valve gets any trash in it, or the spring breaks, or weakens or the ball sticks, your oil pressure will be really low when the motor warms up. One way to check this is inspect the oil pump. Remove any trash etc. One other way to fix this (don't try this at home) is hammer a cold motor and try to flush out any stuck stuff with high oil flow. Don't blame me if it blows up.

[diveplane]

sounds scarey :>(