What would cause a stock high mileage SB Mopar 360 engine, that had always run fine, have zero oil press one day, following a day of driving with normal oil pressure? I recently ran into this, so I put a mechanical gauge in place of the electric one, disabled the ignition and spun the motor over and it showed 40 psi pretty quickly. Great, I figured the original factory sending unit was bad so I bought a new one, but put it in a T fitting so I can have a mechanical back up gauge. Started it up and all was good...for that day. Let it set for 2 days and now it again has zero pressure on both gauges. What the heck is going on here? I've never seen or heard of this. Is there something in the oil pump itself that can cause this pressure then no pressure condition? I'm stumped. Need input!!

I would replace the filter

If an old engine, it could be some built up crud in sump messing with the flow.

chunks of nylon from the timing gears have been known to do this exact thing. The nylon flakes off over time, the pick sucks it up tot eh pickup screen, flow is cit off, you shut down and the flakes float away only to do it again later.

I suppose a sticky bypass in the pump itself could do it too

The sticky bypass valve is what I'm leaning towards, but I have not eliminated any of the possibilities as of yet. Either way I'm going to have to pull the pan to see what's in there. I will report my findings whether I fix it or not. Thanks for the input.

As said I would suggest changeing the filter first. EDIT then I would take off/check/clean the 90 deg filter adapter if it has one on your app. then I would preoil with it still off & see if it gushed out the hole especially if you can preoil it at a time when it has zero psi & all that is left is the pickup plugged (likely at that point) or pump bypass. I'm thinking clogged pickup but holler what it ends up being. With a flat circular plate you might also preoil it & see if it flows out the small holes. Actually I would grab a helper & have him undo the filter/plate/bypass while you are preoiling & do this at a time when it is showing zero psi, anything to avoid pulling the pan if you dont have to tho I ain't thinking of anything up top that would make this intermittent once it makes the horizontle turn back to the vertical feed.