Low oil pressure?

Duane said
Well this is only my opinion but I believe without the plug the oil would loop and have no resistance so it builds no pressure. With the plug in there it forces the flow path to go through the filter and then into the motor where it encounters resistance. The pressure then would equalize on both sides of the plug which may be why the plug does not blow out. Just my opinion. The book may be wrong, I just wanted to share another reference.

I really cannot explain why that statement is there in that book. It really does not make sense, once the pump is runinng and all air is purged. BUT, if the plug is missing, then the anti-drainback feature of these filters, will be mostly defeated; oil can all drain back out through the pump and pickup and leave them 'dry'. I suspect there will be a delay in pressure build up at start-up in this case, and that may be the real issue that the FSM contributors were trying to address. Chrysler has used anti-drainback filters in all sorts of engines for a long time.

The filtered path and the direct passage (that normally gets plugged) are 2 parallel paths into the main oil gallery. The oil will not flow in a circle for these 2 simple reasons:

Even with large passages, ANY flow will create a pressure drop in the passages; so pressure will be lower at the top, where these 2 paths rejoin; it may be just a fraction of a psi lower, but it WILL be lower. To say that oil will flow backwards through either passage means that the oil has to flow from that lower pressure point back to a higher pressure point (the pump output). For those who understand hydraulics, look at it in terms of where the pressures are higher or lower, and it will make sense that this circular flow will not happen.
If the oil flows in a circle and does not enter the engine, then it means that the pump flow drops to zero once the circular passage is filled with oil. These oil pumps are positive displacement pumps, which means that they WILL pump flow, even into the wide open air, with no back pressure. So the above idea cannot happen.

Duane, to answer the plug retention question, the pressure will never equalize on both sides of the plug. When it is in place, there will be a pressure drop from the inlet side of the filter (below plug) versus the outlet side of the filter (above plug). The ledge against which the plug is seated is what keeps the plug from moving, or blowing out, as you say.

Gravity does not enter into this in any meaningful way once the pump is running. You have about 0.5 oz of oil in that vertical passage where the plug resides, and that would crate a pressure of difference of around 0.2 psi top to bottom.