I am no expert on anything, just thinking in type, in the hope it might help you Jeremy..
Just thought to cover one of my original thinking re the possibility of your pressure relief valve perhaps being the issue..?
As I initially typed, I thought that unlikely as being the cause of reducing normal oil pressure, I imagined it would require to be stuck in some degree of being OPENED, g iven I expect in normal operation the valve would be fully closed..?
As I typed in my earlier ramble, the only time I think the valve would be forced opened is when the oil pressure rises enough to force the valves plunger off it`s seating and thus opening a relief passage to discharge the excess of oil (pressure) back to the sump..? The plunger spring should automatically shut off the relief passage as the pressure begins to drop back to normal. I suspect the relief pressure to be set not much above 65 PSI at a guess.
For the relief valve to be stuck opened by any degree it would require to have experienced oil pressure above it`s "relief" setting, which is more likely to occur with cold engine oil and high revs combined.. Other than that chances are that if the oil pressure did not by some chance rise above the relief setting, it seems unlikely that the valve might be stuck opened..?
I did read somewhere where a recently rebuilt engine suffered low oil pressure relative to some swarf finding it`s way into the relief valve, but without the full story..???
I have known of relief valve settings being adjusted by installing shims under the nut holding the spring and plunger assembly in place, which is of no use to you.
In terms of the pump perhaps having lost it`s ability to self prime, I have read of the pump gears being packed with grease to assist priming, though think there is a potentially simpler method which I have used successfully used on two occasions, once on a marine diesel and the other example typed of below.
If you can forgive a bit of a nostalgic ramble...The first car engine I ever rebuilt (BMC A Series) suffered zero oil pressure on first start up, Lorne may remember that I shared the story many years later on eMog... Fortunately I had smeared all the bearing surfaces with oil during the rebuild.
The problem was suspected to be caused by the pump requiring to be primed. In an attempt to resolve the issue I removed the oil pressure relief valve spring and plunger and brass retaining nut all of which were in the side of the engine block, which then provided me with an opened passage to the oil galleries, being a less than well equipped yoof working in a lock up garage with no power and a couple of miles from home, the only tool that came to hand was a lever action oil can (every Halfords will have one) trying to create a seal from the nozzle of the oil can to the now open access to the oil gallery was achieved by using a bit of rag as a seal.... Hardly engineering practice.. but it worked, and that in time the increasing resistance felt on the lever action of the oil can, confirmed that oil was going somewhere, and not leaking out past the rag sealing to any great degree.. Much to my surprise, I was able to pump enough pressure into the gallery to quickly view the dash and see the needle that formerly has not moved, now dropping back towards zero as the result of my pressure inducing via the oil can alone.
In old style non-electric gauges, when the ign was turned off after running, one could witness the oil pressure reading dropping, not so on electric instruments as turning the ign off kills power to the instrument.
Reassembled the oil pressure relief valve set-up and fired up the engine....problem resolved... Boy what a relief..
OK so possible differences for your situation...? I guess access seems likely to be an issue, although some oil cans come with a flexible nozzle. Should you seek to use my method, perhaps if you removed the oil gauge sender unit, it might provide easier access to the oil gallery. as opposed to the relief valve assembly.? ... Also that your gauge is electric would require the ign to be on for you/someone to check if the oil can method is indeed increasing pressure in the galleries..?
I guess that the main thing is that the pump should be primed to some degree....?
As you have run the engine with no oil pressure till it rattled.....seems to suggest definite loss of oil pressure, thus logically not a gauge or sender issue.
With everything reassembled and oil tight...
Rather than risk creating further damage, you might consider ensuring your battery is well charged, and removing the spark plugs to then spin the engine over on the starter motor, which should be enough for you to be able to monitor pressure rise on the gauge..?
Perhaps best to disconnect the ign coil primary, to reduce the risk of damage caused by sparks seeking a path to earth..?
On the other hand if you are feeling confident you could just go for it and fire it up...????????
Wishing you the best of luck regardless whether you might use my method or not... Looking forward to reading you are back up and running Jeremy.
