Yeah I didn't really say that correctly. Yes the pressure should drop to zero as soon as the pump stops, unless there are trapped air pockets in the system, and even then the pressure would drop very quickly. I was trying to show that there should have been no reason for the gauge to indicate pressure. I assume the most everything downstream of the filter head drains back into the pan, but I don't see that as much more then a quart or two. If that is correct and the filter and pump stay primed then the passages should fill up quickly and the pressure should start to rise once the main passages are full.Yes I would say its bleeding off... But not pressure as pressure is gone as soon as the pump stops. But the oil from the lines drains back to the pan, which is why your supposed to wait 15 minutes before checking your oil after you shut the truck off...
The style pump that the oil pump is is more effective as rpms increase, meaning you cant slow it to 70 rpms and get 1/10 the flow as 700 rpms... It doesn't produce the suction needed.
Since the pump is a positive displacement type the only thing that changes how much volume it puts out per revolution is any internal leakage (or air or aerated fluid). This leakage rate will depend on how tight the tolerances are in the pump, the pressure difference between the pump inlet and outlet and the viscosity of the liquid being pumped. That being the case, at startup with cool, relatively thick oil and little or no pressure on the pump's discharge I would assume the internal leakage is not that bad, though maybe thats not a valid assumption. Do you have a pump curve (graph of the pump flow rate) or other pump output data for our oil pumps?
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