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Hey guys, I am lost right now and would appreciate any input or thoughts as to what my issue could be. After losing oil pressure, I replaced my oil pump with a Melling high pressure unit. Upon startup I had great pressure and went for a drive, after a mile I lost oil pressure again. It gradually kept going down and I shut it off and pulled over as soon as I could, by that time it hit about 10-15 psi cruising and you could hear the lifters. Same exact thing it did last time. What could my issue be?
Sounds like the oil pump was not your issue. Oil pumps create flow not pressure. Oil pumps *withstand* pressure. Pressure is caused by the resistance to flow through a restriction such as the bearing clearances in your engine (crank mains, rods, cam bearings, lifters). If you have low pressure and the pump isn't the issue, (its usually not unless it sucked up pieces of bearings first and it wasn't the issue to begin with) then you have a reduced restriction in the system such that flow from the pump is being lost too easily back to the sump either through a leak between the pump outlet and the oil galley, or the bearing clearances have increased due to premature wear. The sound from the lifters and the fact that the oil pressure sender fails to indicate max range oil pressure not zero is pretty damning evidence for your engine. The oil has no reason to pump up the lifters because it can too easily fall right out of the crank mains or rod bearings.
EDIT to clarify my note about the pressure sender may be unclear...the failure mode of the pressure sender is taht it shows MAX pressure all the time when it breaks. It will not break in a way that shows 0 psi. It was specifically designed in this way to not cause panic when it eventually fails.
Sounds like the oil pump was not your issue. Oil pumps create flow not pressure. Oil pumps *withstand* pressure. Pressure is caused by the resistance to flow through a restriction such as the bearing clearances in your engine (crank mains, rods, cam bearings, lifters). If you have low pressure and the pump isn't the issue, (its usually not unless it sucked up pieces of bearings first and it wasn't the issue to begin with) then you have a reduced restriction in the system such that flow from the pump is being lost too easily back to the sump either through a leak between the pump outlet and the oil galley, or the bearing clearances have increased due to premature wear. The sound from the lifters and the fact that the oil pressure sender fails to indicate max range oil pressure not zero is pretty damning evidence for your engine. The oil has no reason to pump up the lifters because it can too easily fall right out of the crank mains or rod bearings.
EDIT to clarify my note about the pressure sender may be unclear...the failure mode of the pressure sender is taht it shows MAX pressure all the time when it breaks. It will not break in a way that shows 0 psi. It was specifically designed in this way to not cause panic when it eventually fails.
It is possible to have a bad Oil pressure Sender and have it at zero, this happened to me.
Regarding the actual issue, because it is getting a reading and you do in fact hear the lifters getting loud, it's safe to say it really is low pressure. I also had this problem and it turned out to be a cam bearing that walked out bleeding all my pressure. It would get down to about 6 psi at idle. Did you do any other engine work or just an oil pump?
It is possible to have a bad Oil pressure Sender and have it at zero, this happened to me.
Regarding the actual issue, because it is getting a reading and you do in fact hear the lifters getting loud, it's safe to say it really is low pressure. I also had this problem and it turned out to be a cam bearing that walked out bleeding all my pressure. It would get down to about 6 psi at idle. Did you do any other engine work or just an oil pump?
O ring at the pick up tube to pump, barbell at the back of the engine, or the oil tube pick up in the pan issues are just a few thoughts...
Help! low oil pressure problem
