Greetings and welcome to Just Answer. My name is Mark.
Only making a suggestion here because I have been here and seen it done. There is an 0-ring that seals the pick-up screen to the oil pump. It is VERY IMPORTANT that this 0-ring is installed correctly, or the exact situation you are encountering will happen. At an idle, the oil pump will just suck air and you will lose oil pressure. Once you pick-up the idle, the oil pump spins faster. Yes, it is still sucking some air, but it will pick-up the oil too. Now the lifter noise goes away until you bring it down off idle.
I know you start out your post with your vehicle losing oil pressure suddenly, and then finish up with the oil pump being replaced, but I have seen these o-rings split and fail on an otherwise normally running engine.
I am going to assume your mechanic took an oil pressure reading BEFORE he replaced the oil pump. There is a lot of work to replacing the oil pump on one of these, and if your mechanic is not familiar with installing the pick-up screen correctly, he could have repaired your original issue, and created another.
I have seen these 0-rings fail and create the kind of issue you're talking about. The repair is not the oil pump, as much as it is the pick-up screen o-ring. If initially the mechanic takes an oil pressure reading and it is low or non-existent, the first step is to remove the oil pan and then remove the pick-up screen. If one finds the 0-ring to be the issue, then there is no need to replace the oil pump, as it requires removal of the engine front cover.
Below, I have included the repair and pictures of the pick-up screen and pump. I would ask your mechanic if he indeed remembers installing the 0-ring SECURELY and CORRECTLY. I would then put an oil pressure gauge on the engine and verify you have no oil pressure. I know by your description with lifter noise and such, you do not, but you want to make sure.
If you had an engine bearing issue, the characteristics usually are different than what you are describing. Oil pressure may come up a little bit upon RPM increase, but lifters don't normally bleed down as much as you're indicating. Oil pressure is created and maintained by bearing clearance. As engine bearings wear out, oil passes through more easily and thus, lower pressure. With worn out bearings, there usually is some engine bearing noise associated.
I am not saying you don't have more serious engine issues, ... I am only stating by what I am reading here, it sounds to me like the o-ring issue. It is worth a check see. Oil Pump and Pick-up Screen
- Remove the oil pan.
- Remove the engine front cover.
- Remove the oil pump screen bolt and nuts.
- Remove the oil pump screen with O-ring seal.
- Remove the O-ring seal from the pump screen.
- Discard the O-ring seal.
- Remove the remaining crankshaft oil deflector nuts.
- Remove the crankshaft oil deflector.
- Remove the oil pump bolts.
Important: Do not allow dirt or debris to enter the oil pump assembly, cap end as necessary.
- Remove the oil pump.
- If required, clean and inspect the oil pump.