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So after looking at the set up of the oil pump, I came up with this:
1) Previously the plastic chain guide railing broke and was broken into small pieces.
2) I removed most of these pieces, but a small piece was likely pulled into the pump through the oil galley circled in the picture due to the close proximity of the railing and the galley opening.
3) As the pump moves the oil and forces back the pressure valve the piece probably went past the plunger and is lodged between the plunger and where the plunger rests against the back of the passage, thus keeping the pressure valve stuck in the open position.
4) As the car sits for a while the oil slowly flows back down to the pan and allows the plunger to close a bit more, but not fully due to the piece that may be preventing it from fully closing. So when I start the car up again after it sat for a while the oil pressure will initially be up again until the valve starts to fill and the additional oil and the possible broken piece of railing cause the valve to open more than normal, thus dropping the pressure of the oil.
This is of course just a theory based on the symptoms, which is only the low pressure reading. I suspect that these cars will put out well over the amount of oil than what is required to keep the components lubricated. 3 bar is about 44 psi to begin with and that can be achieved at about 1500 rpm, so I wonder what the pressure reaches at 4000 rpm. Anyways, once I get the oil pan gasket, and probably the mounts, I'll drop the pan and see if this is indeed the issue. Also, I did test the cluster gauge with an ohmmeter and everything is working as it should.
**Edit**By the way, note the mesh filter at the opening of the oil pump. It feels like heavy duty chain mail. I understand now why the plastic railing was used: to prevent catastrophic damage to the engine components same as the plastic ends on the radiator. I found the later out when my engine reached about 120C while I was unable to pull over or shut it down and I heard this loud explosion under the hood. The radiator plastic had over pressured from the high temp and coolant was everywhere.
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1990 190E 3.0L
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