You guys are correct about the bypass valve at the filter NOT being the culprit. I have to apologize about that; I was picturing a different engine plumbing in my head when I was writing. Sorry.
Do consider the pressure relief valve in the pump itself. It has a metal-to-metal valve, that, if not seated correctly, will result in a steady stream of oil constantly being dumped back into the pan. The spring behind it controls the maximum pressure, which you did not say was low. So there is probably a constant leak on the pressure side considering the pressure goes up in relation to the rpm; it's just lower than it should be all the time.
The pump casting could be cracked or the sealing surface between the pump and the block isn't sealed. You should use a hard-paper gasket there for street applications. On some engines, the factory doesn't install a gasket there, but the L6 should have one. You can remove it only if the surfaces have been planed or ground flat. I've never seen a rebuild shop do that on a stock motor. Even a little nick in the metal can screw it up.
There is also an outside possibility that the top cover on the pump has blown it's gasket. That thing is very thin paper, and rarely goes south, but is still a possibility. All in all, you will likely have to drop your pan to find the reason for the low pressure.
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David
newbie #4153
