Strange oil pressure problem on rebuilts 996 3.4 engine

So last year I rebuilt the engine from my 2000 996 C4. I did not due the Nickies but pretty much everything else, bearings, rings, gaskets and on and on. Last November I finished the rebuild and put the engine back into the car. I used the Driven BR 15-50 Breaking Oil. Before starting it I turned it over with the battery and it was quickly showing 3-4 bars just spinning with the battery. The I started it up and cold it was showing about 5 bars. Ran it a bit while I addressed some loose coolant hoses etc. Once I got those sorted out I got it warmed up a bit but did not drive it. The seats, particular the Driver's seat leather had totally disintegrated over the years, and I had picked up a seat of 997 seats, so I parked the car and started putting them in the car so I could drive it. This turned into quite an adventure as they were full memory seats and had to be adapted to work in the 996. The passenger seat was really easy, but the driver's seat was much more of a challenge. I finally got all the functions working including the lumbar and put the driver's seat in a week or two ago. Started it up and everything seemed to be fine but truthfully, I was not closely looked at the oil pressure gauge. I took it for a drive around the block and the brake fluid light can on a I found one of the lines to the rear had rusted away so I could not drive it until I fixed that. It took several days to do that as finding a took that could properly make bubble flare was a challenge but that is now done. So then I went to drive it and the oil pressure gauge took several seconds to so any pressure and now only goes up to 2 or 3 bars. Far lower than it was before and too low to drive much. I check the sensor and one wire was not connected. Connecting it help some but it still takes a few second to show pressure and it is still low. Looking for any insight on what the problem my be. I have a new sensor and I am going to put it is and see if that makes a difference, maybe the old sensor has failed. I am also thinking about putting in fresh oil as I have a case of the Driven 15-50 sitting here. Any comments?

Pull the main oil pump by-pass valve out, and make sure it isn't sticking or the spring is broken... If all is OK there, then install a real oil pressure gauge to see what it says..

Porschetech3 Thanks for your response. I have the 997-bypass valve that I put in years ago. II did check it and the spring is fine. So I removed the pressure sensor and put in a gauge. On start up it goes up to 3 bars or so within a few seconds. Put when I raise the revs at first the pressure drops about a bar or so, then comes back up to 3+ bars but with more revs it does not go up further. I think I am going to remove the pan and see if there is anything blocking the pick up. I am also going to check the installation of the scavenger pumps as back in 2009 when I had the heads off to fix a cracked head I had them reversed and that caused problems as the scavenger pump were not returning enough oil to the sump. once I changed the orientation of the scavenger pumps that problem was gone. I can't believe that I installed them wrong last fall having made this mistake back then, but it is easy enough to check it. Any other ideas you might have would be appreciated.

I don't think the scavenge pumps are your main issue here because you have low pressure on start up....Scavenge pumps inop would not show up on cold start due to low oil flow and thich oil, it would take a little while before that problem would show( but is easy enough to check and verify).. Checking the pick-up tube screen is a good idea, but I think your problem is a "crushed oil filter".. I have seen many times that people being super careful on rebuilds will use too much assembly lube. Too much assembly lube can stop-up certain oil filters and they will crush...so I would check that first..

That is an interesting idea about a crushed oil filter. I should not that I have had the LN Spin on Oil Filter Adaptor and have been using the NapaGold 1042 for years. But I think I will run over to the Napa store in the morning in the morning and grab a new filter. I will take less than 10 minutes to swap the filter and if that fixes the issue than that would be great. If not then I will drop the oil pan, that will take maybe 10-15 minutes to take it off, of course putting it back on will take a little longer than that. Again, thanks for your insights.

+1 on the crushed oil filter. Had a similar experience on my second rebuild. Max pressure was reduced to about 3 bar. I think I went a little heavy with the ARP head stud lube. Jake's theory was the oil pressure relief valve got stuck closed, and high pressure crushed the filter. Sure enough the pleats were collapsed when I cut it open. Replaced the filter and oil (switched from the break-in oil to HR5), and was back up to 5 bar.

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I have seen this occur in very cold weather with too thick of an oil (10w60), but it has never been with a Napa Gold 1042. If I remember correctly, it has only been with K&N Gold and M1 oil filters which are too restrictive.

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Good idea to check the orientation of the pumps. The other things I have have customers report back as causes for low oil pressure are a blocked oil pickup or missing piston squirter. These are the most common mistakes behind the orientation of the scavenge pumps being wrong.

Bingo on the oil filter! I put on a new Napa Gold 1042 and within a second or two I had 5 bars at idle. I am going to put back on the old oil pressure sensor and drive it for a while. Thanks to everyone for their help and suggestions. I will keep you posted.

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Great to hear it was a simple fix! Out of curiosity, where did you get the defective Napa Gold filter and does it say where was it manufactured?

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Glad to help.... Its always good when a $10 part/10 min labor can fix a low oil pressure issue, and I don't think you had any bearing damage either, you were very cautious and observant

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it was last fall when I bought the filter and I am pretty sure it was at the local NAPA store. It says that it was made in the USA. I am going to see if I have a pipe cutter that is large enough to cut the cover so I can see what the filter looks like. Looking into the filter I can�t see anything that looks unusual. Before I put on the spin on adaptor I always used to cut open the old cartridge filters to look for debris, etc.