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I have about 20k miles on my rebuilt motor and now have low oil pressure light coming on at idle warm. I have dropped the oil pan again re-cleaned the pickup tube (not much was on it) replaced the o-ring it and the crossover tube. Rolled in new crankshaft and rod bearings. (The bearings looked good but I had already bought them so did it while I had the pan off. I forgot to plasti-gauge to confirm clearances). I swapped out the one at a time the oil pump and the oil pressure relief with used spares. Nothing has looked bad but still had low oil pressure at idle when warm. I installed a glowshift oil filter sandwich and a digital oil pressure sensor. I ran the car for a little while last night and was not happy with the results.

The oil pressure at idle fluctuates from 25 psi to 5 psi. When I drive the car it goes up to 50 or so psi before somewhat stabilizing but it is not a consistent value like I would expect. I do not have a high degree of confidence in the gauge since this is the first time I have used it. It seems to me right now that as the engine rotates somewhere in the engine there is something that is allowing a lot of oil loss. If it was a bearing I would expect the oil pressure to be steady at a lower value. I do not really want to have to pull the engine again and go back through it if I do not have to. I am hoping someone might have an idea as to what could be causing these symptoms. I am leaning toward something like a valve tappet being bad. I do have some tick that I think is a tappet but that could be caused by the low oil pressure and not the cause. I am not sure if a failed hydraulic tappet could even cause low oil pressure. What ever it is seems like it changes through out the engine rotation.I am honestly at the end of my experience to figure this out.



A little history. 2006 Saab 9-5 wagon. I bought the car with a known blown engine. It ended up being a busted piston, bent rod, blown turbo, etc. I pulled the engine and rebuilt it myself.
At a local machine shop I had.
1)Crankshaft checked and polished.
2)Block cleaned, checked and honed. (New balance shaft bushings installed
3) Head rebuilt
I put new parts on the engine as I went including Forged pistons. All new bearings seals, new oil pump, new timing kit, oil trap and hoses. As well as all new sensors (oil pressure, crank position).
I have also redone pretty much the entire suspension, sub frame, struts everything.
Recently heater core, followed by radiator and coolant cap.
My intention was to have a car I would not have to work on for a long time and be a reliable comfortable replacement for our 2001 9-5. I feel like am very close to having the car we wanted but I am about at my wits end.
 

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I'd take a good hard look at the oil pressure relief valve and maybe even just replace it. I had a similar issue with my 02 when I first re-did it that every now and then the bing-bong would happen and the oil light would go on. If I shut the engine off and re-started all was well and then some unpredictable time in the future it would happen again. I replaced the oil pressure relief valve and never saw the problem again.

you've already replaced everything else that I would suspect and you are right that if you had a bearing issue you would have it all the time when the oil was hot. Same with the oil pump.
 

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I did swap the relief valve with one I had from another parts engine. I did not seem to change anything. I am planning to pick up a mechanical gauge and verify pressure readings. I may try putting the original back in and see if there is any change from one to the next.
 
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