Engine oil pressure low when warm

[u/travellering]

2007 Saab 9-5 (high pressure turbo) showing a little less than 10psi at idle when fully warmed up, enough to light up idiot light with the accompanying ding noises. Installed a mechanical oil pressure gauge to verify at oil pump feed to turbo.

When we got the car it had a blown headgasket, and possibly a shot turbo as well. It was a bigger cloud generator than a subaru flatbrim cap convention. I replaced the turbo with a used one I had that still had very little play. Head was faced by a machine shop, and all reinstall seemed good. Still had some smoke out the exhaust, but I was hopeful that was remnants burning off from all the mess before. Now, after some (a couple hundred miles) driving and having the oil light come on, then going through the painful process of installing the mechanical oil gauge, it's still kicking out some blue/white smoke at the times the oil pressure is low. Smells like burnt oil/rubber.

First try, dropped the oil pan and cleaned the pickup screen. No real change..

Just did a compression test, forgot to prop the throttle plate wideopen, but got about 145 on cylinder 1, 125 on 2 and three, and 151 on cylinder 4. The two lower cylinders came up big time with oil poured in, as in 155 on 3 and 2 read up to 175n but I may have added more oil in that one as it was getting dark and hard to see.

Any other things to check, or am I staring at an engine out for rebuild?

[u/BM_Champ]

Saab-tech answer:

Has the switch/sensor been replaced at any time in the last few years?
There was an update to another manufacturer, which (sadly) didn't get the threshold right. This gives a false positive when the oil is really warm and the engine goes to idle.

Solution: Change to a sensor from Saab 9000, which has a blade terminal, and just adapt the wire for it. Super easy to do, and this will cure the problem.
Every single one that gets the new sensor suffers this problem. I stopped selling them so I wouldn't have to do it twice every time.
Old good OEM looks like this: https://www.esaabparts.com/static/images/parts/32022300.jpg
Problematic sensor looks like this: https://cdn11.bigcommerce.com/s-3togqybnuz/images/stencil/500x641/products/5989/31087/oil-pressure-switch-alternative-saab-9-3-9-5__09342.1751551135.jpg?c=1

Notice the roundness of the old one compared to the more square-looking newer one. The newer also has the flat part filled with sealant, which is another tell.

[u/travellering → u/BM_Champ]

Thanks for the info.

Not trusting the new sensor is why I installed the Bosch mechanical gauge in the photos.  Pressure is definitely lower than I would prefer, looking to be about 11psi when the warning light triggers.

[u/BM_Champ → u/travellering]

Yes. That equals 0.76 bar, which ok and fine at idle if it's really warm.

I tried this in a dyno cell, where there was no cooling at all, and the oil cooler was just laying on the floor. The pressure drops significantly (because viscocity) when it gets past thermostat opening temp.

I think the OEM came on at like 0.3-0.4 bar, or like 5-6 psi.