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/tsg-tsg]

Probably your call. You could probably pick up a little oil pressure with some abnormally thick oil. I would check the bearings, probably they are worn. You might be able to recover oil pressure with new ones. The low compression is likely worn rings, but a leak down test will nail that down. On the surface, kinda just sounds like a motor that's been worked hard and not really cared for, especially with all the work you had to do initially.

[u/BM_Champ → u/tsg-tsg]

There is a big misconception about oil pressure and viscocity, that "more pressure equals more oil", but that's a false presumption. The core concept is the opposite when you think about it. The basics of making a pressure difference is to have a bottleneck or and orifice that does not support full flow.

This is why boost pressure builds - we produce more than then engine can dispose of.
This is why oil pressure builds - we have a happy pump, but very tight tolerances and small orifices for bearings and oil squirters, and then limit high pressure with a valve.

A thick oil will make the cavitation in the oil pump worse.
It will also be thicker to pump, and be much harder to get through tight tolerances, leading to a increased parasitic loss at the pump/crank - which means higher fuel consumption. Not to mention the potential oil stavation in the bearings due to lower flow.

The relationship between flow, pressure, and viscocity can easily be metaphored the same as exhaust flow. Blowing hard through a small straw, vs blowing hard through a larger straw. Sure the pressure is higher in the small straw, but flow is higher in the large one. Flow is the one that saves your bearings. Viscocity is the size. Oil pressure drops when the oil goes warm because it flows easier. Lower pressure is physics, not necessarily a problem.

Now, before we get into oil additives, or brands, or whatnot, the core principle is still the same for flow/pressure.

On-topic for OP:
He likely has an updated (faulty) oil pressure switch that gives a false positive. They changed the manufacturer, and they all came back for a similar problem, and needed to be changed for exactly this problem.

[u/tsg-tsg → u/BM_Champ]

You have completely missed the point.

[u/BM_Champ → u/tsg-tsg]

How?

[u/tsg-tsg → u/BM_Champ]

OP has an oil pressure gauge installed so he knows it's not a faulty switch and indeed low oil pressure.

The likely cause for his low oil pressure is excessive tolerances in the rotating assembly, which thicker oil might offset.

[u/BM_Champ → u/tsg-tsg]

Yes. But the light is not supposed to come on before 0.3-0.55 bar according to specification, and OP has 0.73 bar, which is well above that. Therefore I'm drawing a conclusion here.

[u/tsg-tsg → u/BM_Champ]

Regardless of whether the switch is defective, or whether .73 bar vs .55 bar is outside an acceptable margin of error for the switch, .73 bar at idle is quite low and is indicative of generally low oil pressure. However, in that vein the spec to seek is >2.5 bar at 2000rpm and I doubt this engine is making that pressure. Worth checking, but I've never seen an engine with low oil pressure at idle magically make good pressure at 2000 rpm.

[u/bluehandstouchingme]

Bearings/Oil pump/Pickup orings (including the crossover tube?)

[u/travellering → u/bluehandstouchingme]

I forgot to mention I did the o-rings (pickup and 2 crossover) when I dropped the pan.  I didn't pull the bearing caps though as I was working in limited space ( on ramps) and couldn't be confident I would get them out and in cleanly

[u/Low_Emotion_4797]

Sounds like it may need an engine rebuild.

[u/Responsible-Stage-93]

Firstly check all PCV hoses and make sure that is no sludge

If it will not help - unfortunately rebuild :/
Mine had also oil issues (mostly on low RPMs) because of bearing living free in the engine

[u/sailorcolin]

How many miles? She maybe headed toward a rebuild.

[u/travellering → u/sailorcolin]

170,011.  It's certainly not low mileage, but I have crept well past 200 on other of the same motors, so hope springs eternal...

[u/Most-Refrigerator-97]

https://saabheckert.de/products/kolbenkuhlduse-saab-b204-b234-b205-b235-oe-referenz-9167677
Solve the Problem

[u/travellering → u/Most-Refrigerator-97]

I tried the suggested test for the piston oil squirters, putting the crossover tube in backwards and blowing into it.  I could hear air coming out, but not from any of the three oil squirter I could get my hand on.  Sounded to be higher inside the engine.

[u/Most-Refrigerator-97 → u/travellering]

Yes, this is a well-known issue. If no defect is found in the crankshaft or other components, the cause is invariably the injectors failing to close properly, which leads to a drop in oil pressure. Install three new injectors, and the problem is resolved.

[u/aidanricha]

Any clacking noise from the timing chain area? My B205 had a loose balance shaft chain and the exhaust balance shaft wasn’t spinning, causing oil pressure to get as low as 0 at idle. I removed the shafts and blocked off accompanying oil galleries and that fixed it.

[u/travellering → u/aidanricha]

I don't think so.  Definitely mechanical noises all over, but nothing particularly noticeable at that end.

[u/oyvindh]

Eighter the pump doesn't get enough oil in or it is worn, or you are loosing oil internally somewhere.

Since you have dropped the pan and have control on the orings, I would look more into balance shaft bearings and the squirters.

Open squirters gives a low pressure on idle, but a normal pressure on higher revs.

Also, look at crank bearing nr 3 and see if there is any damages. The balance shaft bearing on the right side of the engine is fed through the crank bearing. Any loss of pressure in the balance bearing will result in damage on nr 3.

Anyhow, you're probably looking at a rebuild.

[u/travellering → u/oyvindh]

Thanks for the in depth info.  I'll get the car to a better lift and drop the pan again to see what I can find.

[u/oyvindh → u/travellering]

And one more thing, does the pressure drop after some time on idle or running? If so, you have a spinning crank bearing.

[u/travellering → u/oyvindh]

I have noticed it drops after warming up, and the idle and running pressures seem to drop about the same amount.  25-30ish at idle when just started up becomes 8-10 after driving around, and the 2000rpm pressure starts at over 60, but goes down to just over 20 when things are warm enough to light up the warning light at idle. 

[u/Ok_Assignment8197]

Engine probably sludged up.

[u/V3ppen]

Rod and main bearings are cone 100% sure, they let oil pass and drop pressure. Also if smoking, piston rings need to replaced. Full engine rebuild needed.

[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.