2009 jcw N14 128000 mls, top end rebuild, valve stem seals chains etc, new oil pump & chain, oil filter housing, vanos solenoid,

I can't see any visible leaks & engine runs fine, no fault codes, done a pressure test..... 12psi (0.68 bar) @ idle. 20psi (1.37 bar ) @ 3000rpm

lost for ideas any thoughts welcome????

 

Are you sure the pressure gauge you are using is accurate?

Also, do you have the factory oil pressure specs? 12PSI ain't necessarily low. If the oil pressure display of my BMW 230i accurate at hot idle oil pressure barely registers.

At hot idle my Hellcat engine oil pressure was 38psi. At hot idle my Scat Pack engine oil pressure was 30psi. And my Porsche 996 Turbo hot idle oil pressure was 1.5 bar (about 21psi). All 3 engines filled with 0w-40 oil (Pennzoil in the case of the Hellcat/Scat Pack) and Mobil 1 in the case of the Porsche Turbo. The BMW 230i is filled with 0w-20 oil the same oil that is in my 2023 JCW. But the JCW doesn't show oil pressure.

Hot in the above case is with the oil at 212F. My Hellcat and Scat Pack also displayed oil temperature (and coolant temperature) along with oil pressure. So I always made it a habit to "check" oil pressure at idle at 212F. With the Turbo I just waited until I was confident the engine and engine oil was up to ~212F.

With the BMW oil temperature seems to race through/past 212F pretty darn quick and settles down at around 220F, although that is a WAG as the temperature display is a bar graph.

Pic:




The flip side is at higher RPMs oil pressure in all 3 engines was around 60psi. But at higher RPMs the BMW 230i oil pressure is still quite low. To get it to 20psi requires some throttle/load on the engine and then the oil pressure goes up but as engine speed builds and load diminishes oil pressure comes back down.

If the oil pressure is truly low one WAG is at 128000 miles the bottom end probably should have been refreshed as well. My thinking is the low oil pressure is from excessive main and rod journal bearing clearances.

 

Here is the official spec for N14 oil pressure:


This measurement should be taken at the pressure switch location, on the head, next to the vacuum pump. Remove the switch, and install your pressure gauge.


While your idle pressure is within spec, your pressure at 3000 rpm would give me concern. Since you replaced the oil pump, maybe you got one with too much clearance, or the oil control solenoid isn't working correctly.

 

BMW state…
0.7 bar @idle.
1.7 - 3 bar @ 3000 rpm

tested with snap on gauge & I’ve now got a mechanical gauge fitted permanently, both give the same reading… am thinking problem is with head somewhere partially blocked?

 

0.7 bar at idle agrees with what njaremka posted. That sort of signals the bottom end is "good". Not 100% reliable but if the clearances were bad enough to effect oil pressure at higher RPMs they would certainly affect oil pressure at hot idle.

If you are seeing at a steady 3K RPMs oil pressure varying from 1.7 bar to 3 bar that's quite a variation. A blockage is possible. But the variation suggests something else.

But a leaking pressure bypass valve can also result in low oil pressure.

Another explanation is if the engine is fitted with piston oil jets the valve that controls the amount of oil the jets get may not be working right and supplying too much oil to the pistons. And the low oil pressure is the result.