What does the low oil pressure warning mean on a BMW 7 series?

Answer 1

I've also been suffering the low-oil pressure warning light over the past few months, usually when the car has been running for some time and is good and warm. Whenever the idle slips down to around 500-550 or so (hard to tell exactly looking at the tach), the RED oil-can light flickers or shows almost steady, and I get the warning and chime.

In many other threads I've read with great interest all of the possible causes and fixes, which are very helpful. But I've noticed some of the same people have re-ordered their troubleshooting routines and/or stated, in different posts, what they find to be the "most likely" cause.

The only thing that's NOT arguable is that the oil pressure itself really should be checked, because the possible solution lists are pretty evenly divided by the result ("low pressure" or "good pressure") and it makes no sense to attempt solutions from both lists, as the forums might unwittingly suggest. However, it also appears LOTS of folks do just that, because it's an appointment and also maybe expensive to get a mechanic to check the oil pressure properly.

Given how many say they replace the oil pressure switch with no luck, and given the fact that the "false" warnings only appear at idle--when we would expect oil pressure to drop (but not too much)--I do wonder if conditions that create what I'll call "slightly low pressure conditions" are really the most common culprits. But I'm not here to judge...YOU are.

Help me get this compilation right. If it's all good, I'll try to take extra time to add part numbers and BMW specifications.

BMW E39 LOW OIL PRESSURE WARNING TROUBLESHOOTING GUIDE

1) Confirm correct oil level using oil pan dipstick. If level ok, proceed to (2).

2) Check the oil pressure using mechanical gauge (attach gauge in place of the oil pressure switch) according to BMW specifications.

If oil pressure is good, then proceed to #11.

If oil pressure is borderline/low, then assume one of the following is causing a slightly low-pressure condition:

3) Oil too thin. Try increasing viscosity, perhaps by use of slightly heavier (5w40 or 10w40) grade. Some Festers like glenn2m advocate insisting on LL01 approved oils (for instance, Euro formulation of Castrol). DIY job. Meanwhile, some folks like Catso would caution you that if you are having to increase viscosity to solve engine problems, you may have other problems, like "restricted oil pickup, worn pump, excessive bearing clearance, lots of sludge, etc."

4) Clogged oil filter. A filter with more than 10K miles will have at least some clogging. DIY job; OEM filter recommended.

5) Faulty oil filter housing, which could mean: Oil filter housing cap (often cracked), cap gasket (big o-ring), and (on I6 engines) the cap shaft with the two tiny o-rings. If the shaft tip is broken off or if o-rings are missing, then oil is misdirected. DIY jobs; OEM parts recommended. Note: Always use the correct socket for the cap to remove, and only hand-tighten when done.

6) Worn Vanos seals; per our friend CN90, they allow oil to be misdirected into the VANOS housing and not back to the engine. About 4 hours labor at dealer; complex DIY job.

7) Oil pump bolts backed out/loose. (Generally an issue on V8, not I6). DIY/dealer job.

8) Insufficient hot idle, usually below 500. (Consider checking for vacuum leaks, checking idle control valve, VANOS seals, etc. DIY/dealer job.

9) Sludge. Plenty of people on the Forums are happy to lecture you on that topic!

10) Blocked oil intake on oil pump. Pull the oil pan (good luck on an I6!) and see: Debris in the sump may be blocking the oil pump's screen. Per CN90, could even be broken timing chain plastic bits and pieces. DIY/dealer job.

11) Bad oil pump. Not a high-failure part, however, unless it is fighting sludge. A failing oil pump would also create low oil pressure at other than just idle. DIY/dealer job.

If oil pressure tests GOOD, then assume a false indication to the dashboard red light, and troubleshoot beginning here:

11) Bad wiring between the oil pressure switch and the dash light. Trace it, look for breaks/shorts etc. DIY job; take your time with the harness.

12) Corrosion at the oil pressure switch pin connections. Pretty common due to poor design allowing moisture migration. Unplug the harness (at the sensor), give it a good spray of WD-40 and reconnect and see what happens. Suggest clean/lube with dielectric grease. DIY job.

13) Bad ground for oil pressure switch. Per Qualitas, "As a test, run a wire from the ground portion of the switch and run it back to the negative side of the battery. See if that helps. It seems as though these sensors/switches etc really like good ground." DIY job.

14) Bad oil pressure switch. DIY job; part is relatively cheap; enjoy this rare break.