Low oil pressure-BMW 530I

My Bmw has 150,000 miles on it and has been religiously maintained. I'm now getting the "low oil pressure" readout along with the warning bell.
This happens when the car is warmed-up and then parked for a few minutes. Then it occurs at every stoplight. Once cooled down, it does not recur until it is stopped and then restarted. Is this mechanical or just a tired engine??
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What model/year? IIRC, the V8's from the E34's (93-96?) had a problem with oil-pump bolts backing out. If it's a 6-cylinder M30, it's more likely to be a spun bearing on the cam or crank.
Floyd
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fbloogyudsr wrote:

Ot it could just be a bad sensor. Hook a real oil pressure gauge to it before you panic.
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JimV wrote:

Yes, no panic, but at the same time this is not something to be delayed looking into. You could do some serious engine damage if there really is low oil pressure (such as was mentioned with the oil pump bolts).
-Fred W
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It is a 1995 530I. A local told me that 1t had 18.7 PSI at 2500 RPM. Is this low? Why would it be sporadic?
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As I said earlier, I would have the oil pump bolts checked: http://www.strictlybmw.com/newsletter/importantnotices/v8oilpump.htm This problem could cause both low, and intermittently low readings. Definitely don't drive it until you get the problem (whatever it is) fixed.
Floyd
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Charles McMillen via CarKB.com wrote:

We can assume by "local" you mean local mechanic?
More important is what is the oil pressure at idle (600-800 RPM)? The oil pressure light sensor (a switch) turns the light on at 7PSI. If the pressure is less than that with the engine warm - you have a problem.
As far as sporadic - it sounds to me as if you described a pattern, not a sporadic happening. It is OK when the engine is cold (oil is thicker and clearances are tighter = more oil pressure), it shows up after running the engine, warming up the oil, stopping the engine (which transfers more engine heat into the oil - thinning it) and then restarting the engine. It then shows up when you come to a stop light (where we can assume the engine is idling.)
The oil pump is engine driven. It puts out more pressure as the engine speed increases. Oil - especially if you're not using a good quality multi viscosity - or if the oil has been in the engine too long - tends to thin out as it gets hotter - thin oil passing through the same passages as thicker oil = less pressure. Engine bearing clearances get wider due to the expansion of the metal as the engine warms up. The engine is probably warmest when you shut it down hot and then restart it while it's still hot. Wider bearing clearances allow oil to pass though the bearings with less back pressure - lowering the oil pressure. ALL of these can be combined to lower the oil pressure.
Given these symptoms, I have two conclusions:
1. Your oil pressure sender is bad - and triggering the light. It is turning on at more than 7PSI of oil pressure at idle.
2. Something else is worn/wrong in the engine causing the problem. The oil may be the wrong grade/viscosity, or may be old. The oil filter may not be a stock BMW one - and it's causing problems. The oil pump may be having the mounting bolts backing out problem - causing an internal engine leak that lowers the oil pressure. The engine may be just plain worn out due to lack of maintenance. OR all of the above.
I'd try to eliminate #1 first. The part is less than $20USD. It is easy to install.
If this doesn't solve it - try an oil change with fresh oil of the grade specified for the engine and a new BMW filter.
If that doesn't solve it - your mechanic is going to need to do some investigation on the possibilities from #2.
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I'd replace the oil pressure switch first. On both my E28 520 and E34 525 they failed - more than once with the E28. Cheap enough not to worry if it isn't the fault.
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*Ah, I see the f**k-up fairy has visited us again

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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