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I have changed the oil a couple of days ago, and I've noticed that the low oil pressure light stays on longer than normal. Normally, it goes out almost immediately (<1s) after starting the engine, while it sometimes takes up to 4 seconds now after the oil change. This only happens during a 'cold start', not when the oil is already warmed up (then it goes out immediately).

The oil level is fine and stable, there are no leaks, and the oil is a high quality 5w30 oil that is specifically recommended for the car; the oil viscosity is identical or nearly identical, I believe, to the previous oil (it was either 5w30 or 5w40). The oil filter I used was a high quality Bosch filter. The car works perfectly. Again; this only happens during a cold start; never during a 'warm start' when the engine oil is already warmed up, nor during driving.

I don't think it's a big deal, but I would like to understand why this is happening. I pretty much rule out a malfunctioning oil pressure sensor, since it only started happening after the oil change. Oil viscosity also seems to be okay, oil levels are fine, the oil and filter quality are both okay,...

I've thought about the clean oil being thinner than the dirty oil causing this rather subtle issue, but I have no idea whether this is true, let alone whether this can cause such a warning.

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have you solved the problem? thanks –  Peter Aug 8 at 15:03
    
@Peter I haven't really done much to it yet. The warning light still stays on for a couple of seconds; I'll wait till the next oil change to see and see what that does. –  Ben Aug 9 at 18:01

2 Answers 2

up vote 2 down vote accepted

It sounds like you are getting drain back. When your engine sits without running for a period of time, most all of the oil in the oil passages drains back into the crank case. When you restart your engine, the oil has to be brought from the pan back up into the oil passages, then you see your oil pressure raise. Since I don't know which year your BMW is, I cannot tell you if there is an oil filter for your car which has an anti-drain back feature. If so, this will ensure oil stays in the oil passages and you'll see oil pressure right off the bat, or at least within the second you are accustomed to. If you recent changed which type of filter you use, you may want to go back to that style/brand of filter (unless that filter is a Fram filter --- plenty of stuff out there on it, but here is a good example with pictures of exactly what I'm talking about).

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The car is from 2001. How long does the car need to sit without running for something like this to happen? I use the car daily. It's the first time that I did the oil change myself, but I'm pretty sure that Bosch filters have always been used; they seem to be the standard here. If the drainback is really the problem, is a 4-second delay enough to cause premature wear and is it thus something to worry about? –  Ben Mar 30 at 21:11
    
I've checked the website, and they do claim that there's an anti-drainback feature in their filters (see boschautoparts.com/filters/pages/…). Is it possible that an incorrect fit causes the drainback feature to fail (even though I'd think that an incorrect fit would mean that it would leak oil)? –  Ben Mar 30 at 21:36
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If left overnight, the drain back has enough time to occur. It will actually continually do this, it's just that it happens over time. Say if you left it sit for six hours, it may take two seconds to fill the passages versus the 4 seconds you are suggesting right now. I don't want to tell you that damage will not occur, but it is very slight. Over time it will add up, though. It could be that the anti-drain back is not working correctly in your current filter. You may give it until your next oil change and see what happens then. –  Paulster2 Mar 30 at 21:49
    
Looking into this issue a bit further, is it possible that the oil filter is upside down (it's a paper filter, not a canister)? I've never heard about this happening, but it looks like the only possible explanation; since it's a valve, I'm assuming that it wouldn't work if the filter was installed upside down. If so, is it sufficient to simply turn the filter and try again? –  Ben Mar 31 at 10:19
    
ADDENUM to the previous comment: I tried changing it, but it only clicks in one direction, so I'm assuming that I did mount it correctly. Very strange... –  Ben Mar 31 at 11:48

It's also possible that your old oil had built up sludge in your oil galleries - more likely if that oil was based on castor oil - and the new oil dissolved a bunch of that sludge out, making the oil galleries effectively much larger.

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