[AMPowerJ]

Here is the deal ...

I have had two days of interstate road trips of greater than 100 miles each way. I had the service performed at 1200 miles and yesterday I had about 1500 miles on the car. The oil gauge in the gauge cluster was reading full.

On my trip, while on the interstate at speed, the oil level dropped one tick and then another. I was slightly concerned that I would burn that much oil but thought oh well.

This morning when I cranked up the car it read full again. Huh? So I decided to try something. Sure enough on my trip today it dropped two ticks again while at speed but this time when I arrived at my destination I let the car idle and reset the gauge. Bingo ... full reading again. Got back into the car to drive home and the same thing happened. Down two ticks AGAIN on the interstate. Just got home and pulled into the garage, reset the gauge, and all full again.

So what gives? Does the gauge only provide an accurate reading while idling and stopped? Has anyone else noticed this behavior? Think this is normal for the POS gauge?

[elp_jc]

You're full buddy. Once you understand how it works, it's pretty accurate. First of all, every time you turn your ignition ON, the oil level resets. Problem is it DOES NOT register until oil is at operating temperature, and the moment that happens you're probably under way, which means less oil in the crankcase. Oil level won't be checked again until next time you cycle the ignition folks.
Just like you found out, the correct way to check oil level is always on the same spot (I always use my garage), with engine at FULL operating temperature, and AT IDLE. And yes, best method is to do a 'reset' (that way you don't have to turn engine off and back on again) since it's as accurate as turning engine off and on when oil is at operating temperature.

By the way, the 'reset' function forces an oil reading even if oil is not at full operating temperature, but it's not as accurate under those conditions...and it takes a lot longer that way. Hope this clears any confusion.

As a final note, I plan to always keep my oil full. Manual says to wait until you get the 'add +1L or risk engine damage' warning, which seems retarded to me, so I won't do that, just as I won't wait 15K miles to change the oil either. If you add 1/4L when the oil level drops one notch, it shluld go back to full (or add little by little after that until full again). Once you know how to check it, it can be treated almost as a regular dipstick (obviously not as accurate, since it registers 1/4L at a time, but close). That way I never have to carry oil in my car, or worry if I have to take a trip, but to each his own. Good day gang, and hope this helps.

[AMPowerJ]

Thanks! Have you noticed the oil reading dropping while traveling and then coming back up when checking at idle specifically?

[elp_jc]

Originally Posted by AMPowerJ

Thanks! Have you noticed the oil reading dropping while traveling and then coming back up when checking at idle specifically?

No, but that only means you're borderline full. Engine oil level drops slightly when engine is at 3K rpm rather than 550 rpm, so when your sensor takes the reading above idle, it registers the next lower level than at idle. Does that make sense? Remember (theoretically speaking) if you are .01L or 0.25L below full you'd get the same reading, and by the same token, if you're .01L or 0.25L above, you'd still get full (above 1/4L you get the overfilled message, I believe). So you're barely full and I'm probably 0.2L over, or something like that, since I added 1/4L the last time (I was probably a hair below full before that last 1/4L).
By the way, my '08 jumps from 1/2 to full, but newer cars supposedly added the 3/4 mark, so in my case I can be .01L to 0.49L below full and get the same reading. That's why I'll always keep it full. I'd have paid $100 for a freaking dipstick, but now that I know how this stupid thing works, I'm okay with it . Take care.

[Cleveland]

G-forces at speed push the oil in the pan away from the oil level sensor...

[AMPowerJ]

Yeah that makes sense but going 80mph on the interstate in a straight line doesn't really create any G forces to move the oil around.

I can understand it dropping one tick but two ticks means even if I am borderline full that oil seems to be going somewhere and then coming back. I wish I knew the mechanics of how the sensor actually measures the oil amount and maybe that would explain it.

BTW - If anyone could come up with a relatively inexpensive < $200 dipstick mod they would make a mint.

[elp_jc]

Originally Posted by AMPowerJ

I can understand it dropping one tick but two ticks means even if I am borderline full that oil seems to be going somewhere and then coming back.

Your car might be like mine, which drops from full to 1/2; no biggie. And the oil has nowhere to go.

And Cleveland has a point. Just like I explained, you never know where the heck the measurement will take place, and sometimes it takes 20 minutes to happen. You could be avoiding an obstacle . As long as it's consistent at idle, don't even worry about it brother. Take care.

[Cleveland]

Originally Posted by AMPowerJ

Yeah that makes sense but going 80mph on the interstate in a straight line doesn't really create any G forces to move the oil around.

I can understand it dropping one tick but two ticks means even if I am borderline full that oil seems to be going somewhere and then coming back. I wish I knew the mechanics of how the sensor actually measures the oil amount and maybe that would explain it.

BTW - If anyone could come up with a relatively inexpensive < $200 dipstick mod they would make a mint.

Motor oil is a viscous liquid, and therefore subject to any gravitational forces that are acting upon it. The gas in your tank reacts in the same way. The tank is baffled to minimize the sloshing of gasoline from these gravitational forces as well. (especially when you are running low on fuel)

The M3 is a performance oriented car. But it cannot completely stop the oil from running away from the sensor at speed. These forces are exaggerated more during cornering of course, but don't think for a minute that straight line speed doesn't move the oil in the pan around as well. (just enough to show a slight difference)

This force combined with some crankshaft windage inside the motor (another mitigating factor) are the two most likely reasons for this phenomenon IMO.

[chuglobal]

I bought an E90 M3 a little more than a month ago. I noticed the at the Oil Gauge bar was Full but not ">Max" which is optimal. Over the last month I watched the bar go from Full > 3/4 > 1/2 > 1/4 after 2K+ miles and a "spirited" mountain run.

I read the manual on checking the Engine Oil Level however I have a few questions as I didn't want to wait until the "+1L" showed up. First, I added 1/2L + drove home from the dealer + reset the Gauge on the hwy and the bar went from 1/4 > 3/4. Next, I added another 1/4L + reset the Gauge in the garage still at opertaing temperatures however the bar remained at 3/4. Finally, I added another 1/8L + reset the Gauge in the garage 1hr later idling up to operating temperatures but the bar is still only at 3/4.

Why did the Oil Gauge bar not change from 3/4 after adding more than 1/4L? Do I really need to drive it around for the sensor to work? Can we really add 1L from 1/4 bar and not get the >Max? I added almost 1L however the Oil Gauge did not go from 1/4 to Full ... I figured adding 1/2L to 3/4L would have definitely hit the Full? Thanks.