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Discussion Starter #1 (Edited)
I started my car (2005 325i) earlier today and the low coolant light flashed yellow on the dashboard. I shut the car off and turned it back on and the light disappeared. Fearing that it would overheat and leave me stranded if I drove it to work, I refused to drive the car until I had the issue sorted.

So today I opened the expansion tank cap today and the red float thingy was below the minimum level mark on the cap; it fully submerged in the expansion tank. There was barely any coolant in the tank. I looked in horror as I thought "you waited THIS LONG to tell me my coolant level was low?!!?" What was strange was that my car didn't overheat despite the level being so seemingly low for a while.

I got ready to top off the coolant until something weird happened. I did this without warming up the engine. Following the standard bleed procedure, I turned key to position 2 (car ON, engine OFF), set the heat to 91 degrees F on the lowest fan speed. But as I was taking out the bleeder screw, there was a hissing noise coming from the hole the screw goes into. Once the screw was fully out, the red float thingy rose all the way up and out of the tank; it was at the maximum mark. The coolant level in the expansion tank rose up to just below the bleeder hole.

I screwed everything back up and took the car for a short 2 mile drive. The temp gauge stayed dead center.

I had a shop replace almost the entire cooling system on this car about 2 years/20k miles ago (with the exception of the fan and clutch) with OEM parts. I also told them to fill the coolant level somewhere between the min and max mark as indicated by the expansion tank cap.

I'm guessing that some air bubbles may have gotten trapped in the cooling system and I let them out by removing the screw? Would this fool the coolant level sensor into thinking the car is low on coolant when it's not?
 

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'02 330i 5-Speed
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I'm guessing that some air bubbles may have gotten trapped in the cooling system and I let them out by removing the screw? Would this fool the coolant level sensor into thinking the car is low on coolant when it's not?
Welcome to BMW ownership. ;) All 4 of my BMWs have "burped" coolant from time to time, with no evidence of leaks otherwise. I check the level weekly, top off as needed, and don't sweat it.

FWIW, there's a margin of safety built in the coolant system, which is why you were able to drive around without overheating with it so low.
 

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2003 330i
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I wouldn't blame the car for a lack of notification of low coolant. You ought to check it once in a while, and not rely on nanny devices. To your questions: Perhaps.
 

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Normally you don't need to turn on the ignition and/or to open the bleeder screw when topping off coolant after a low coolant light - this is the procedure to fill the system after a cooling system job.
To top off coolant, wait until the engine is cold, then remove the expansion tank cap and add coolant until the level indicator is between min and max. Then all you need to do is to screw the cap back on.

What you described is normal - upon removing the bleeder screw, you let some coolant from the upper radiator hose to drain into the expansion tank, hence the level went up.
 

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Normally you don't need to turn on the ignition and/or to open the bleeder screw when topping off coolant after a low coolant light - this is the procedure to fill the system after a cooling system job.
To top off coolant, wait until the engine is cold, then remove the expansion tank cap and add coolant until the level indicator is between min and max. Then all you need to do is to screw the cap back on.

What you described is normal - upon removing the bleeder screw, you let some coolant from the upper radiator hose to drain into the expansion tank, hence the level went up.

This..... Plus.....Think of the expansion tank as extra coolant. The tank can be empty and the coolant system will work fine as long as the radiator is full. The problem with BMW's is there is no way to check the radiator level like there is on other manufacturers. If you don't check it regularly the low coolant light may come on when it is cold and go out once the engine warms up and the coolant expands.
 
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