Disappearing coolant?
 

Got this sudden urge to check my coolant, and the expansion tank was just about empty. No visible leaks anywhere. Tank is new, as are the upper/lower hose, temp sensor. The rad is relatively new, still has its barcode from when the PO replaced it.

Going to pick up some coolant today and top it off. I'll admit I hadn't checked in a while. Thinking to get a 1.2 bar cap. Any thoughts?

1.2 bar cap is a good idea in any case. Perhaps the person who changed rad, sensors, tank might not have followed the heater bleed procedure to ensure the system is properly filled and mostly de-aerated?

Set heat to max temp, run car to normal operating temp then run car for 5-10 minutes (run inside blower to a normal speed). Then shut car off, de-pressurize system and top up as required. It might be that simple.

IF NOT:

Easy way to check many common leaks is to use a cooling system pressure tester - it installs where the pressure cap would be. You pump up the system (engine off) and you watch the gauge to see how it holds pressure (normal case) or where you have a visible or audible leak (typical). Head gasket leaks may not always be shown here - there is a test fluid you can buy to detect combustion gas in coolant.

1.2 bar cap from GAS ($30) or 1.4 bar factory cap from E30 ($15) both would suit you fine.

I hadn't checked mine in a while either and was about 8 oz from full mark, just poured a few oz of distilled water and its been fine since then.

Thanks guys! Going to hunt down that e30 cap this week. Exhaust doesn't smell like coolant, but will see about testing for a head gasket leak. Might not have been bled properly when the new waterpump was put in, either. I knew I should have checked. :P

No light, just had an urge to check. Sensor was replaced last december w/the tank

How far down was the indicator/float/needle from the top of the reservoir's opening ?

Sitting at the bottom.

As with all BMWs...check the usual coolant system components that fail. I had an elusive check coolant warning on my e38 for a good 6-9 months (don't drive it much). I kept an eye out for dried white coolant spatter and/or residue that may have been flung from the spinning fan or at hose couplings...and never could find anything...but kept getting the warning.

Then during a front brake pad job the culprit finally showed a tell tale sign...there was a small leak from the underside of the water pump..

Thanks, will be looking. Tank/sensor replaced in December, rad hoses and temp sensor in march/april, water pump was replaced in June. No drips thus far. Just topped it off, got it sitting on the uphill driveway, will bleed once the engine cools a bit.