Notes

Hey, I have a question about the cooling system and the bleeding of it in my 2001 318Ci (N42B20).

Now I have to state I don't actually have any indications of any problems, the car has never overheated in the 4000miles I've put on it since I've had it, and the temp guage has always been dead center.

However I've noticed that my coolant expansion tank slowly gets lower after driving some distance (a rough measure I've found is about 200ml every 500miles).

I've had my system pressure tested by my mechanic who thought there might be a leak somewhere, but he said the pressure test came back fine, so he just re-filled my coolant tank and let me get on my way.

I initially just topped it up a few times each time it got low, but recently went to actually bleed the system via the bleed screw. But what I've noticed is that as soon as I start turning the screw and the air starts bleeding from it, the coolant level in my expansion tank rises back up again. I've done this about 3 times now: Coolant tank gets low, I turn the bleed screw, coolant tank fills up again, I close the bleed screw when coolant tank is back to indicated level.

My question is: why is the coolant tank filling up when the system gets bled. Surely with the air escaping the system the coolant tank should be getting even lower during bleeding? And secondly, how can I bleed the system properly if after 5 seconds of bleeding my coolant tank is back up to max level, without actually overflowing the coolant tank which might then make it too full causing it to overpressurize and crack when I run the car?

As I said, I've not actually had the coolant cause any issues (apart from maybe the AC not running quite as cold as it should be below 30mph), but it's just a little something that's been bugging me.