[Mk1_Oz]

I have a 2016 330i with the B48 engine and am experiencing some coolant loss that I cannot account for.

12 months ago the car over-heated and I got the "stop now" message. The water pump (or a plastic part thereof) had failed so I purchased a genuine BMW replacement pump and installed it along with a hard plastic pipe (I believe this is the pipe that is known to fail often) that had either failed or that I had broken whilst undertaking the repairs. All fitted. No leaks and all good.

Since that time the car has been loosing coolant. It probably took 3-4 months before I got the first low coolant message but the lost of coolant volume seems to be increasing such that the low coolant message comes up more frequently. I am refilling with BMW blue coolant.

Last week I replaced the coolant tank cap hoping that was the cause but I am still loosing coolant.

Last week I also did a coolant system pressure test. I did this BOTH with a cold engine and a warm/hot engine to 15psi. I left the pressure for 10 minutes and saw zero pressure loss which I assume means zero coolant leaks. Maybe I should leave the pressure on for much longer?

I am concerned that the over-heating may have damaged the head gasket or warped the head causing a leak into the combustion chamber (I have no signs of milky oil at the oil filler cap). I would have thought that the pressure test would have shown this up though?? Maybe on over-run the partial cylinder vacuum creates a pressure delta >15psi and is enough to suck in coolant from a faulty head gasket??

Has anybody seen this before and can shed some knowledgeable light on the subject??

I don't want to take the car to the dealer as we all know they will just change every electronic module and probably half the engine mechanicals claiming that was the issue $$$$$$.

Car runs great. No smoke from exhaust. No coolant smell from exhaust detectable. The secondary (small) coolant tank is remaining steady.

[Mk1_Oz]

Just ordered some coolant dye and UV light. It used about 150ml in a 150km drive!!