Coolant escape from expansion tank

[ProfCJJ]

Hi all

Its been a while seen I have been on here - a series of issues in personal life and a move coupled with building works meant that my Z4 became a bit of a "work horse" for a while - just being used for my commute and somewhat forgotten ... anyway, things have settled down now and I am catching up on some projects including the Z4.

This current problem is a bit of a weird one and I am not sure what the problem actually is (indeed, if there IS a problem!)

So, my Z4 was at its MOT and apparently the MOT tester had left it idling whilst he did all the checks (it passed the MOT thankfully) however while he was writing up the paperwork and load of steam started gushing out from under the bonnet ...they opened the bonnet and said it was coming from the expansion tank. Of course everything was super hot, so they pushed it outside to cool down and when cold enough opened the expansion tank and it was of course empty. MOT tester doubted the fan was turning and kindly connected it to his generic tester and sent the code to turn the fan on and it did .... anyway, they filled the coolant tank up (wrong coolant, the pink stuff, but doesnt matter, it was an emergency) and I drove home keeping my eye on the temp - only 2 miles - with no incident.

When home I topped up the expansion tank with water, did the automatic bleeding thing, and checked with Bimmerlink for any major errors ... only error present was "Low coolant" (obviously) cleared that and no new errors. Ran the car to temp, left it idling - no problem. No leaks from coolant tank or radiator as far as I can see ... next day the coolant was still under a bit of pressure (slight hiss when I opened coolant tank cap) and the level was the same.

Worried that it might "give" at any moment I have only been driving locally, a few miles here and there, each time with Bimmer Link recording a bunch of temp and coolant behaviour parameters ... all seems "normal".

Yesterday I got deionised water and BMW coolant and comprehensively flushed the system and filled with BMW coolant (50/50 ish). Bled the system and ran the car up to temp with no issues.

So - no apparent leaks, water pump is working, fan seems to be working, no coolant loss or overheating issues ... so what the heck happened? and what do I do next??

I am a bit worried to go on a 2 hour trip away somewhere and for the system to fail right when I need it, but everything seems to be OK ... what do you all think? what would you do?

I do have one question - what if the fan is is only "partially" working? If I look at Bimmerlink and monitor the e-fan PWM I can see it goes to 7% when car started (and fan doesn't turn) then it goes to 16%/ 20%/ 25% as the car idles (and when I turn aircon to max) - and the fan seems to respond accordingly ... only occasionally it seems to hesitate and jiggle a bit before spinning (I know, clutching at straws here!!)

It's the only thing I can think of: the car was stood idling with aircon on for an "extended" period of time and if the fan "stuck" the car could conceivably overheat? (although no error messages are recorded of that - only coolant empty).

So I am stumped!

Any thoughts?

Best

Christopher

 

[enuff_zed]

It could be that overheating caused the expansion tank to overflow and therefore ehen it cooled it was empty.
If that was the case, possibly the electric water pump is starting to misbehave.
Do you have any codes recorded?

 

[ProfCJJ]

It could be that overheating caused the expansion tank to overflow and therefore ehen it cooled it was empty.
If that was the case, possibly the electric water pump is starting to misbehave.
Do you have any codes recorded?

Yes I assume the expansion tank vented and that's where all the coolant went ... the question is why? The water pump seems to be working - I can monitor it functioning in Bimmer Link and it also works fine in bleeding the coolant system repeatedly when I flushed the system.

The only thing I can think of is that the coolant level had somehow, over the years, been getting lower and lower and I hadn't noticed until it became critical ... I find that hard to believe though as I would imagine I would get a "coolant level low" error on the dash before anything catastrophic happened surely??

No error codes whatsoever were recorded (other than "coolant empty" obviously).

 

[enuff_zed]

Hence why I said that the water pump may be 'starting to misbehave'.

 

[RobbiZ4]

If I look at Bimmerlink and monitor the e-fan PWM I can see it goes to 7% when car started (and fan doesn't turn) then it goes to 16%/ 20%/ 25% as the car idles....

I've placed this info on my BimmerLink Dashboard some days ago and was irritated as well. My current explanation is, that this value represents the electrical fan in the DME housing and not the coolant fan. But I did not yet find an evidence for it.

The BimmerLink Dashboard is a nice tool to monitor all the cooling parameters in parallel while driving.
Have a look at my current configuration for motor & coolant temperatures, the water pump's speed, the electrical heated thermostat and others:

 

[B21]

Coolant pump often fails in weird ways as the brushes wear out..not all data / faults are recorded properly..

 

[ProfCJJ]

Well, after my first short trip after flushing the coolant I had to top up the expansion tank slightly - I assume that is to be expected. And now after driving some more the level is stable at Max. No codes, no overheating, nothing of note ... I need to do a 200 mile round trip tomorrow, I guess I can "proceed with caution" very weird!

 

[B21]

My bet is the issue will return FWIW..