Very slight brown goo in coolant reservoir

[pashley300]

Hi people, hopefully you can help me out here.

I own a 2009 E87 120i (N43 engine) since October 2022.
A couple of weeks ago I noticed the coolant level was very low as I checked the reservoir.
As I’m planning to do the water pump and thermostat these next days I decided to simply top up with a bit of demineralised water for the time being, relying on the existing (blue) coolant for the additives.
Didn’t notice anything unusual then.

I then drove the car for two weeks including motorway driving and checked the level again today in case I had a leak.
The level had remained intact but this time I noticed a tiny bit of brown goo under the reservoir cap and on the plastic edges. Just enough to have a bit on you finger.
There’s nothing in the coolant itself which still appears clear and bright blue.

I change the oil every 5000 miles if that and never noticed anything wrong with it.

What are your thoughts? A very slight oil/coolant cross contamination? Or simply some crap in the system that was brought back up after having raised the fluid level? To be fair it hadn’t been flushed for several years.

I’ll still be doing the pump these next days and flush the whole system. I’ll see if the goo comes back but I still wanted to hear your thoughts about this.

Thanks guys.