Having done a few bits on the car lately myself, I've been paying close attention to it - far more than my usual "if it works, don't fix it" approach.
In the last month i've replaced radiator, top/bottom hose, oil modine hardpipe, plus modine removed and O ring replaced. Old coolant was the pinky orange long life stuff.
Now its all back together and running using Motul expert coolant (clear blue colour). New radiator has a rad cap but it's been plumbed into the expansion bottle correctly.
I've got about 7.5 litres of coolant in, pretty sure there's no airlocks. Hot air through vents inside, both hoses get hot, fans kick in as required etc. Car drives fine, doesn't overheat.
Gave it some stick last week after it had been put back together and all seemed fine. Went to take it for a drive last night and checked coolant before starting the car - level in header tank was slightly down so topped up. (might have been that it settled and purged some air after first drive week before?).
However, before topping it up, i noticed this on top of my nice new coolant...difficult to get pics on a phone but you get the idea.
At the time of taking these pics, the car had stood untouched for just over a week. As soon as it was started, the contaminants on the surface mixed/disappeared.
Now... I used some degreaser to clean up the oil modine (it was filthy after a slow oil leak for a year) and some could have got inside (although only a tiny amount I expect through the open hose ends). I also fitted the new rad without flushing it through first - didn't even think of it at the time. Could it be a remnant of grease from manufacturing?
Can't see any major signs of coolant leak although the header & rad caps were slightly damp around the sticker. Don't know if its pressurising too much? Guess it can't hurt to try a new header tank cap as i've read of a lot of problems if these fail.
Please don't say its duff head gaskets
If I hadn't started doing bits on the car myself I'd never have even checked and i'd be sitting here quite happy not knowing
