Re: Ahhh - Stupid Mistake, Help!
I think that's the coolant expansion tank, mine's got a green screw-on cap (although there's a blue 'flick-off' cap for the washers).
I would drain and replace the coolant (50/50 water/anti-freeze) mix.
There should be no trouble in itself, but you can't be to sure how the existing coolant and recently added screenwash may react with each other.
Better to be safe than sorry, it's an easy job and should take no more than 30mins.
Try and get the best quality coolant mix you can, as it acts as a corrosion inihibitor as well as anti-freeze. I use Paraflu in the V6, but used BlueCol for years in my Saab turbo.
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