I was just doing an oil change this evening and I had a light situated under the truck so I could see better. It so happened that it was casting light through my overflow bottle, and this is what I saw.

Does any one have any idea of what this is? I don't think it's oil, as it should float on top of the coolant, which I proved in a cup on my work bench to make sure since I was unsure of the density of the coolant. Washer fluid just blended with the coolant so I don't think it's that either? Has anyone else noticed this, or would anyone care to take a flashlight to the bottom of their bottle to check?

 

How many miles are on it?

I bought an old '83 Chevy many years ago that had a lot of miles on it. When I pulled out the over flow bottle to clean it out, it had this greenish/brown sediment type stuff in it. It was really slimy too. The only thing I can think of is that over the miles, sediment builds in the bottom of the bottles whether it be the green or pink anitfreeze. I can only guess that it's a combination of the anti-corrosive stuff in the antifreeze, scale, rust and all the other junk that's in an engine that might sluff off over the miles.

Since it seems to be quite a bit, I'd flush it out.

 

I only have 30k miles on mine.

 

is it just a stain, or is it actually a liquid ?
stain is probably mold or rust residue.
you can remove it, clean it out, and see if it comes back.

 

It looks as though it's liquid. I shooked the bottle as much as I could and it moved around some inside the bottle.

 

its just corrosion from the radiator, nothing to worry about, they all do this, just sometimes the rad, gets to hot and over heats tossing the crude into the overflow bottle

 

If that's the case then I'm just going to remove it and flush it out. I don't want it in there.