Many people here have noticed that gray scum builds up in the coolant overflow bottle, purportedly from exhaust gas blow-by, even in healthy motors. I've removed my coolant overflow bottle for the purpose of cleaning it out thoroughly. Any recommendations on the best way to clean/break up this gunk? I was going to fill it with warm water and simple green and just shake it till the cows come home and see if that works, but if someone has a better alternative (brake parts cleaner? gasoline? ...?) I'm all ears. Thanks!

 

I did this when I rebuilt my engine. Took out the tank, filled it about half full with hot water, added a half-cup of bleach and a half-cup of CASTROL SUPERCLEAN, and a good squirt of DAWN dishwashing liquid. Shook it around, let it sit for a few hours, shook some more, let it sit again, shook it again and then let it sit all night.
Damn thing came out absolutely looking like brand new!

 

I removed the container, got some pea gravel from my walkway & added waterless hand cleaner.

I covered all the holes with my hands & shook the hell out of it for a couple of minutes. Dumped the whole mess out & rinsed it out with water.