I just bought a 95 prelude and was checking the cooling system; and when pulled the radiator cap off there was some gray buildup on the cap and in the radiator. I never seen anything like this before and was wondering what would cause this? The previous owner said that rather then using coolant he used water and added water wetter to it. The car runs great and doesn't overheat. I was thinking of flushing everything out and adding some new coolant, but I don't know if that would cause any problems by doing that.

 

Could be something to stop a coolant leak.

 

Flush away brotha...flush away.

 

It is possible that it could be some kind of stop leak, so I'll flush it out today