Welcome Dave...
What an awesome boat you have there!... I could only dream of driving that boat!...:driving: After reading your post, I went in to look at the manuals and anything else I could get my hands on that would give me a little insight into your boat. At the moment, with it being brand new, it's kinda hard to find specific information. But I think I found what you need.
The engine in your Challenger is the 1503 4-TEC supercharged SCIC engine. The cooling system is a closed loop system and the manual recommends an ethylene-glycol and distilled water solution with a 50/50 mix. It also states that you can use any coolant that is specially formulated for aluminum engines. Which means, yes, you can buy this at your local auto parts store. Many of todays car engines are built with aluminum blocks, so I'm sure they'll have it. Just tell the parts counter guy you want antifreeze for an aluminum engine.
Ethylene-glycol, is the standard coolant you get in Prestone. Pick up a jug and see that it meets those requirements. I would use the distilled water though. Reason being, regular tap water has suspended solids in it that can precipitate out of the water and adhere to the metals of your engine. Distilled water has been flashed to steam then re-condensed into pure water with no impurities.
As for the level in your expansion tank, if it's just below the line, then you should be fine. The level of coolant has to be above the highest point of the engines cooling system to keep it flooded for freeze protection. But don't hesitate to top it off. When you took it out and ran it, it circulated and heated up, so that's probably why you got the small drop in level....
Man, take care of that jewel.........Post as much as you like, I just don't want to hear you post with an engine problem.....not for another 10 years anyway!.........
