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I have an older C5. The inside of the overflow tank is so dirty/stained that you can't see through it to see the coolant level. Is there a way to clean this?
But I wouldnt do it in the car. It comes out easily, two or three 10mm bolts (look for the one under it on the engine side towards the back). You could record the level before you start, clamp off the two lines, remove the tank and steam it or whatever, refill with dexcool to level (or whatever you are using) and reinstall.
"If" you can successfully remove it, you can fill it full of soapy water and ice cubes so dislodge the grit and grime in there. I have done this with coolant reseviours as well as plastic brake and clutch res's as well.