sorry I must didn't ask the right question the substance is inside the radiator reservoir tank
what are you asking?anyway there could be several things in,floating or on the bottom of the reservoir...flush it out and then add new coolant to the full line and check it again in a few days.
Agree. Its probably nothing to worry about although it can build up in the bottom and start causing problems with sensors. Like Kelly said, flush it out and top up the coolant. Be sure to use the recommended GM coolant which in the case of the Cobalt is Dexcool. Do not mix other types of coolant or you will create new problems for yourself.
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