get a bottle of blue devil, flush the coolant, dump that stuff in, drain 1/2 and add new coolant. i used it in old old engine that was just eating coolant. it stopped eating it, but if you flush the coolant out so does the blue devil and it starts to eat coolant again. its a clear thin liquid, not tar. i took that engine apart after i blew it up. saw nothing bad from the BD. ps only reason i did a coolant flush was cause of years used, so it held up for like 4 years or so. till the flush, then i had to add more and it stopped asap once again.