Loaned out my 06 impala LS to my roommate for a month. He told me afterwards that the coolant was really low so hetopped it up about 8 times. i checked it and sure enough he's been topping up with green coolant, not dex-cool. It slowly leaks while running and then dumps about half a gallon until it cools down. Now pretty sure im leaking out the water pump or thermostat or both. I bought each brand new just in case along with gaskets and a new belt.

Obviously i want to get it all flushed out, but i was told it would be unwise to flush green coolant through my brand new water pump and tstat.

 

I don't know why it would be unwise other than having potential material (metal failure) in the flush. Since you are replacing both, you are going to be draining the system if you have no damage on the old parts, just do it.

 

what he said.
coolant crossover gaskets are common leak points in addition to water pumps... so pressure test it