Listen to what NITROUSN has advised. These pellets are cheap, effective, will not clog heater cores and will do the trick. You do not have to drain the system, and you do not have to run without antifreeze when it's below freezing outside. GM specified these for use in the Northstar engines and others where porosity was an issue. I've use them countless times and usually keep some in stock at the shop.

We had a friend who had a suburban that would lose a half gallon of coolant a month for years. They spent thousands of dollars replacing this and that at the advice of three different mechanics. Nothing worked. They were about to get rid of it when they were over at the house and told the story. I asked if they wanted me to give it a shot. They agreed. I took off the radiator end of the top radiator hose, crushed up two of these pellets and put them in, topped off the coolant and drove it around for a half hour. They still have the vehicle to this day and have never had to add more coolant.

Certainly worth a try