Hi, do not do this:
Years ago, actually several decades ago, I would buy cooling system cleaner in a can. It says to run it for an hour or two, but I would leave it in the system for about a week. Then I would park my car in the street, remove one heater hose from the heater core, let that hose hose hang down towards the street, and connect my garden hose to the heater core, turn the water on, start my engine and rev it up intermittantly for about thirty minutes. Or until only clear water came out of the heater hose draining into the gutter. Then re-conncet my hoses and add coolant. If you do this today, you will probably go to jail,
but it really worked great.