Flushing cooling system with heater core and radiator bypassed? (Page 1/1) cebix JAN 10, 08:04 AM 85 duke auto w/AC. I'm replacing the radiator and heater core and want to flush the system. There are products that you supposedly should add to the coolant, let the engine run warm for 15 minutes or so and flush it down. Since I already have the heater core bypassed and don't want to introduce any new crap from the current radiator I'm thinking - can I also bypass the radiator? Connect the radiator hoses and tranny hoses with two pipes and run the engine that way idling without the thermostat? Can the engine overheat quickly this way or should it be manageable? Or should I forget about these flush "products"? 2.5 JAN 10, 09:01 AM I don't recommend using chemical flushes. They can lead to leaks in older cars. I just drain and refill. edfiero JAN 10, 10:59 AM When you removed the old radiator and heater core, how did they look? Pretty clean, or full of gunk? If they were clean, you can forget the power cleaning just flush with water. If you had a lot of gunk, then I would go ahead and use a cleaner. cebix JAN 10, 03:08 PM I wouldn't describe it as gunk. More like sand and small rocks. You can look up my other recent thread to see what came out of the heater core and reservoir tank - probably stop leak leftovers and just mineral residue. I don't know how much of that circulates around or is just stuck in the radiator and heater core. I guess though if I'm replacing both the radiator and heater core there's no more small channel passages in the system and just a water flush might do the trick.