You can use a radiator flush or just drain and fill. Make sure you get the correct temp.thermostat. Get what it calls for, I just don't remember what I installed in my 89 4.2 but I believe it was a 195, double check. Also when you fill the system it has to be "burped" to get the air out. With the cap off, let it get to operating temp., the thermostat will open, and air will be pushed out of the system. It will spill out but it's worth doing. Close everything up and only fill the expansion tank when cold as not to overfill it.