Changed heater core now it will crank but not start?
You will need to see what it is not getting and repair that. In most cases it has to do with what you last worked on. So double check your work to make sure you didn’t leave something loose or disconnected. Then check your fuses(ECM fuse). You will want to determine if you have spark and fuel. If you have spark, see if you can get it to start on starting fluid sprayed into the air filter. If not you may need to test your fuel pressure and for injector pulse.