Heater doesn't work so good - 74 Duster

As many of the replies have mentioned, flush the core by itself. The way they're made and positioned makes them a "dirt trap." The flow rate is lower, vs the rest of the system. The top enter and exit causes dirt to settle in the bottom of the core and slowly block tubes over time.

I've flushed a few small blocks, several times, draining the block, etc. Then disconnecting the core hoses at the block and doing it w the garden hose yields a big blob of dirt into a 5 gal bucket and you have heat.