I removed the heater core from the radiator end the top and the one right under it. While engine is cold. Get water hose (I use the nozzle) sprayed water through the lower hose blowing the crud out the top hose. I did this a few times until the water was clear coming out. The Heat is working. Next I plan on flushing the radiator because if it was in the core it's in the radiator also! Hope this helps
Now this is far fetched and not sure if this can work on a I6 engine. But anyway for future reference on any vehicle you may need to do this too for cheap.....
Friend has an early 90's V8 dakota that puts out poor heat. He modified the line from near the thermostat to the heater core. Used rubber hose that went form the stat and routed it to towards the exhaust manifold. When at the manifold he went to a copper pipping line that ran right next to manifold (using the heat from the manifold to heat the copper pipe) thn back to rubber to the heater core. He also used a ball valve to adjust flow/ heat to the core. Works great!!
He plans to wrap the manifold and copper pipe together in heat wrap to keep the heat in.
most newer vehicles require certain temp thermostats, check to see if yours is opening all the way.. lots of if's with a heater for sure, but a thermostat controls what heat goes into that heater core... there again, if the heater core, hoses, or anything else is plugged, well..... another thing, if you have to change that heater core, definately look into the fan motor upgrade.. WOW, what a difference it made on mine..
I want to die in my sleep like my grandfather, not screaming in terror like the passengers in his JEEP!...