[KimMG]

to flush the heater core, remove hoses, hold garden hose to one of the inlets, run water through core until clean, then hold the hose to the other inlet and backflush. If there is no hot water flow through the core, the core will not heat up providing heat to the passenger compartment.

Your old spark plugs, were they black? Tan? White? The condition of the spark plugs can tell if you are burning oil, running lean, running rich, etc.

Are you losing coolant? How long did the cooling system hold 16lbs. after you pressurized it? Did the pressure slowly leak down?