I cannot find my old thread on my cooling system woes but here are the steps I did to clear out most of the crap from the heater core. Removed heater core hoses and applied static water pressure at both ends. Water streamed out a solid brown color first for 2 minitues. The water flow was slow comming out..it was plugged. Then continued untill it streamed out. Then used calcium lime rust "CLR" remover in the cooling system. That made a BIG difference in the heat output from the heater core. Before I would guess the heater temp at the vent was 70F when the engine was full warmed. After using clr, then flushing with regular engine coolant flush, flush again, then use crystal dish washing agent and flused again after driving it twice the temp at the vent has gone up to 120-125F A big improvment. The rear heater core which never really had any heat related problems was 135F. This is all the result of a owner who did not know better and never flushed his coolling system after 20 years then had it flushed by a shop about a year ago because his radiator sprung a leak..I wonder why! So, the last shop did a poor job and I had to complete it. It kinda amazes me that these engines are this tough to not be flushed and will still run. On the positive side, the engine idles more smoothly, it seems to have a slight amount of pep when excelerating and it warms up alot faster now. I plan on installing a WVO some day and need all the hot water I can get.