sumo
dn ʇǝƃ ʇuɐɔ & llǝɟ ı
had similar problem with my 89. Had blockage/trapped air in the heater core. I replaced the tstat and had the same probs. I ended up taking the hoses off the core and shot a garden hose througn one of the hoses. Cleared out the blockage. Seems to be fine now. You can get one of those prestone flush kits from any auto store