heat works fine until I stop at red light or pull over then blows cold on 1993 Toyota Previa

blower works great. Not overheating at all, heat works great till I stop at red light or pull over then blows cold. When I start driving again, it starts blowing heat until I stop or pull over... Any Suggestions??? Possible causes??? Am I harming the motor by continuing to drive when it has this ailment???? I did add antifreeze, but that wasn't the problem. Otherwise, Runs Perfectly... Someone said I need to replace the water pump.. is that correct?? ( I'm just another fella, not a Rockefeller ) Thanks in advance for your help....

Asked by for the 1993 Toyota Previa
it could be a partially restricted heater core, feel the hoses going to the core, if they are hot, it is probably not restricted, it they are cold, it probably is. backflush the heater core until clear, then refill the system and bleed all air. if not, it could be the impeller of the weater pum is damaged and not flowing the coolant properly. though, usually this will cause it to overheat in the summer
OK. Changed water pump 2 days ago. Heat worked fine. Tues. night white smoke BELLOWING out of exhaust pipe. Went out to check fluid levels this morning, and van wouldn't turn over, was making loud grinding noises,,, I've had van for 60,000 miles and never once did it overheat right up till this morning. No water in oil, never lost coolant, never had a problem. Changed oil every 3000 miles, etc.etc. Motor is seized, isn't it??? How or why did this happen. Symptoms were SO sudden. Check engine lite never came on. What would van be worth for parts?? Any ideas????
headgasket . pull the sparkplugs . look for white ashy color on a plug or water running out of cylinder .
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