Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
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The best way to unclog heater core is by flushing it out with a garden hose. You will want to disconnect both of the hoses going into the firewall then use a garden hose to flush it out from both directions. I'd expect this to take you around a hour or two and a shop would charge about a hour labor to do this. Let me know if you have more questions or if there is anything else I can do to help. Thanks.
No water would not get inside by doing flush. If coolant was leaking inside right now then that would mean heater core is leaking. The hoses go into firewall right behind engine, pretty much in the center. You can make a adapter with a slightly larger radiator hose or just duct tape garden hose to heater core hose. Let me know if you have more questions, thanks.