need help/advice on how to replace leaking heater core on a 93 blazer and labor time to do it

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If the vehicle has A/C, it does NOT need to be discharged. If you have not replaced a heater core, it would be best to obtain a Chilton or Haynes manual. Plan on the job taking about 6 hours to complete, with experience, plan on about 2.5 hours.

After replacement, remove the radiator cap and check the coolant voltage with a digital voltmeter. (Negative probe to ground, positive probe in the coolant.) If the voltage is greater than .3 volts, check and/or replace the negative battery cable and/or body ground cable. Electrolysis will "burn" pin holes in the heater core tubes if the voltage is greater than .3VDC. Hope this helps.