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Your waterpump seems to be fine. By turning the heater on, you are effectively increasing core size and airflow. If the pump wasn't, then turning the heater on wouldn't matter because there would be no water being pumped.
In my Jimmy I replaced the radiator, thermostat, hoses, and water pump. It still ran hot on the road. Turned out the fan clutch had gotten weak. With the new thermostatically controlled clutch ($29 at a local parts store vs. $150 from GM) it stays right on the thermostat of hot days and actually 10 degrees cooler on cool days.
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44 Willys MB
52 M38A1
64 Corvette Coupe
68 Camaro 'vert LT1 & TH700
69 Z/28 355 12.6's @110
69 Chevy Short Step 4 1/2"/7" drop
72 Jimmy 4WD 4spd 4" & 35's
02 GMC 2500HD 4x4 Duramax
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