Plugged radiator?
Or a solid mass of dead bugs etc blocking airflow through the rad. The AC condenser is in front of the rad. If any of this is blocked, temperature could climb under load.
Clutch fan malfunction?
2007 GMC 3500 dually ext. cab 4X4 LBZ Dmax/Allison - 2007 Pacific Coachworks Tango 306RLSS
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