It could be that the fan isn't turning on at the right time to cool down your radiator. When driving at speed this wouldn't be a problem, but when idling at traffic lights it would be. Could it be the thermostat or something with the fan clutch? I'm not sure how the radiator fans work in a KJ.
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2008 Liberty Sport 4x4 Automatic
Jeepin By Al 4" Lift Kit
BFG KO2’s 245/75R16
Full MOPAR skid plates
Rock Lizard Skink Super Sliders
AtTheHelm Front Recovery System & Roof RackM116A3 Trailer build thread: viewtopic.php?f=72&t=77997