Temp fluctuations in my 99 ford ranger 4.0, from normal running temp to high temp. But temp stays normal at idle.
Your problem probably is thermostat.In idle mode its make antifreeze run,when thermostat in a close position,on small circuit to heat engine faster,but when temperature of coolant reach preset level (each car have a different setting) thermostat will opens and let coolant run on full circuit what allow to cool your engine in full capacity.Try to replace thermostat.
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