Could needing freon cause the engine to run hot when the air conditioner is on?

Answer 1

No, that's most likely the result of low coolant, burned out coolant, or a thermostat in need of replacement. A defective water pump may also be to blame, or possibly even loose belts

As previous OP stated, but to elaborate, the condenser is located in front of most radiators, when you turn your a/c on the condenser will produce a good amount of heat, almost "preheating" the air going through the radiator. Your cooling system must be in good order to dissipate the additional heat. Also most vehicles if equipped with electric fans will have i.e. one or two cooling fans that will come on when the ecm or high pressure switch on the a/c indicates high pressure due to excessive heat in condenser (high head pressure). Also, look at condenser for bent fins, leaves or debris between condenser and radiator, proper electric fan operation and if it's a belt driven fan, some have temp operated clutches, the hotter they get the more air the fan will pull, if this fan clutch has leaked oil, it will cause this issue. Shut vehicles off HOT and look at the fan, if it keeps spinning freely and slowly comes to a stop, its shot, if it stops after a few revolutions its most likely in good shape. Year, make and model would help BUT for most part, if it runs cool until a/c is on, then you have a marginal issue, most likely fans, radiator getting plugged or possibly thermostat, however some VW and other vehicles use plastic impellers that fracture and others that impellers slip on the shaft, again - year make model helps...