A medical device system and associated method classify candidate pacing electrode sites for delivering pacing pulses to a patient's heart. A control unit is operatively coupled with a plurality of electrodes for controlling the delivery of the pacing pulses, and a sensing module senses a cardiac signal generated in response to the delivered pacing pulses. A processor is configured to deliver a first pacing pulse at a first timing interval to a first pacing site, determine the cardiac signal sensed in response to the delivered first pacing pulse and determine a first morphology corresponding to the determined sensed cardiac signal, compare the first morphology to a threshold morphology and adjust the first timing interval to a second timing interval different from the first timing interval in response to the comparing, and determine the optimal pacing site in response to the adjusting.