2 degrees before it starts to ping, under a load. I don't think the timing and a lean burn is your problem. I would look at a plugged radiator or a stuck or missing thermostat being the culprit. Borrow a digital thermometer and start looking for cool spots in your cooling system. I did see one far out scenario one time, a waterpump had rusted the impeller fins completely off of the pump, not moving any water at all. That one sucked to diagnose. Good luck,
Scott
