I've got a 69 bonneville with a 1972 400. It runs great around town and on the highway up to about 80. Anymore than 80, after awhile it heats up to the point of the engine pinging/knocking. Slow down and turn on the heat, and it goes away? The temp reads 225+/- 5 degrees. Any ideas? Radiator/block dirty, water pump. The water to antifreeze mix is 50:50.
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