I had the transmission fluid flushed /replaced on my 96 lumina at a Valvoline. The next day my son was driving home and the Service Engine Soon light came on and the car was misfiring badly. Had it towed to the shop. They found coolant in the oil pan and the engine was locked up. I am now replacing the engine with one from the salvage yard. I can't see that changing the transmission fluid would have been the cause. Any ideas before I drop in the replacement engine?