I have a 1992 Geo Tracker U.S, 1.6L 8v sohc 2wh dr. overeating at highway speed(65+)mph but at 45mph or lower including idling it seems fine. It does not steam and no water in the oil. I have changed the waterpump and thermostat and hoses but it seems like it is not circulating well. could it be plugged in the block or maybe as simple as a radiator cap? The check engine light stays on all the time but no codes. Could that be the NAG light? Any suggestions is greatly appreciated. Thank You for your time!! I forgot to mention it is blowing coolant back through the reservoir out the overflow.