I have a 1990 GMC Sierra 1/2 ton, fuel injected, 350cui, 5.7L, a/c, a/t (4spd) that has started running way too hot! The radiator was replaced less than 1 yr. ago. I have just replaced this past week: the upper and lower radiator hoses, thermostat (195 degrees), pcv valve, radiator cap, and overflow tank cap. What could possibly be causing this? the fan seems to be working fine. Could it be the timing? It doesn't start running on the warm side until about <a style='text-decoration: none; border-bottom: 3px double;' href="http://" onmouseover="window.status='10 miles'; return true;" onmouseout="window.status=''; return true;">10 miles</a> down the highway. Or in town after about 10 miles. With or without the a/c on, doesn't matter. The coolant level in both the radiator and the overflow tank have not gone down and I don't know what else could be causing this, other than possibly it jumped time. The only reason I think this is because it backfired on me the other day when getting on the highway, and it has not ever done that before. Any suggestions? I don't have a scanner to check the onboard computer and I would like to be able to check it out at home rather than taking it to the shop....but...HELP!