My brother's 2013 Silverado with 5.3L. About 160K miles on it. Said it started running hot today while he was driving. Said he pulled over...but didn't shut off engine, and the temperature slowly fell back to 210 (normal). So...only runs hot while engine is under a load. Told him to bring it to me since he wasn't far away. Cooling fans kick on/off as they normally would. Thermostat seems to be opening and closing normally. Another symptom...coolant level is low and no signs of external leaks. I don't have a pressure tester and no access to one until about Tuesday. I go ahead and remove the spark plugs to see if anything stands out. Sure enough, 7 out of 8 look to be burning perfectly normal. This one, however...cylinder #1... Something has this one cooking. Is this a tale tale sign of coolant getting into this cylinder? Or is simply from overheating? I should add...he had these spark plugs changed about 60K miles ago. He said 3 months later, the truck had a misfire...bank 1, cylinder 1...this same cylinder. He changed just the plug and the misfire went away. It wasn't misfiring today...but it looks like it was just a matter of time.