So truck has 75,000 miles been running great.... was in a line for Fast Food friday... was mildly hot out (87 degrees)... when then warning light came of for over heating.... pulled over and opened the coolant tank and Air came out...

When driving the truck stays at 190-194 but when i parked and let the truck idle it would heat up to 215-228... wouldn't let it get above that... so i turned off the truck... not sure if it is the Thermostat since it cools fine when driving... the Fan Clutch seems to work.. any thoughts... ??