This morning my '98 1500 with the 5.9 seemed as if the thermostat never opened up. Got very hot then fluctuated down, along with the air coming out of the heater finally being hot. Pulled over and checked everything out and lines to heater core are spotty with temp. but seemed okay to continue. Started doing it again so I gave it a quick good rev in N and got another surge of coolant flow (also felt through the heater) and engine back to operating temp. Thinking excessive air inside or bad stat/pump? Also, the check gauge light came on when hot but I believe it might've actually got that hot...

 

How old is the stat? It's the simplest and cheapest thing to check / replace first. Other issues could be radiator, pump, cracked head, etc. Any leaks?

 

Stat and radiator/core were changed about a year ago and pump about 50k miles ago. Has never had any concerning leaks. Also, drove it home a few minutes ago and no problems this time...