Heater core shutoff valve…revisited

[racerop]

Originally Posted by LuckyLightning

I just drove the truck today for a few hours in the hottest part of the day. I think what I saw was the temp rising before the thermostat opened up. I saw it happen again today - the temp would rise until the thermostat opened up, then go back down to normal. Interesting thing was it happened every time I turned off the truck for more than a few minutes (such as when pumping gas). I assume this is normal operation, but I never noticed it before.

I had a car have the same symptoms as what you are describing. I would flush/refill with new coolant and replace the thermostat. The thermostat is probably sticking while opening.