Coolant guage goes back to cold after being shutdown for 20 minutes

[ElectroMechanoMan]

That sounds more like a thermostat problem to me. The coolant isn't really getting to the oversized radiator unless the thermostat is allowing it to do so.

Both engines ran with temperature gauge oscillations from the time the trucks were new.

 

[dumkid]

Both engines ran with temperature gauge oscillations from the time the trucks were new.

Diesels usually do this. They don't make much heat while cruising or idling but they do heat up under load. So the temp gauge will wiggle a bit.

Gas cars make enough heat at idle to keep the thermostat open

 

[belbers9715]

Sounds like my current truck. 2015 F150. My transmission temp gauge goes all the way cold if it sits for 10-15 minutes. The reason is the gauges on the dash are essentially "idiot lights". They do not actually measure temperature, they simply show if parameters are in-spec.

 

[The motor guy]

Sounds like my current truck. 2015 F150. My transmission temp gauge goes all the way cold if it sits for 10-15 minutes. The reason is the gauges on the dash are essentially "idiot lights". They do not actually measure temperature, they simply show if parameters are in-spec.

Possibly. The one on the newer truck could be real.