Summer is gone here and by night temperature drops at about 16°C not that cold..and even engine temp drop down to 60°C or less when descending long grades without the touch of the gas... so this is normal or I have sm thermostat issue? maybe other cooling problems..? I think truck must stay at 90°C degree even w no gas...

thanks all, Rob

 

Mine is an 06 and it drops about 10 to 15 degree when coasting down hill. With little to no fuel when coasting and RPM being up, then your engine becomes an air pump and it will cool off quickly.

If the thermostat maintains a constant temp when driving normally I would not worry about it too much.

 

I have a similar problem, but only when I tow my 5th wheel. If I go downhill on a pretty good grade, the water temp will drop to 140 degrees & I doubled checked the oil pressure, which is up where a cooler engine would be.

I changed my thermostat & put in Cummins coolant 3 years ago, the thermostat is supposedly an updated version. The funny thing is, it only does it when I tow the trailer & it won't drop like that without the trailer.

 

Yup, mine does the same thing. It'll go all the way down to 140 or so if I'm coasting down the backside of a pass or something. Doesn't matter if it's loaded or not. My 06 did the same thing.