Hey Scotty.
Got a wired problem and I think its the thermostat.
I have a diesel van, brand new got 8k miles on it. Winters coming and morning's are cold. While the car is warming up and not fully to operating temp I watch the heat gauge move in the opposite direction while going down hill. If its cold enough out and its just gotten to operating temp it will reverse too. If I come to a stop and its idling it slowly moves backwards as well. I live in a very hilly area, so from my place to the highways mostly downhill.
If I drove it up to my place it warms up rather quickly but if I turn around just as the needle touches op temp and head downhill it can loose about a quarter of its temp rolling down. While in gear, standard transmission.
What's going on there?
It's not unusual for a modern small diesel to cool down (or not warm up) when going down hill. There is almost no combustion to warm it up. Like the Renault K9K CDI engine, that also cools down when idle without the radiator fan running.
Cool, never saw that happen before. My volvo xc90 is up to temp 1/4 way down that same road same temps D5 engine, but the 2.0 Ford ecoboost Na ah. Takes father time to get up to temp.
D5 is a nice engine! If you want a precise temp reading you can use a scan tool like the BlueDriver, in some cars the dash temp gauge can be a bit slow or sometimes be biased to show "normal" engine temp.
Hmm that’s strange it must be really cold outside for it to move toward the cold direction, cold air and downwards force of air could do that but it should get to operating temp eventually, I don’t think it’s anything to do with anything on your vehicle especially if it’s only got 8k miles on it, but check your thermostat just In case it isn’t stuck open
In harsh winters this behavior, which is pretty normal and even a good sign of fuel effectiveness, can get annoying. An easy and time-proven solution is a sheet of cardboard placed in front of the radiator in the most cold days, to limit airflow through the radiator 😉
Depending on climate specifics and vehicle type, partial coverage may be enough.
