'08 GL320 T-stat Question
 

(Hopefully) a quick question...

Outside temp: 18*F (car sat outside overnight)

After 45 minutes of driving around (city-ish), the highest the coolant temp would reach (using a scan tool plugged into the OBDII port) is 182*F/83*C. Sitting in a parking lot (engine idling) it would drop to 160*F/71*C within 5 minutes. I did not stay in the parking lot any longer to see if it dropped lower.

It also took a good 30 minutes for the idle RPM to drop to a normal level (meaning, it stayed elevated - about 1,000 to 1,100) for at least 30 minutes.

T-stat sticking open or is this normal?

At idle, the fuel to air ratio of a diesel is something like 150 to 1.

Gasoline engines are always ~15 to one (stoichiometric)

Cooling at idle is to be expected with a diesel. That's why you see the canvas radiator blinds on big trucks and why some of them have electronics to force a fast idle should they cool down too much.

Still, that fast idle should subside ...

in cold weather it will take significant amount of time for the engine to reach operating temp. due to its high efficiency the diesel engine does not produce a lot of heat when not revved very high. so at idle it will actually cool off if you have the heat running and it is cold outside.

you do have however auxiliary electric heater that will warm up the hvac air for you even if coolant temp is low. so if you are getting warm air out of the vents even if the coolant temp is dropping or low- the system is working as expected.

if you want to help engine reach operating temp faster turn heat in recirc mode. it will revert back to fresh air in 10 minutes or so, but you can repeat a few times for best effect.

I was getting the normal amount of heat as I sat there - or, at least, there wasn't an obvious difference. Thank you guys - sounds like it's pretty much doing what it should. I was surprised at how long that high idle took to calm down. But maybe I was just being more "critically aware" since I was monitoring things closer to help further diagnose another issue I posted about.

Yes - I have a "winter cover" that I made for my old '98 Suburban with a 6.5L diesel. But that engine is a dinosaur compared this 3.0L. Everyone withing a half block radius knows it when that beast starts up on a cold winter morning!!!!