So lately I have noticed my idle increases from about 600 RPM to 800 RPM while sitting out in the sun in park. It does it automatically without any input from me whatsoever. Is this common? I don't recall it doing this before but maybe it was and I just didn't notice. It definitely lowers the engine temperature from about let's say 209° to as low as

194°. Anybody else experience this?

 

In my years of idle time in the west I haven't noticed my idle going up after a long sit time but then maybe I just haven't notice/ paid attention. I'll pay attention now.

Maybe with your truck being a F550 it's a fail safe feature being a "real" work truck. 😉

 

Yes my 2013 6.2 did it, and the 2023 7.3 does it also.

Only when it's fairly heat soaked and sitting at a light or something.