Elim

Hey Dave, I am glad to hear it is now back to the way it was, and what I have experienced as "normal" on both this truck, and the 6.7 Cummins I had before this one. When we build up enough heat in the engine, and the thermostat finally opens. All of that ice cold fluid from the radiator hits the warm thermostat/engine. That causes the stat to slam shut and have to build heat again to re-open. Rinse, and repeat. I think since we can not get enough heat into the coolant/radiator it just constantly opens, and closes causing the swings up, and down. I know we all know how a thermostat works, but this guy shows it with a small torch, and then spraying it with water to show it opening, and closing. I think this is exactly what is happening to you/us. http-~~-//youtu.be/T8cGKdICJmc I just put on some grill inserts today. It is not really a full winter front, but should help restrict some of the air flowing through the radiator. I will report back when I get some drive time if it helps at all. Take care!

Hey Dave,Just letting you know you are not alone. Mine does the yo-yo temps too. I only climb to 204ish before it opens the first time. After that it goes up to about 199ish, and drops to about 188ish. I also felt that only my top hose and that very top corner of the radiator was warm/hot. The cap, and the other side were cold to the touch as well. My coolant is still full, and clean. It is not leaking, or blowing out anywhere. I think it is just that we are not getting enough temp into them in the colder weather without pulling/hauling something to keep the stat open.I can only guess it is normal because I have never had any problems with temps leading up to this, and only noticed it swinging this much after it had gotten cold. My other Cummins was the same with temp swings as the thermostat opens, and closes. I forget if it was exactly the same or not though. I think it was about the same because I remember a bit of concern initially like I was with this one.I do not know what is going on now that you can not hit 180 though unless the bypass hole in the NAPA stat is larger allowing more coolant to pass through without the thermostat having to open, and your not getting it warm enough to get up there. I am by no means an expert though. Lets see what everyone else says.Good Luck!

*BUMP* Reminder for oldschool. Can you please hook us up with the numbers when you get time? Thanks!