I had just bought 2 thermostats when engine blew. Kit came with 2 new ones. I watched them get put in. Still have 2 new ones. The old ones were from last Truck,I had just replaced them when engine blew. Got them back for spares. History repeats itself. Good to have spares. But the rubber seals dry up. Only got 28k on inframe. I want it to seal rings up good. It’s always ran cold, same with last one.but not this cold.
There are seals that are inside the housing that wear out and let coolant leak past
through the rad and over cool the engine. they are down inside the housing.
More often than not its the seals leaking and there is not a problem with the thermostats.
Ok, finally got to warm weather. It’s 48 degrees. Idling at 1000 rpm. Fan just came on just about 200, Went down immediately to 190, fan shut off, only tan about 15 seconds. Temp went down a little more, now headed back up. Seems normal. Just running cold in cold weather.At an idle, and even cooler running down the road. Runs cool enough that Cab heat is noticeably less. Last pic is with manual fan switch on, after about one minute. That seems a bit too cool.