My Detroit 60 will drop engine temp to 50 idling in cold weather.

have you tried checking your thermostat? When my heads started going bad it would cool down a lot during idle so I think your heads are good if it doesn't happened all the time.

 

My best educated guess (and I'm very uneducated) would be the temp sensor maybe has a loose connection of moisture in the plug that's freezing etc. My temp sensor needed replacing and hiw I knew was it would sporadically go from overheating to under heated

 

Get a Thermostat. Make sure its a 190 degree one. They are simple to replace but you'll need some antifreeze to replace the lost portion. Meanwhile, put some cardboard in front of it and check the fan to see if it is running all the time. Possible the clutch is bad and fan is stuck in drive mode.

 

Do you get heat from your heater fan in cab. Could be a sensor problem. Your wires when they get cold shrink, this could give your ecm a false reading.

 

I do not know what year your engine is. mine is a ddec 3 95 year. the temp gauge on dash gets it reading from the sensor in the thermostat housing. found this out with a continuity tester. the water temp sensor in the side of the head is what sends temp to e.c.m. the sporadic reading could be a bad ground, or a sensor going out. check the wires at the sensors. they break & sometimes are holding on by 1 or 2 wire strands. on mine when I change my thermostats, I drain all the coolant in 1 gal jugs so I can reuse. you will loose a lot of coolant if you take thermostats out without draining most of the coolant. this again is on my engine, don't know about yours & how different it may be. if you know your engine, like I know mine, the smell, the feel, e.t.c. & know it is not getting hot & coolant level is normal. than it is most likely a wire, I.M.O. could be a common ground used on several sensors, or like I said bad wire connection at one of the sensors would be where I would start.

 

if i idle my s60 in 20 degree weather it needs to be at 900 rpms to stay at 120 coolant. They don't run hot while idleing. To save bull gear you should idle around 900 anyway. Why it overheating i dont know. Are you sure your coolant mix ratio is correct?