ISX CM2350 trouble maintaining temp

[Moose]

With the colder then usual temps down here in the past couple weeks I'm finding that it cannot hold 180 at idle past an hour. My local work is slow so I'm having to rollerskate it regionally and I found out this the hard way up in TN on night. I woke up to no heat, 12 deg ambient outside and 140 degree coolant temp. It has done it every night I've had to idle since it's been demandated and cold. It acts exactly as if the t-stat is sticking open like the older ones with the bad bridges, but this is not the case. Some history: De-mandated (fully), t-stat recently changed again (frivolously).

Now, I know with the modification already done I'm losing a large amount of heat rejection with the EGR cooler now producing zero heat while its in bypass loop but there's either something else providing cooling or I'm just not producing enough cylinder temps anymore. I'm looking for advice or history as to what everyone has possible dealt with before.

I'm thinking it's one of two things:

It looks like the coolant still flows to and from the trans oil/water exchanger along with the heater core when the t-stat is closed/bypass. I could solve this with an oil/air conversion. There is normal flow through the vent tubing even with the t-stat closed but I do not want to mess with this flow.

At idle (700) the exhaust temp is around 230F and will go as low as 200F. Raising the idle up to 1500RPM and locking the fan in I can get 360F exhaust and it will come up to temp VERY slowly. Driving the truck it will reach temp and hold it. Barely if empty and it's typical 185F coolant/215F oil temps when loaded.