3126 CAT in Freightliner M2 - no heat in cab?

As the engine warms feel the upper radiator hose. Should remain cool until 180 or so and then get hot. Heater hoses to the firewall or core should be hot to the touch and barely hold a bare hand to them. If the upper hose gradually warms with engine running and stays only warm the thermostat is stuck open and continually circulating coolant through the engine and is overcooling. With the engine at operating temp of 180 to 190, then look to poor coolant flow to the heater core or dash, blend door issue.

 

Agree 100% with the above. 125 degrees is definitely running cold.

 

They operate by compression ignition. That requires some heat and the reason for rough running cold starts. Temperatures have increased for emissions but the older diesels still liked around 185 degrees.

 

In today's world of emissions .........engine operating temp is critical to many key operating points. Given your location, you may or may not notice a complaint depending on the application of the truck. If your in northern
Michigan your gonna notice loss of heat before someone operating in Georgia. The biggest loser of low engine operating temp on any engine is fuel mileage.

I would never recommend removing a thermostat from any engine. They are designed to operate with them. In some cases, removing a thermostat can actually cause an engine to overheat.

 

As the others have already said it is most likely a thermostat failure. They generally fail in the open position so the engine doesn't overheat, and you can limp it home to repair it. Normally your coolant temps should be 180F to 210F, a bit more will not hurt, as the system is pressurized so doesn't boil over at 210 or 215F like it would if not pressurized. Good chance to change rad and heater hoses as well if they are older and need replacement.

 

You don't remove a thermostat because the coolant will flow thru the radiater too fast and not cool. You could drill a hole in it to start the cooling process sooner.