I know that I havce a coolant leak somewhere in the system, but even when that coolant is topped up the heater doesn't work properly. We have had some cold weather lately (-10 to -25) and after driving for 15 minutes with the engine temperature up to 70deg the heater is barely putting out any heat, usually only enough to make the cab bearable.

After driving the truck across town, I felt the upper radiator hose: it was warm (more than luke warm but not too hot to hold on to).

Now I am stuck and I am not sure where to look next, maybe the thermostat?

Cheers,
Vince

 

yeah you might check the thermostat, or the valve to the heatercore in the firewall.