Temperature guage stays cold

Sounds like you need a new thermostat. The thermostat helps the engine warm up faster and keeps the engine at normal operating temperature which is usually 195°F. Hope that helps! Jim

Yep replace the thermostat first, make sure you get the right one for the engine, a good parts store should have the right one in stock. If the heater core is leaking you should disconnect it and loop the hoses together to prevent engine damage from loss of coolant. If the core is plugged that maybe an indication of further cooling system problems. Did you just buy this rig? If so them maybe the previous owner was trying to keep engine temperature low due to a plugged radiator.