If it's losing coolant, that can only go outside (leak) or inside (crankcase or heater box). If it's not pumping coolant, I would think the reservoir would become full from the blow-off. Sometimes ya get real lucky, and it's a radiator cap not sealing & pressurizing, but I'd expect staining. I'm assuming the cooling fan comes on ? How does the oil look ? Any coolant odor from outside or at the tailpipe ?
