Coolant problems

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You may have a cracked head or a dropped liner/blown head gasket. Does your truck have a bleed hose from the thermostat housing to the header tank? If so, put the header tank end in a bottle of water and run the engine with the fan belt removed from the crankshaft pulley (to take water pump pressure out of the equation). If you get bubbles in the bottle of water the cause is either a blown air compressor head gasket (unlikely) or a cylinder head/liner issue.