* Water Pump Failure: This is the most probable cause. A failing water pump can't effectively circulate coolant through the engine, radiator, and heater core. The lukewarm upper hose points directly to this, as it's not getting sufficient hot coolant flow.
* Thermostat Stuck Closed: A thermostat that remains closed prevents coolant from circulating through the radiator until it reaches a very high temperature. This would cause the engine to overheat and the upper hose to remain cool until the engine is excessively hot. However, given the lukewarm hose, this is less likely than a failing water pump. It's possible it's partially stuck.
* Radiator Issues: While less likely given the lukewarm hose, a severely clogged radiator could restrict flow. However, a completely blocked radiator would likely cause the upper hose to be very hot, not lukewarm. Internal blockage is possible.
* Blocked Heater Core: A blocked heater core would prevent hot coolant from reaching the heater, explaining the cool air. However, a blocked heater core alone wouldn't explain the lukewarm upper hose and overheating. It's more likely a symptom of a larger circulation issue.
* Head Gasket Failure (Less Likely): A blown head gasket could cause coolant to leak into the engine cylinders or oil, leading to overheating. While this *can* cause overheating, it usually presents with other symptoms like white smoke from the exhaust, milky oil, or loss of coolant. The lukewarm hose makes this less likely initially.
Troubleshooting Steps:
1. Check the coolant level: Low coolant can cause overheating. Top it off if needed (use the correct type for your engine). Look for leaks around the engine, hoses, and radiator.
2. Feel the lower radiator hose: If the lower hose is cold while the engine is hot, this strongly points to a water pump failure.
3. Check the thermostat: If possible, remove the thermostat and inspect it in a pot of boiling water to see if it opens properly.
4. Check for air bubbles in the system: Air pockets can hinder circulation. Bleed the cooling system if necessary (your vehicle's manual should explain how).
Recommendation: Given the symptoms, start by replacing the water pump. It's the most likely culprit and relatively inexpensive to replace compared to other potential repairs. If the problem persists after the water pump replacement, then you can investigate the other possibilities. Because this is an older vehicle, consider replacing other worn components while you're at it – such as hoses and the thermostat – to prevent future issues.
Important Note: Overheating can cause significant engine damage. Do not continue driving the truck extensively with these symptoms. Have it towed or have a qualified mechanic diagnose the issue.
