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I have a 99 V-10 Triton that keeps over heating. I have replaced the water pump, thermostat, and heater core, flushed the entire system and put a dual electric fan on it. It was good for awhile. That is before the heater core was replaced. Since that time, if I drive above 65 mph it will overheat. Any ideas?

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There are a few causes of overheating that wouldn't be fixed by your previous repairs.

  • System not pressurising: The radiator cap needs to seal pressure in. If it doesn't, your coolant will boil at a lower temperature than intended, preventing it from flowing through the system properly. Check your rad cap.
  • Coolant temperature sensor: sometimes, they get funky. Perform an electrical test on it, make sure it is reading within range.
  • Poor air circulation through the radiator: Have a look in front of the radiator for obstructions. Leaves, dead bugs, shopping bags, bent fins, etc.
  • Poor coolant circulation: given the age, there's probably gunk in your cooling system. Just flushing it can help, but it may just loosen crap that will detach and plug something later. Drain the system and fill it with something that dissolves calcium, lime and rust buildup. Run the engine until it reaches temperature and turn the heater on full blast and set to max heat. Then drain and fill with proper coolant.
  • System is overworked: are you towing something? Are you running the AC full blast? Are you running high revs? Are you doing all 3 together? These are an easy fix: STOP IT! :D
  • Air trapped inside the cooling system: use the bleeder valves and "burp" the system. With the engine off, squeeze tubes until coolant slushes around quietly.
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The clue is that the heater core was replaced; it sounds like a classic case of the system being air-locked. You can get the scenario where a bubble of air is trapped below the level of the coolant in the header tank. This means that the coolant doesn't actually circulate even though the level looks good.

Your best bet is to allow the car to cool. Remove the radiator cap, switch the HVAC controls all the way to HOT then let the engine run with the cap off. Squeeze the radiator bottom hose and watch of air bubbles "burp" out of the radiator cap. Keep doing this and slowly adding coolant until all of the air is flushed out of the system and hopefully that will solve the problem.

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