why does my f150 six cylinder truck overheat even after replacing the thermostat and antifreeze?

It only overheats when the outside temperature is at least 80 degrees. When it begins to get hot (after about 20 minutes of highway driving), I have to turn the heater on full fan and full heat in order to keep the truck in a safe operating temperature range. What else could be wrong besides the thermostat?

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  • 1 decade ago
    Favourite answer

    Those are classic signs of a bad radiator - the build-up of heat is almost always a bad radiator and the sensitivity to outside temperature is a dead give-away. That running the heater, basically a very small radiator, helps means I would bet money on it (I don't gamble, but I bet when I know the outcome). A flow test should confirm the diagnosis: remove the hoses from the radiator and put a garden hose in the upper hose port. The radiator should be able to pass the full output of the hose if it is not blocked. The last radiator I changed, for a friend, was so blocked I could hardly blow air through it.

    If you have been using tap water in the radiator, either to dilute the anti-freeze or to top up the system, that explains a lot. The hard water minerals deposit out in the radiator tubes where the coolant is cooled, blocking them. No flush can clear them - believe me, I've tried - so the only economically practical solution is replacement. Most aftermarket radiators are good and radiators are not terribly hard to change. Dilute your antifreeze with purified water (less than a dollar per gallon at most supermarkets) and you won't go through this again.

    Source(s): 35 years maintaining my own cars
  • Anonymous
    1 decade ago

    The first thing, and easiest after double checking the coolant level, is to check the outside of the radiator. Blockage on the outside of the grill, radiator or between the A/C condenser (if equipped) and radiator will not allow air to pass through.

  • 1 decade ago

    check your Radiator, it needs to be clean and clear for the air to pass through, the only other thing is,to check your fan is cutting in, without this it will overheat in most conditions

    its a good idea to completely flush your cooling system,use a garden hose and let it run until clear water comes through.

  • 1 decade ago

    Could be several things. Could be the Temperature sensor or the heater core. Could also be a clogged radiator.

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  • Anonymous
    1 decade ago

    ITS A FORD!!!

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