The 1966 model year is the favoured year among many Mustang collectors. This year's Mustang came with three engine options, a straight-six 200, a 289-cubic inch V-8 with a two-barrel carburettor and the 289 with a four-barrel carburettor. Overheating is one of the most serious problems that your Mustang's engine may face. Knowing how to determine why your car is overheating goes a long way towards knowing how to fix it. No matter which of the three engines available in 1966 or what might have been put into it since, diagnosing your Mustang's cooling system problems are the same.
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Coolant leaks are the most common cause of engine overheating. The most obvious indicator of a coolant leak is an empty radiator and coolant overflow, if equipped with a sealed system. Open the radiator cap and check for the presence of coolant. If the level is low, refill as needed, start the engine and check for leaks in hoses, the engine block, intake manifold and thermostat housing. Replace any leaking hoses or gaskets. Also feel the floorboards under the dash against the bulkhead -- sometimes incorrectly known as the firewall -- for wetness. If the floor is wet, you'll have to replace the heater core because it's leaking.
Cooling System Condition
Coolant that has lost some of its resistance to boiling due to contamination needs to be changed. Change coolant every 15,000 miles or every year, according to most coolant manufacturers. Open the radiator and look at the coolant. Normally it should be bright green. If the coolant is rusty, flush the system. Check the visible radiator cooling tubes for obstruction by sludge and rust build-up. Excessive build-up should be professionally cleaned, a process that requires taking the radiator apart after removal from the car. Ensure that old coolant is disposed of properly, as it is highly toxic and poisonous. Don't place it where children or animals can get to it.
Obtain an automotive cooling system thermometer. Remove the radiator cap and refill the radiator as needed. Insert the thermometer into the radiator opening and start the engine. Run the car until it reaches normal temperature. The thermostat should open at 82.2 degrees C. Observe the coolant in the radiator. You should see coolant moving once the temperature climbs above 82.2 degrees C. If the coolant water does not circulate, the thermostat is stuck closed.
Head Gasket Leaks
Head gasket leaks are rare, but not unheard of. Head gasket leaks allow hot exhaust gases to enter the cooling system, boiling the coolant. There are two ways to test for this. The first is with a special kit and dye. The easiest is just to place a balloon over the radiator opening after removing the cap and starting the engine. Exhaust gases in the cooling system will cause the balloon to inflate.