Radiator Demons: Cooling System Malfunctions Often Overlooked or Misdiagnosed

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If your ride is overheating but full of coolant - and you don’t see any sign of a coolant leak - you might be under attack from invisible radiator demons. Relax, they’re not actual demons. The truth is they are only radiator malfunctions that are often overlooked during routine maintenance and testing. These malfunctions can leave you with a blown cylinder head gasket, cracked cylinder head, or damaged engine block if not quickly recognized and repaired.

Coolant Flow Restrictions
Even though your radiator is not pouring coolant, it could be malfunctioning because of a coolant flow restriction. Older vehicles and vehicles with high mileage are prone to this type of problem. Portions of the radiator core become partially or completely clogged with debris and block the flow of engine coolant to/from a particular radiator tank. This can cause the engine to overheat.
Vehicles which are more likely to overheat at highway speeds (than at low speed or idle) usually suffer from this condition. Diagnosing a radiator with restricted flow is made much easier if you have access to an infrared thermometer with a laser pointer. A radiator with a restricted flow condition will have portions of the core that are much cooler than other areas. Typically one lower corner (or even an entire lower half) of the inboard radiator core will be significantly cooler than the remaining area. Lower portions of the radiator core are more susceptible to this condition because sediment and debris from old coolant tends to settle there and become solidified.
Flushing coolant at regular intervals will help to prevent this condition.
The most effective course of action for correcting this problem is radiator replacement.

Air Flow Restriction
Since the automotive engine cooling radiator uses air flow across finned hollow tubes (rods) to decrease engine coolant temperature, a constant supply of clean ambient air is critical for proper operation. When the vehicle is at idle, the engine cooling fan draws ambient air through the radiator core, thus cooling engine coolant as it flows through the finned hollow rods and returns to the engine block, cylinder heads, heater core, etc. As vehicle speed increases, ambient air is forced through the grille, condenser, and radiator. Road tractors and other vehicles with large grilles or open grille designs frequently utilize a zippered cover to restrict air flow manually during winter months. This raises engine temperature, decreases the time required for the engine to reach normal operating temperature (when the thermostat opens), and the resultant temperature of the cabin heater. If your vehicle is so equipped; make sure that all restrictions are removed during summer months. Failure to do so may result in engine overheating. 
The most common inadvertent cause of air flow restriction is debris trapped between the radiator and air conditioning condenser. Debris may include leaves, pine-straw, dirt and other natural obstructions. These items often become trapped between the radiator and condenser when the vehicle is parked under trees, in the yard, or along the side of the roadway. Simply remove the upper fan shroud and use a flashlight to inspect the area between the radiator and condenser. If there is debris present, use an air blower or other slim tool to carefully remove it. In extreme cases, it may be necessary to tilt the radiator away from the condenser (or even remove it) in order to eradicate the debris effectively. If radiator removal proves necessary, consider replacement as a smart alternative to reinstalling a sub-par component.