The cooling systems used in Holden HK, HT and HG models were designed to maintain consistent engine temperature under a wide range of driving conditions. Whether fitted with the 161 or 186 cubic inch six-cylinder engines or the 253, 308 and later 350 V8 engines, these vehicles use a pressurised cooling system with full-length water jackets surrounding each cylinder to ensure even heat distribution. Coolant circulation is handled by a belt-driven centrifugal water pump, while engine temperature is controlled by a thermostat and cooled through a cellular tubular radiator assisted by a belt-driven engine fan. When operating correctly this system keeps the engine within its designed temperature range, preventing overheating and ensuring efficient combustion. However, age, corrosion, worn components or incorrect adjustment can lead to cooling system problems. The following guide outlines the most common symptoms found on HK, HT and HG Holdens and their most likely causes.
Engine Overheating
One of the most common cooling system complaints is overheating, where engine temperature rises above normal operating levels. This can occur when coolant circulation or heat dissipation is reduced.
Common causes include:
• Lack of coolant in the system
• Fan belt loose or broken
• Fan belt contaminated with oil
• Thermostat stuck closed
• Water pump inoperative
• Cooling system clogged internally
• Incorrect ignition timing
• Brakes dragging and increasing engine load
• Incorrect carburettor adjustment
• Radiator pressure cap defective
• Radiator pressure cap main valve gasket damaged or missing
• Radiator core air passages blocked with insects, dirt or debris
Correcting overheating usually begins with checking coolant level, inspecting the fan belt, verifying thermostat operation and ensuring the radiator core is clean and unobstructed.
Over-Cooling
In some situations the engine may fail to reach its correct operating temperature. Running too cool reduces efficiency and can cause poor fuel economy and excessive engine wear.
Common causes include:
• Thermostat stuck open
• Extremely cold climate conditions allowing excessive cooling air through the radiator
Replacing a faulty thermostat normally resolves this issue, while partially covering the radiator area was sometimes used in extremely cold climates.
Loss of Coolant
Loss of coolant can lead directly to overheating and engine damage if not corrected quickly. Coolant leaks may occur in several areas of the cooling system.
Common causes include:
• Leaking radiator
• Loose or damaged hose connections
• Leaking water pump
• Cylinder head gasket leakage
• Cracked cylinder head
• Cracked cylinder block or loose expansion plug
• Engine operating at excessive temperature
• Radiator pressure cap defective
• Radiator pressure cap main valve gasket damaged or missing
• Leaking heater unit or heater hoses
Diagnosing coolant loss normally involves inspecting hoses, radiator tanks, water pump and engine gaskets, and pressure testing the cooling system.
Circulating System Noisy
Unusual noises from the cooling system often indicate wear in the pump or drive components.
Common causes include:
• Water pump seal squealing
• Rough or worn pump bearings
• Fan blades loose or bent
• Fan belt noisy in pulley
• Fan belt inner plies loose
Correcting these faults usually involves replacing worn water pump components, tightening fan blades or adjusting and replacing the drive belt.
The cooling systems fitted to Holden HK, HT and HG models were designed to provide reliable temperature control under demanding Australian driving conditions. However, decades of use mean many vehicles now require careful inspection of hoses, pumps, radiators and thermostats to ensure proper operation. While this troubleshooting guide helps identify common cooling system faults, the complete factory workshop manuals provide detailed procedures, specifications, diagrams and adjustment instructions required for correct servicing.
While this guide helps identify common cooling problems, the Holden workshop manuals available from Car Manuals Direct include the original specifications, adjustment settings and repair procedures used by Holden mechanics when these vehicles were new.