Holden HQ–HZ Engine Overheating – Causes and Remedies

Holden HQ–HZ engines are tough units, but like any older powerplant they can suffer from overheating if the cooling system isn’t in top condition. Below is a breakdown of the most common causes, symptoms, and diagnostic steps to help owners keep these classics running cool.

Common Causes of Overheating and possible solutions

When an HQ–HZ engine starts to run hot, the issue usually comes back to one of the following:

Lack of coolant in the system
• Refill the radiator and overflow bottle
• Pressurise the system and check for leaks
Fan belt issues
• Belt loose – adjust or replace
• Belt worn, cracked, or oil‑soaked – replace immediately
Thermostat faults
• Thermostat sticking closed – replace
• Incorrect heat range – fit correct specification
Water pump problems
• Pump not circulating coolant – repair or replace
• Leaking pump seals or noisy bearings – replace
Cooling system blockages
• Rust, scale, or debris in passages – flush and reverse‑flush
• Radiator fins obstructed – clean externally
Incorrect ignition or valve timing
• Re‑time the engine to factory specifications
Brakes dragging
• Check brake adjustment and ensure calipers/wheel cylinders are not binding
Wrong engine oil
• Incorrect grade or viscosity can increase operating temperature
• Change to the correct oil for climate and engine condition
Lean fuel mixture
• Overhaul or adjust carburettor to correct mixture
Restricted air cleaner
• Clean or replace the filter element
Exhaust restriction
• Partially blocked exhaust or collapsed muffler – clean or replace
Fan drive clutch (where fitted)
• Defective clutch reduces airflow – replace

When the Engine Won’t Reach Normal Temperature

Sometimes the opposite problem occurs—an engine that runs too cool or shows an incorrect temperature reading. Common causes include:

Thermostat stuck open or wrong heat range
• Replace with correct specification
Faulty temperature sending unit
• Replace the sender
Temperature gauge faults
• Gauge not reading correctly or installed incorrectly – inspect or replace

Diagnosing Coolant Leaks

Coolant loss is one of the most common contributors to overheating. Typical leak sources include:

Radiator or transmission oil cooler leaks
• Repair or replace
Cylinder head gasket failure
• Replace gasket and check head for warping
Incorrectly tightened head bolts
• Re‑torque evenly to specification
Cracked or warped cylinder head or block
• Repair or replace as required
Cracked intake manifold
• Replace manifold
Heater or engine hoses
• Replace damaged hoses
• Ensure clamps are tight and sealing properly

Visual Inspection Checklist

A quick visual inspection can reveal many cooling system faults. Check for leaks at:

All hoses and hose connections
Radiator seams and core
Engine core plugs and drain plugs
Cooling system gasket edges
Transmission oil cooler
Heater core and heater hoses
Water pump housing and weep hole
Dipsticks for signs of coolant in engine oil
Overflow/supply tank for signs of oil contamination

Thermostat Testing Procedure

If you suspect the thermostat, you can test it with basic workshop tools:

Remove the thermostat from the engine
Place it in boiling water until fully open
Insert a feeler gauge into the throat and allow it to cool and clamp onto the gauge
Suspend a thermometer in a container of cool water and heat the water gradually
Suspend the thermostat by the feeler gauge in the warming water
When the thermostat opens and drops off the gauge, note the temperature

Correct Thermostat Specifications

6‑Cylinder Engines

Opening temperature: 177–182°F
Fully open (at least 10 mm): 202°F

V8 Engines

Opening temperature: 177–182°F
Fully open (at least 10 mm): 202°F

In Closing

Overheating in HQ–HZ engines is almost always preventable with regular checks, proper coolant maintenance, and attention to early warning signs. These engines respond well to routine care, and keeping the cooling system healthy is one of the best ways to ensure long life and reliable performance.

While this guide helps trouble shoot common problems of the Holden HQ-HZ engines the Holden HQ-HZ 1971-1978 Workshop Manuals available from Car Manuals Direct include all the original specifications, adjustment, torque settings and repair procedures used by Holden mechanics.

Safety, Copyright and Source Notice

Safety Disclaimer
Working on motor vehicles involves hazards. Always follow correct procedures and consult a qualified technician.

Source
Information compiled from period factory service literature.

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