Ignition and misfiring issues are common in older Holden HQ–HZ models, especially those running the 173 or 202 six‑cylinder engines. These problems can cause rough running, poor fuel economy, and hard starting. Below is a practical guide to diagnosing and fixing ignition and misfire faults in these classic Holdens.
Common Ignition Problems
Ignition faults are often the root cause of misfiring or poor performance. Check the following areas first:
-
Distributor Points Burned or Corroded
• Clean or replace points
• Ensure correct gap and dwell angle -
Incorrect Point Adjustment
• Adjust points to factory specification -
Worn Distributor Shaft or Bushes
• Excessive play can cause erratic timing — overhaul distributor -
Faulty Condenser
• Replace with correct capacity condenser -
Ballast Resistor Faulty or Out of Circuit
• Inspect and replace if resistance is incorrect -
Loose or Corroded Primary Circuit Connections
• Tighten and clean all terminals -
Ignition Coil Weak or Failing
• Test coil output and replace if voltage is low -
Incorrect Ignition Timing
• Re‑time engine using timing light and factory marks
Spark Plug and Lead Issues
Misfiring is often caused by poor spark delivery. Check these components carefully:
-
Spark Plugs Fouled or Worn
• Clean or replace plugs
• Set correct gap (usually 0.035–0.040 inch for 173–202 engines) -
Incorrect Plug Type
• Use plugs recommended for Holden six‑cylinder engines -
Loose or Corroded Plug Leads
• Clean terminals and ensure tight fit in distributor cap -
Damaged or Cracked Plug Leads
• Replace leads showing wear or arcing marks -
Distributor Cap Cracked or Carbon‑Tracked
• Replace cap and rotor button
Fuel and Air Mixture Related Misfires
Even with good ignition, poor mixture can cause misfiring. Check the following:
-
Carburettor Dirty or Passages Blocked
• Overhaul and clean carburettor -
Choke Malfunctioning
• Adjust or repair choke mechanism -
Vacuum Leaks
• Inspect manifold gaskets, hoses, and fittings for leaks -
Fuel Pump Weak or Failing
• Test output pressure and replace if low -
Fuel Filter Blocked
• Replace filter regularly
Engine Mechanical Causes
Persistent misfiring may indicate internal engine problems:
-
Low Compression
• Check piston rings, valves, and cylinder condition -
Valves Sticking or Burnt
• Free up or grind valves as needed -
Leaking Manifold Gasket
• Tighten bolts or replace gasket -
Incorrect Valve Timing
• Check timing chain and camshaft alignment
Electrical System Checks
Older Holdens often suffer from voltage drop or poor grounding:
-
Battery Weak or Partially Discharged
• Charge or replace battery -
Poor Earth Connections
• Clean and tighten engine and body grounds -
Alternator Output Low
• Test charging system and replace regulator if faulty
Quick Diagnosis Tips
- Listen for irregular firing or popping through the exhaust — indicates ignition or valve timing issues
- Check spark quality — a weak yellow spark suggests coil or condenser problems
- Inspect plugs after running — black soot indicates rich mixture, white deposits indicate lean mixture
- Use a timing light to confirm steady ignition advance
- Check vacuum advance operation on distributor — replace diaphragm if leaking
Remedies Summary
- Replace worn ignition components (points, condenser, coil, leads)
- Adjust timing and dwell angle correctly
- Clean and tune carburettor for proper mixture
- Ensure strong battery and clean electrical connections
- Maintain correct plug type and gap
- Regularly inspect distributor cap, rotor, and vacuum advance
Final Thoughts
The Holden 173–202 engines are reliable when properly tuned, but age and wear can lead to ignition and misfire problems. Regular maintenance, correct timing, and clean electrical connections will keep these classic six‑cylinders running smoothly and efficiently.
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.
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Source
Information compiled from period factory service literature.
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