Rough Idle

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Original submission by T Feb 19th 2007:

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The yellow Circle shows the Bulkhead Connector where the Coil pink and Yellow Wires Connect. Click to Enlargen. Photo by HX_HZ_Monaros.

Rough Idle in Straight 6 Cylinder Holden Engines:

Many factors can contribute to making an Old Holden Engine Idle roughly. As in any repair situation a good diagnosis can save you a lot of time and money where the results are easy to interpret. It's always best to test the simple things first. No-one wants to tear an entire Engine to pieces when the only thing wrong was a loose Coil connector.

Air Leaks:

Air Leaks are a big contributing factor in this problem. In pre-PCV Engines an air-leak was an easy problem to diagnose. After PCV came along the air leak could then be caused by almost any part of the Tin Ware.  

Exhaust Leaks:

Periodically check the exhaust manifold nuts. They can work loose through thermo-cycling. Thermo-cyling is the process of a bolt expanding with heat, becoming loose, then twisting partially undone each time it heats up as a result.  

Loose Inlet Manifold:

Periodically check the tension of the inlet manifold nuts. The gasket shrinks over time and permits exhaust and inlet leaks.  


An Engine that is getting an excess or a shortage of Fuel will produce a rough Idle. The Power Valve is supposed to be drawn clear and shut at Idle and only engage as the Throttle is opened. A leaking Power Valve can cause a rough Idle.  

Ignition and Ignition Timing:

While excessive Spark Advance can cause an Engine to kick back, excessive Retard can cause an Engine to heat up. Any part of the Ignition Timing that wanders can also help cause a rough idle. Loose and worn Distributor Balance Weights are notorious for it.  

Mechanical Wear:

Valves that aren't sealing properly or have loose Valve Guides can contribute to rough idling. Broken Piston Rings and Pistons will cause Compression and Power loss contributing to the problem. Re-using old rockers and hydraulic lifters also causes unreliable valve clearances which results in rough idling.  

The HZ Rough Idle:

HZ 202 Rough Idle Submitted by T on Mon, 19/02/2007 - 08:18. As mentioned, retorque all the Manifold Nuts, EGR Carby Mounting Nuts etc. HZ's are dependent on good PCV for good idle. Any tinware leak screws them up. Check for loose screws or oil leaks around Rocker Cover, Side Covers and any looseness of the Sump. Pinch any rubber hoses shut with pliers and note the idle speed as you do so. Pinching shut the Brake Booster and Trimatic Vacuum Lines should not affect the Idle. If they do, there is a leak in them. Run carby cleaner down the Idle Air Bleed Passage of the Carburettor (located near the Choke Butterfly on the Float Chamber side of the Carburettor). The Engine will slow as you do this then speed up as you take the spray away. There's a vertical stud on the Engine side of the Inlet Manifold which holds the 2 halves together. If the stud works loose it can cause an airleak. A 9/16 AF spanner tightens the nut. Its 2 fellow bolts are located under the EGR Valve. T 

Bulkhead Connector:

On HQ and later model Kingswoods there is a Connector next to the Wiper Motor that provides the Ignition Feed to the Coil +ve. Pink and Yellow Wires connect together there. The Pink Wire is active when the Key is in the ON Position and the yellow Wire is active when the Key is in the START Position. This Connection is known to burn if the Connector has been allowed to come loose. 

Plug Drop Test:

Plug Drop Test A plug drop test will tell you which Cylinders are weak.



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