Plug Drop Test

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Original submission by T Feb 19th 2007:

Plug Drop Test:

What is a Plug Drop Test:

In a well tuned and well running Engine each Cylinder should produce the same power.

Under the circumstances where an Engine lacks power or has a noticable rough Idle
the problem may be caused by one or more Cylinders not performing their share of the work.

Shorting each Spark Plug Lead prevents individual Spark Plugs from sparking. This prevents its
Cylinder from firing which should result in an equal drop in RPM for each Cylinder. The Cylinder that doesn't have an equal drop in RPM has a problem.

A Plug Drop Test will tell you which Cylinders are at fault.

If the Plug Drop Test reveals that all Cylinders have the same RPM drop and the Idle is still
rough, then the cause is something common to all Cylinders.

How to Perform a Plug Drop Test:

  • Note*; this is for Old Holdens and not Engines fitted with Catalytic Converters nor LPG.

    By shorting out each plug lead at the Engine, one a time while the Engine is running, each
    Cylinder should slow the Engine by the same amount. The Cylinders that make little or no difference have the biggest problems. The good Cylinders slow the Engine noticeably.

    Stop the Engine and make sure the Ignition is turned off.

    Remove each Spark Plug Lead one at a time and twist some wire onto the Spark Plug Terminal
    leaving a V shape protruding. Fit the Spark Plug Leads to the V and make sure they are tight
    enough not to fall off the Engine. Work one Spark Plug at a time so you won't become confused
    over where each Lead belongs.

    Do not allow any part of the Ignition to touch your body while the Engine is cranking or running.

    Start the Engine.

    *Caution* Note that the Ignitions on Modern Cars are very high potential so use a Screwdriver
    with a Plastic Handle and a stated Voltage Rating. A Wooden Handled Screwdriver must
    definitely *not* be used.

    Short each Spark Plug to ground by first placing the Screwdriver on the Cylinder Head then pressing the Screwdriver shank against the Wire you twisted onto the Spark Plug. Keep your Fingers away from the metal Shank of the Screwdiver.

    Record the RPM drop for each Cylinder.

    Once the test is complete shut the Engine off, remove each Spark Plug Lead and the twisted
    wire then replace each Spark Plug Lead directly to each Spark Plug.

Comparing Each Cylinder's Response:

If you have difficulty judging the speed difference by ear, use a tachometer to compare the speed drop of each Cylinder.



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