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Spark plugs come in an abundance of designs.  Many claims used in selling spark plugs are vastly overstated or false.  Very sound reasons also exist, why spark plugs should be replaced.   Many reasons for replacement are not common knowledge.

Evolution of spark plugs has been slow

The basic spark plug has changed very little from the days of the Model T Ford.  Design changes have been evolutionary  but not revolutionary.  For instance in the mid 1960's the resistor spark plug was introduced.  Resistor plugs were originally designed to thwart radio interference.  Today the resistance is also used to provide feedback to electronic control circuits.

Materials used to build spark plugs have also changed.  Steel and tungsten have largely been replaced by platinum and iridium for plug gaps.  Washer type seals on the plug bases are more often a conical seat today.  Actual spark plug gaps have also become much wider and are no longer adjustable on many plug types.  For instance the gap on iridium plugs should be checked, but never adjusted.  If the gap is improper, the spark plug should be returned or replaced.

Each of these design changes and countless other engine enhancements have resulted in spark plugs that can lasts up to 100,000 miles.  Many useless gimmicks have also been tried, in an attempt to lure sales.

Multiple electrodes, strange electrode shapes and claims of pulsing, offers no benefit over conventional spark plugs

a worn spark plug gap and a new spark plug

The function of a spark plug is to provide a gap, where a spark may jump and ignite the fuel-air mixture in the engine.  As the high voltage spark jumps across the metal gap, the electrodes wear.  Metal is eroded and the shape gets rounded.  The wider gap, with rounded edges of worn electrodes are more difficult to jump.  This is one reason spark plugs must inevitably be replaced.

On a modern vehicle, replacing worn spark plugs will not enhance fuel mileage or solve running problems

Older vehicles were not computer-controlled and lacked the ability to adapt to changing conditions.  The spark for the plugs was generated by mechanical or crude electrical means.  Since the spark intensity was set and limited, engine performance decreased as the spark plug wore.

Today this is no longer a factor.  An ignition spark is created by electronic means and can be increased as needed.  As spark plugs wear the ignition can increase output to compensate.  A computer-controlled engine will NOT experience degradation of mileage or performance as spark plugs wear.  Instead of performance loss, worn spark plugs cause components like plug wires and coils to be overloaded and fail.

worn spark plugs can damage plug wires and ignition coils

To prevent other much more expensive problems, spark plugs must be replaced when they wear out.  The best guide is the manufacturer’s mileage recommendation, though these are sometimes at the extreme end of service.  Not replacing spark plugs in time can result in damaging ignition coils and other components.

carbon has caused a track on this misfiring spark plug

Worn electrodes are NOT the only reason for replacing spark plugs.  High resistance, tiny cracks in the ceramic and other things can cause a plug to “carbon track.” Carbon tracking is where the spark fires between the top of the plug and the base, across the ceramic shell.  This leaves a characteristic carbon track on the insulator.  It also causes the plug to misfire.

A misfiring spark plug is extremely damaging to an engine.  When the spark plug fails to fire, the fuel in the cylinder is not burned.  This unburned fuel enters the catalytic converter, where it drastically increases the temperature.  One misfiring spark plug can destroy an expensive catalytic converter in a matter of minutes.

hot compression gas can destroy a coil-on-plug very quickly

Spark plugs can also leak compression around their bases.  When this occurs, extremely hot compression-gas may destroy coil-on-plug type of ignition coils.  These coils can easily cost $100 to $200 each and there is usually one for each engine cylinder.  Leakage at the ceramic results from age and can be made much worse by pinging and detonation.

Best is to have spark plugs inspected, when mileage approaches the recommended interval.  Eighty-percent is a rule of thumb recommendation for inspection and sooner if a problem is suspected.  For instance if the recommended replacement interval is 100,000 miles, one or two plugs might be removed for inspection around 80,000 miles.

Some engines still use copper type plugs and have recommended change intervals around 30,000 miles.  Shorter change intervals are sometimes used to prevent the spark plug seizing to an aluminum cylinder head.  Other engines such as the Ford 3-valve V8 have a known issue with spark plug seizing and breakage.  In these engines, defective two-piece spark plugs should be replaced to help prevent this extremely expensive issue.  At 30,000 miles they can often be removed.  Waiting until the recommended 100,000 mile intervals may require engine removal and disassembly to repair.

Waiting for symptoms, such as dropping fuel mileage, before replacing spark plugs may be an expensive lesson.  If your vehicle has 80% of the mileage recommended in the owner’s manual for plug replacement, play it safe.  AGCO can inspect and if needed replace your spark plugs.  AGCO, it’s the place to go!

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