Engine: Spark Plugs

What it Does

The spark plug is fired by the vehicle’s ignition system. This is done to ignite the air & fuel mixture in the cylinder, which is how the engine generates power.

Why This Matters

Gasoline engines (excluding diesel engines) require spark plugs to run. The spark plug should deliver spark for each combustion cycle that occurs in its cylinder. Since spark sets off the in-cylinder combustion event, without spark, the engine cannot make power. Since most vehicles run 4,5,6,or 8 cylinders, loss of spark for one cylinder will not disable the engine, but it will cause it to misfire and run rough. Loss of spark on multiple cylinders can disable the engine.

Possible Problems and Concerns

Misfire (due to weak or no spark):

A misfire induced by a failed spark plug will result in unburned fuel being present in the applicable engine cylinder and the vehicle’s exhaust system. Continued driving in this condition can result in loss of cylinder lubrication via wash-down, and catalyst deterioration or severe failure per intrusion of unburned fuel.

Ignition System Stress:

A worn or failing spark plug can put excessive stress on the coil that is attempting to fire the spark plug. If this condition is left for too long, requisite repair work may include the spark plug and the coil that fires it, and maybe even the spark plug wire between the two.

Overdue Spark Plug:

A spark plug that has been run past its proper lifespan may cause the engine to misfire. This often results in testing costs that would have been avoided if the spark plug had been replaced at a proper maintenance interval. Spark plugs are a standard maintenance item (even the ones that run very long intervals), and appropriate maintenance in this area can be a money and time saver.

Carbon Tracking:

If a carbon track develops between the spark plug and the ignition coil or spark plug wire boot, electrical energy can travel down the carbon track instead of jumping the spark plug gap. This results in the engine misfiring. Good workmanship standards help to prevent this kind of failure from occurring. If it occurrs, both the applicable spark plug and its insulating boot should be replaced.

Wrong Part (Heat):

Installing a spark plug with the wrong heat range can push engine in-cylinder temperatures to the point of internal mechanical damage occurring. Just because the spark plug looks like decent match from the outside doesn’t mean that it has the proper electrical characteristics.

Wrong Part (Dimensions):

If too a long of a spark plug is accidentally installed, it may get smashed by the cylinder’s piston.

Varying Longevity:

It is very common for multiple spark plugs to fit the same engine and operate in the correct heat range as well. However, rated or approved spark plug lifespans can vary all the way from 30k miles to 120k miles (among plugs that would all work in the same vehicle!)

Recommended Parts Brands

  • OES or factory grade
  • Denso
  • NGK
  • Motorcraft
  • AcDelco (if OE grade) for GM vehicles
  • Motorcraft for Fords

Repair Notes

Longer-life spark plugs generally cost more, but the cost of labor means that it’s cheaper to install $12 plugs every 100k miles rather than $4 plugs every 30k miles. In the case of older vehicles with lower-rated OE plugs, sometimes it’s possible to upgrade to a longer-life spark plug that will operate properly in that vehicle.