Spark plugs are the “window” into your engine

Spark Plugs
Spark Plugs

Spark plugs are the “window” into your engine (your only eyewitness to the combustion chamber), and can be used as a valuable diagnostic tool.

A visual inspection of the spark plugs will display symptoms and conditions of the engine’s performance. The experienced technician can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios.

The following is a small list of conditions that you may find useful when you are checking spark plugs:

Spark Plugs
Spark Plugs
  • Normal

When the engine is running the way it should, normal-reading plugs will look pretty much the same way they did when they were new. Normal, but with red coating. The red coating is a result of the additives in lower-quality unleaded fuel and will be visible on the plug’s ceramic insulation. The red coating is not an indication of any engine problems.

  • Fuel fouled

This may indicate:

  • A too-rich fuel mixture
  • Ignition problems
  • Heat range that’s  too low

Fuel-fouled plugs may have a shiny coating on the tip and side electrode.

First, check to make sure your spark plugs have a heat range that is compatible with your engine (especially if you’ve made performance modifications).

This information is available in your vehicle owner’s manual.

To resolve the too-rich fuel mixture, have the fuel injection (or carburetor) adjusted to correct the air/fuel mix.

  • Detonation damage

This plug condition indicates that your engine timing is off and you probably need a tune-up. Another possibility is that the gasoline you’re using does not have a high enough octane rating. Again, check the owner’s manual to verify the manufacturer’s recommendation on octane level.

  • Worn plug

This is an easy fix – replace the plug. Most plug manufacturers indicate the recommended service life on the packaging.

  • Carbon fouled

If your plug tip and side electrode are blackened, they have been running with too much fuel (or possibly too cool from a stuck-open thermostat). Other sources of the problem may include bad wiring or leaking injectors. In some cases the vehicle has been driven at too slow a speed for extended periods of time. The combustion process is not being allowed to have its natural burning-off, or cleaning, effect.

  • Preignition

The plug is firing too soon, not enough fuel is present in the air/fuel mixture or there isn’t enough fuel in the combustion chamber for a sound combustion event. Check your fuel injection and timing. Take quick action, because a plug in this condition is just short of falling completely apart.

  • Oil ash fouled

Engine oil is getting to your plugs from worn piston rings or valve guides/seals. Get to your mechanic now.

  • Mechanical damage

Piston hitting the plug or foreign material in the cylinder.


The two most common spark plug problems are hot fouling and cold fouling. The “too hot” category includes the preignition and detonation damage. Some performance improvements may be the cause for this type of plug damage. If your vehicle has performance upgrades such as a high-output coil, ignition, exhaust or cams, these can alter the engine’s recommended plug heat range, so you should consider using a spark plug with a heat range lower than the manufacturer’s recommendations.

A few of the symptoms indicated by your spark plugs have simple fixes – others require the hands and expertise of a qualified mechanic. Either way, the main advantage to checking out your spark plugs is for a quick diagnostic tool that gives you a fairly good idea of how well your engine is performing.

What can happen with Abnormal Combustion:


  • Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
  • Caused by hot spots in the combustion chamber…can be caused (or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
  • A change to a higher octane fuel, a colder plug, richer fuel mixture, or lower compression may be in order
  • You may also need to retard ignition timing, and check vehicle’s cooling system
  • Pre-ignition usually leads to detonation; pre-ignition and detonation are two separate events


  • The spark plug’s worst enemy! (besides fouling)
  • Can break insulators or break off ground electrodes
  • Pre-ignition most often leads to detonation
  • Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
  • Most frequently caused by hot spots in the combustion chamber.
  • Hot spots will allow the air/fuel mixture to pre-ignite. If the piston can’t go up (because of the force of the premature explosion) and it can’t go down (because of the upward motion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
  • Most of the damage that an engine sustains when “detonating” is from excessive heat
  • The spark plug is damaged by both the elevated temperatures and the accompanying shock wave.


  • A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead centre)
  • A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons…defective coil, too much compression with incorrect plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
  • Slight misfires can cause a loss of performance for obvious reasons.
  • Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage


  • Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
  • Will cause spark to leach to metal shell…no spark across plug gap will cause a misfire
  • Wet-fouled spark plugs must be changed…spark plugs will not fire
  • Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
  • Be sure to eliminate the root cause of fouling before changing fouled spark plugs.


A visual inspection of the spark plugs will display symptoms and conditions of the engine’s performance. In conclusion the experienced technician can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios.


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