How to Read Spark Plugs

Embody your inner diagnostic god with the following advice!

Spark plug reading could be looked upon with mysticysm but these electric wizards can unlock the key to diagnosing your engines running conditions. From normal wear and tear to potential engine issues, transform yourself into a mechanical detective!

Removing your Spark Plug
Generally a deep reach socket and extension bar will be required to remove the plug itself, they should not be tight, screwed in by hand and nipped up afterwards.
Firstly; let your engine cool. There is nothing worse than removing a spark plug to find the end is red hot only for it to make that fateful journey to the workshop floor!

Regular Happy Spark Plug
The elctrode end of the spark plug should have an even gap with a uniform tip. The colour should be brown or light grey in colour.

Wet Fouling
Ooccurs when a liquid, such as fuel or oil, coats the spark plug, preventing it from firing correctly. The plug will appear wet and glossy, usually with a dark, oily or fuel-like substance.
Causes of Wet Fouling:
• Fuel Fouling: This happens when unburnt fuel builds up on the plug, often due to:
• An overly rich air-fuel mixture (too much fuel, not enough air), caused by issues like a faulty fuel injector, choke, or carburetor.
• Cold starts, where the engine doesn’t run long enough to burn off excess fuel.
• A clogged air filter, restricting airflow and causing more fuel to enter the combustion chamber.
• Oil Fouling: Oil fouling occurs when engine oil leaks into the combustion chamber and coats the spark plug, usually due to:
• Worn piston rings or cylinder walls, allowing oil to seep into the combustion chamber.
• A damaged valve seal or gasket, letting oil enter the cylinder.
• Overfilling the engine with oil, which can cause oil to splash up into the combustion chamber.
Wet fouling prevents ignition by drowning the spark in liquid fuel or oil.

Dry Fouling
Typically results in a powdery or crusty deposit on the spark plug, often appearing as a black, sooty, or ash-like substance. It can prevent the spark plug from functioning efficiently, as the buildup interferes with proper ignition.
Causes of Dry Fouling:
• Carbon Fouling: This is the most common form of dry fouling, and it occurs due to:
• A rich fuel mixture, much like in wet fouling, but instead of being liquid, the unburnt fuel leaves behind a carbon residue.
• Extended idling or low-speed driving, which prevents the engine from reaching the temperature needed to burn off deposits.
• A dirty air filter or improper fuel-air ratio, leading to incomplete combustion and carbon build-up.
• Ash Deposits: Certain additives in oil or fuel can leave behind metallic ash deposits, which are dry and can accumulate on the plug over time. Causes include:
• Using poor-quality fuel or oil with excessive additives.
• An engine that burns oil at a slow rate, leaving behind ash over long periods.
Dry fouling causes misfires or weak sparks due to the insulating layer of carbon or ash on the plug.
More than 10 Ohms of resistance is required between the electrode and the shell for the engine to start normally, when the insulation resistance approaches zero wet or dry carbon can contaminate the spark plug.

Oil Fouling
Oily deposits on your spark plug indicate that oil is entering the combustion chamber and fouling the plug. This condition, can lead to misfires, poor engine performance, and increased emissions. Below are some common causes for oil on your spark plug:
Worn Piston Rings
Piston rings create a seal between the combustion chamber and the engine’s crankcase. If they are worn or damaged, oil from the crankcase can seep into the combustion chamber and coat the spark plug. Symptoms include:
• Blue smoke from the exhaust (a sign of burning oil).
• Loss of compression and reduced engine performance.
Damaged Valve Seals or Guides
Valve seals or guides prevent oil from leaking into the cylinder from the valve train. If these seals wear out, oil can trickle down into the combustion chamber, causing oil fouling on the spark plug. This issue is more noticeable after the engine has been idling or starting from cold.
Blown Head Gasket
A blown head gasket can allow oil to seep into the combustion chamber. Along with oily spark plugs, you may notice:
• Overheating.
• Coolant mixing with the oil (sometimes visible as a milky substance in the oil).
• Loss of engine power.
Overfilled Oil
If your engine oil level is too high, oil can splash up into the combustion chamber or into areas around the spark plug. Overfilling creates excess pressure that forces oil into places where it shouldn’t go.
Worn Cylinder Walls
Over time, the cylinder walls can wear down, which reduces the tight seal between the piston and cylinder. This wear allows oil to bypass the piston rings and enter the combustion chamber.
Crankcase Ventilation Issues
Designed to remove gases from the crankcase and prevent pressure buildup. If it malfunctions or clogs, it can cause excess oil to be forced into the combustion chamber, resulting in oily spark plugs.
How to Address the Issue:
• Check and repair any engine leaks: Worn piston rings, valve seals, or head gaskets may require engine repairs or a rebuild.
• Inspect oil levels: Make sure the engine oil is not overfilled.
Regular maintenance and inspections can help prevent oil fouling and keep your engine running smoothly. If oily spark plugs persist, it’s best to consult aservice agent to diagnose the underlying problem properly.​

Overheating Plugs
Plug overheating occurs when a spark plug’s tip temperature becomes excessively high, usually above 850°C. This can lead to pre-ignition, engine knocking, or even damage to the spark plug itself. Overheating is a common cause of spark plug failure and can lead to a range of engine problems, including reduced performance and efficiency.
Symptoms of Spark Plug Overheating:
White or blistered appearance: The tip of the spark plug may appear white or chalky, with signs of blistering or erosion due to extreme heat.
Electrode erosion: The metal electrodes (the centre and ground electrodes) may erode or wear out prematurely.
Pre-ignition or detonation: The overheated plug can cause the air-fuel mixture to ignite prematurely, leading to knocking or pinging sounds in the engine.
Poor engine performance: Overheated plugs can result in misfires, rough idling, reduced power, and increased fuel consumption.
Melting or damage to the insulator: In extreme cases, the ceramic insulator surrounding the centre electrode can crack or melt.
Common Causes of Spark Plug Overheating:
Incorrect Spark Plug Heat Range
• Spark plugs come in different heat ranges (cold or hot plugs) to suit specific engines. If the heat range is too “hot” for your engine, the spark plug won’t be able to dissipate heat fast enough, leading to overheating.
• A “hot” spark plug has a longer insulator nose, which retains more heat. These are typically used in engines that run cooler. Using this in a high-performance or turbocharged engine, which runs hotter, could cause the plug to overheat.Lean Air-Fuel Mixture
• When the air-fuel mixture is too lean (too much air, not enough fuel), combustion temperatures increase. This extra heat can cause the spark plug to overheat. Lean mixtures often result from issues with the fuel injectors, carburettor, or oxygen sensors.
Ignition Timing Issues
• If the ignition timing is too advanced, the spark occurs too early in the combustion cycle, causing higher combustion chamber temperatures. This can lead to overheating of the spark plug and pre-ignition.
Overheating Engine
• If the engine itself is running hot due to cooling system issues (e.g., low coolant levels, faulty radiator, or water pump), it can cause the spark plugs to overheat.
Low-Quality or Incorrect Fuel
• Low-octane fuel or fuel with improper additives can lead to detonation or pre-ignition, which increases the combustion chamber’s heat and puts extra stress on the spark plugs.
Heavy Engine Loads
• Driving under heavy load, such as towing or driving at high speeds for extended periods, can increase engine temperatures, leading to spark plug overheating.
Effects of Spark Plug Overheating:
• Pre-ignition: The overheated spark plug may ignite the air-fuel mixture before the scheduled spark, causing a harsh knocking or pinging noise, which can damage the pistons or valves.
• Reduced Spark Plug Life: Overheating can cause the electrodes to wear out faster, shortening the life of the spark plug.
• Potential Engine Damage: In severe cases, the excessive heat can lead to piston damage or valve failure, resulting in costly engine repairs.
In short, spark plug overheating is a result of excessive combustion temperatures and can be prevented through proper engine tuning, maintenance, and the use of correctly rated spark plugs.