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What Causes A Spark Plug To Crack?

Spark plugs: those tiny yet vital sparks that set your engine’s fire ablaze. However, they aren’t invincible and can suffer from issues like cracking.

Problems like engine misfires, increased fuel consumption, and potential engine damage are probable results if this problem is left unchecked. But what causes a spark plug to crack?

Thermal shock is often the primary cause of a spark plug cracking – sudden temperature changes that can stress the insulator, mainly when the engine is cold, overheating, or exposed to water. Mechanical damage, chemical contamination, and excessive torque are all also possible causes.

Read on as we go into more detail about each issue of possible causes of a cracked spark. Moreover, we’ll offer advice on how to inspect them for cracks, and by the end, you’ll clearly understand what causes the damage and how to prevent them.

Understanding Thermal Shock And Its Impact On Spark Plugs

Thermal shock creeps in with sudden temperature swings, targeting the spark plug’s insulator. The insulator (that ceramic guardian encircling the center electrode) seems tough, but even it succumbs to thermal shock’s grip.

Good examples are under these circumstances:

Cold Start

When the engine starts cold, especially in winter or after long inactivity, the spark plug chills due to the cold air-fuel mix entering the combustion chamber. This temperature difference can cause the insulator to contract and crack.

Engine Overheating

If the engine overheats, either due to a faulty cooling system, a lean air-fuel mixture, or a heavy load, the spark gets extremely hot. This temperature contrast can lead to insulator expansion and cracking when the engine cools down.

Water Exposure

When water or coolant enters the combustion chamber because of issues like a leaking head gasket, cracked cylinder head, or faulty intake manifold, it rapidly cools the spark plug. This sudden temperature difference can result in insulator contraction and cracking.

A cracked insulator can harm spark plug performance and lifespan in various ways:

  • Reduced insulation resistance may lead to misfires, poor combustion, and power loss.
  • Carbon buildup on the crack may further reduce insulating resistance, raising the possibility of pre-ignition and rapture.
  • Water or coolant seeping into the crack can cause corrosion, rust, and spark short circuits.
  • Gas leakage through the crack may reduce compression ratio and engine efficiency.

To prevent thermal shock-induced spark plug cracking:

  • Choose spark plugs with the right heat range for your engine and driving conditions. Higher heat range spark plugs have longer insulators for high temperatures, while lower heat range plugs cool faster. You can always fall back to your owner’s manual or a qualified mechanic for the recommended heat range.
  • Avoid starting your engine in extreme cold or heat. Gradually warm it up before driving, or let it cool completely before turning it off.
  • Regularly inspect and repair cooling system leaks and malfunctions. Also, monitor the engine temperature gauge and avoid driving when it’s too high.
  • Maintain an optimal air-fuel ratio, avoiding overly lean or rich engine operation. Try to use high-quality fuel – that meets your vehicle’s specifications – as often as possible.
  • Replace spark at recommended intervals or if they exhibit wear or damage. Inspect for cracks and other issues during routine maintenance.

The Consequences Of Using the Wrong Heat Range

As we mentioned earlier, the heat range of a spark plug indicates how well it can dissipate heat from its tip to its shell. Using the wrong heat range can cause your spark to crack or fail prematurely.

If you opt for a spark plug with a lower heat range (hot type) than your engine needs, it won’t cool down fast enough, risking overheating. This overheating result generally causes the plug to go beyond its melting point, triggering it to pre-ignite or crack.

Conversely, a spark plug with a higher heat range (cold type) won’t reach optimal temperature, causing fouling, carbon deposits, misfires, and poor combustion.

As you can see, matching the correct heat range to your engine and driving conditions is imperative to avoid these pitfalls.

How Chemical Contamination Affects A Spark

Chemical contamination significantly threatens spark plugs, causing fouling and eventual cracking. A good example is this forum on North American Motoring, on which substances like fuel additives, oil, coolant, and water can infiltrate the spark power.

Here’s where the contamination originates:

  1. Leaky fuel injectors or carburetors flood the combustion chamber with excessive fuel, diluting the oil film on cylinder walls
  2. Leaking valve seals or piston rings allow oil entry into the combustion chamber, coating the spark plug with oil and fostering carbon deposits
  3. Faulty components like a leaking head gasket, cracked cylinder head, or malfunctioning intake manifold, permitting coolant or water intrusion, chilling the spark plug, and inducing thermal shock
  4. Low-quality fuel or oil containing impurities or damaging additives that leave residues on the spark’s surface

This chemical invasion leads to various spark issues:

  • Narrowing the spark gap or bridging electrodes – resulting in misfires, subpar combustion, and power loss
  • Corroding or eroding the electrodes – diminishing spark plug lifespan and performance
  • Cracking the insulator or shell – causing gas leakage, short-circuiting, or pre-ignition

To safeguard your spark against chemical contamination:

  1. Utilize high-quality fuel and oil that meet your vehicle’s specs, changing them regularly
  2. Inspect your fuel system and engine for leaks or malfunctions, fixing them promptly
  3. Steer clear of excessive fuel or oil additives that can harm your spark or the engine
  4. Replace sparks regularly, especially if contamination or damage is evident

Torque Troubles: Over-Torquing & Under-Torquing Spark Plugs

Torque, the force that can rotate a spark plug about its axis, plays a crucial role in the installation. It’s often one of the most overlooked yet essential steps in a quality spark plug replacement job. It directly impacts the spark’s ability to dissipate heat to the cylinder head. 

You will need to know the torque requirements for your type of spark plug – over-torquing or under-torquing a spark plug can lead to serious engine issues.

Firstly, over-torquing can stretch the spark plug’s metal shell, potentially causing insulator or shell cracks, breaking gas seals, or reducing insulation resistance. This can result in gas leaks, short circuits, pre-ignition, or detonation.

Secondly, under-torquing may prevent the plug from fully sitting in the cylinder head, hampering heat dissipation – leading to various plug damage.

Finally, both over-torquing and under-torquing can affect the spark gap and electrode alignment, triggering weak sparks, misfires, poor combustion, or even power loss.

To prevent torque-related issues, follow these guidelines:

  1. Use a torque wrench to stiffen spark plugs according to the spark’s specifications. If you’re unsure, you could research the model and its requirements or even follow a torque chart.
  2. Inspect spark plug holes for debris and clean them before installation to ensure no resistance affects the torque reading.
  3. Replace spark plugs regularly or when signs of wear or damage appear. Additionally, check for cracks or other problems during routine vehicle maintenance.

The Mechanics Of Spark Plug Damage

Mechanical damage to a spark plug can occur due to various reasons. For instance, the plug might extend too far into the combustion chamber, leading to collisions with the piston, valve, or cylinder head. This can result from compatibility issues, incorrect installation, or engine modifications.

Foreign objects like metal fragments, carbon deposits, or broken valve springs can also strike the plug – especially in poorly maintained engines or dirty combustion chambers. Mechanical damage can also occur from careless removal, installation, or used purposes that it shouldn’t be.

The rule of thumb is to carefully handle any installation and removal, using a torque wrench to follow manufacturer specifications. Also, ensure the plug holes are clean before installation and avoid using the wrong tools.

Finally, if you notice that your MSD ignition box is failing – then it’s time for a quick replacement. A failing MSD ignition box can result in weak sparks, erratic timing, or no sparks at all. Left unattended, it can also harm your engine or catalytic converter.

Checking For Cracks: A Simple Inspection Guide

Mechanical damage to a spark plug can occur due to various reasons. So, to inspect it for cracks, follow these three simple steps:

  • Removal: Extract the spark from the engine using a spark plug socket and ratchet. Be careful when disconnecting the spark plug wires or coils to prevent harm to the threads or electrodes.
  • Close Examination: After removal, methodically inspect the spark for cracks, melting, erosion, corrosion, or fouling on the insulator or shell. If you have a magnifying glass (or even a good cellphone camera) or flashlight, use it to get a better idea of the cause. You can also compare them to what we mentioned before to help you assess what caused the crack.
  • Immediate Replacement: If you identify any cracks, you must organize a replacement for them as soon as possible. It would be even better if you managed to determine what caused the damage to prevent it from happening again and avoid any further harm or repetition on repairs.


Spark plugs deliver the punch that keeps your ride alive – but as we’ve uncovered, they’re not invincible. Cracks that lead to start-up failure, slow fuel consumption, or even engine difficulties can happen.

But with some of the right information, you can ensure your sparks stay in peak form while keeping your engine in top shape.