How to Read a Spark Plug | Spark Plug Photos | DIY Instructions

Knowing how to read a spark plug, can save you considerable time diagnosing (or misdiagnosing) gasoline engine performance.  Spark plugs are the all-important device that delivers necessary Spark to your engine, and when combined with the proper gas-air mixture, provide the necessary combustion to run your motor.  

What Your Spark Plugs Can Tell You

Spark plug condition can provide tell-tale signs about the condition of your engine. 

By simply reading spark plugs for wear and deposits, you can quickly discover whether your spark plugs are either worn out, getting too much gas (rich), too little gas (lean), running too hot (burnt), wet, cracked, and whether it’s time to change them.

Having a bad spark plug or fouled spark plug can also cause all sorts of problems in your Car, Truck, Lawn Mower, Chainsaw, or any other gas-powered engine. Symptoms can include; not being able to start your motor, engine misfires, poor gas mileage, and just overall bad performance.

If you happen to notice any of these issues going on with your vehicle or Lawn equipment, your spark plug is the best place to get started.  

Reasons to check your spark plug first: 

1) A bad spark plug is the main cause of poor engine performance.
2) Spark plugs are very inexpensive to replace.
3) If you haven’t changed your spark plugs in 2 years or more, your problem could likely be a bad spark plug.

Routine Maintenance:

Every vehicle owner, landscape professional, and homeowner needs to regularly check the performance of their equipment. The good thing is, examining spark plugs allows you to “Instantly Read”  the condition of this vital piece of your engine’s operation.

Here’s a short 3minute video from Goss’ Garage on “How to Read Spark Plugs”

Let’s Get Started – How to Read a Spark Plug

After you remove a spark plug and inspect it carefully, you will likely see one of these spark plug conditions:

• Carbon fouled
• Wet (gasoline)
• Burnt
• Oil deposits
• Worn out Electrode
• Broken Electrode
• Normal

Once you remove and examine the spark plug, look for any signs of visible damage. 

Look at the cylinder side of the plug – that’s the end with the threads and electrode. As you

examine it, determine what the condition of your spark plug is by matching it to the photos below:

Carbon Fouled

If you see black deposits on the electrode or insulator of your spark plug, you have a carbon build-up – or carbon fouled plug. Carbon fouled plugs indicate your fuel to air carburetor settings are too rich in gas, or your air filter is dirty and needs to be cleaned or replaced.  You may also consider using a “hotter” or different recommended brand of a spark plug.

Wet

When lawn equipment won’t start, and you’ve pulled the start rope  4 or 5 (or 20) times, you may have flooded your engine and created a wet spark plug. If this is the case, the wet spark plug can’t deliver spark to your engine. You can remove the plug and dry it off, wait a few hours, or try it another day after it dries out.

Burnt

If you see melted electrodes, blisters on the insulator, or any white deposits it is a sign of a burnt spark plug.  It is simply running way too hot for its application, and in outdoor lawn equipment, this can be a result of using the wrong spark plug, incorrect gas/air mixture, or a loose spark plug. Correct the problem, then replace the spark plug.

Oil deposits

If you discover oily deposits that are black on the insulator thread or the electrode it usually indicates oil is leaking past the pistons and into the cylinder of the engine.  If you are very mechanically inclined you can try and pinpoint the problem, or take it to a qualified small engine mechanic to diagnose.

If it’s a major repair you should weigh out the costs, it could be time for a new mower vs. fixing the old one.

Worn out Electrode

If you find a spark plug that looks similar to this photo you’re looking at a spark plug with a worn out electrode.  Replace the plug and you should be fine.

Broken Electrode

If you or someone else installed the wrong size spark plug, and it was too long for your engine, you may find a flattened or broken the electrode.  This type of mistake can cause significant engine damage while installing a spark plug that is too short will cause poor gas usage.

Normal

Congrats – If you see brownish or gray deposits on the electrode like this photo, and your electrode isn’t worn and the ground armature isn’t broke, your spark plug is Normal. You can go ahead and check the spark plug gap, and reinstall it.  

Recommendation:  If your spark plugs all check out Normal, move ahead and check your gas fuel filter and air filter. Make sure they are clean and look normal and replace as needed. These two items can have a big impact on engine performance as well.

Conclusion

Determining the condition of your spark plugs is simple once you know how to read a spark plug. And now that you know what to look for – you just need to get familiar with the simple steps required to change a spark plug.

If you’re changing or inspecting multiple spark plugs on a vehicle, tractor, or other multi-cylinder engines, we highly recommend inspecting and replacing one spark plug at a time. This will help you avoid the mistake of re-connecting the wrong spark plug wire.

Check online or with your local hardware or auto parts store. Once you’ve located and replaced the correct model spark plug(s), gas and air filters, your engine should perform just as it was designed.

Note:  If you’re inspecting Spark plugs on Outdoor lawn equipment, and aren’t very familiar with small engine maintenance; check out this in-depth helpful article we published. 

Note: The contents of this article are designed for informational purposes, and to help you quickly navigate the various conditions.  Under no circumstances should you avoid the advice of your local mechanic or small engine repair professional. Please seek the advice of a certified technician,  if you have any questions or concerns related to the content of this article.

Further Reading

• Check out this handy guide from Briggs & Stratton to locate the correct spark plug or gap requirement for your Lawn equipment.
• Check out the Anatomy of a Spark Plug Check out this article from Wikipedia
• How do you Clean and Change your spark plug? Read about the best way to do it here. 

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