The Art Of Reading A Sparkplug
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The Art Of Reading A Sparkplug

Sparkplugs have a story to tell if you know how to interpret their language. Being able to read their story will help you diagnose the condition of any internal combustion engine.

Did you know that the sparkplug in your small engines and the sparkplugs in your vehicles has a story that they are just dying to tell you? Sparkplugs are like pictures, the visual inspection of their physical condition is worth a thousand words and a shop full of test equipment. Back in the day of the shade tree mechanic, we read what the sparkplugs had to say about the conditions inside an engine. Today, mechanics depends on expensive, computerized, engine analyzers for the same information. I am not knocking high-tech test equipment we still need those test instruments for troubleshooting the on board computers. Never the less, the story that sparkplugs have to tell can still go a long way to helping us diagnose and solve many mechanical problems with any internal-combustion engine.

Degree of difficulty: 1 (Almost anyone can learn to read sparkplugs.)

The story we all like to see.

A sparkplugwith brown or light gray appearing electrodes and a center insulator indicates that the spark plug is firing normally, and everything is normal in that cylinder.

Dry and wet carbon fouling.

There can be many reasons for this type of fouling. Some of the principal causes are worn piston rings; worn valve guides or valve stem seals; a defective head gasket; and an overly-rich fuel-air mixture. An engine cylinder with a dry or wet carbon fouled sparkplug will develop a pattern of misfiring. For a sparkplug to fire, there must be a minimum of 10 Ohms resistance between the center electrode and the shell. Carbon fouling lowers the resistance, and when the resistance reaches 0 Ohms the sparkplug will misfire continuously. If the sparkplug is in a small, single cylinder engine, the engine will not start or run.

The Engine is running hot of has run hot for an extended period.

When an engine runs hot, the spark plug develops a shiny, glazed appearance, like a piece of pottery after being fired in a kiln. When an engine runs hot, carbon and other deposits that have collected on the sparkplug's electrodes and center insulator melt, fusing to their surfaces, giving them a glazed appearance.

Deposit buildup on sparkplugs.

Deposit buildups are indicative to many conditions. They may indicate the use of a sparkplug with the wrong heat range for the conditions under which the engine is operated. They may indicate that the wrong grade fuel is being used in the engine. They may indicate oil entering the combustion chamber because of bad rings, valves, valve stem guides or seals or a blown head gasket.

The problem with lead deposits.

The big problem with lead deposits is that they cannot be detected at room temperature using a resistance tester. Lead deposits appear a light, yellow on the center insulator. Lead deposits form at different temperatures, those forming at 370 to 470 degrees Centigrade (700 to 790 degrees Fahrenheit) have the greatest effect on lead resistance and the firing of the lead fouled sparkplug.

The causes of physical breakage.

Damaged insulators as shown in this picture is usually caused by thermal expansion or by thermal shock. Thermal shock is caused by the sudden heating or sudden cooling of the sparkplug. Thermal shock can occur when cold water is put in an over heated engine. It is always a good idea to wait until an engine cools down before adding cold water, especially after the system boils over.

Normal electrode wear.

This is normal electrode wear. Electrodes wear at a rate of 0.000126 to 0.00063 inches/1,000 miles for 4 cycle engines and 0.000126 to 0.00252 inches/1,000 miles for 2 cycle engines. As you can see, two-stroke engines are harder on sparkplugs than four-stroke engines.

Abnormal Erosion

Abnormal electrode wear as shown above develops when a chemical reaction takes place in the cylinder involving the the lead deposits on the electrodes. Corrosion caused by other components of gasoline also play a key roll in causing this abnormal growth in spark gap.

Electrodes melted and fusing together.

This condition is the result of operating an engine at extremely high temperatures for an extended period. Sparkplug electrodes are made of a Nickel Alloy that melts between 2,200 and 2,400 degrees Fahrenheit. If your sparkplugs exhibit this condition, there is a good chance that other internal parts have been damaged as well. The intake and exhaust valves may be warped. The rod and main bearings may have been damaged by the breakdown in lubrication. It is time to take a very careful look at the engine and not simply install a new set of sparkplugs.

Erosion, Corrosion and Oxidation

This type of wear appears as a greenish buildup on the electrodes and center insulation. This wear is a result of the engine running with a rich fuel-ar mixture for an extended period.

Lead Erosion

This erosion of the electrodes is caused by the lead compounds found in gasoline.

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Comments (5)

oh sure.. you made reading them look easy.. its FINDING them thats hard! lol.. I just leave this to the men folk!

Good article, but not in the category of Home Repairs, more like car repairs. Same goes with your latest articles.

Ranked #2 in Home Repairs

What you say is true, Glen, except for one fact, this article is part of my small engines series (Lawnmowers, snow blowers, chainsaws, etc) and there is no category for those articles.

This is really good to know, but I am like Brenda and think I'll leave this to the menfolk. But I do know how much better my car runs with new and/or clean spark plugs. P.S. I find categorizing on this site to be a challenge sometimes.

Well read, Jerry! My hubby is going to start to wonder how I'm turning into such a little grease monkey! Needless to say it was I who got our fridge back up and running! lol Learning lots and loving it!