Servicing spark plugs

Spark plugs perform hard duty in internal-combustion engines. With their noses in combustion chambers where temperatures can reach 900 degrees C, they conduct 10,000- to 30,000-volt arcs between their electrodes to produce a spark and ignite a compressed mix of air and fuel vapor. And they zap millions of those miniature lightning bolts before they wear out. Auto-racing crews routinely read plugs as part of pre-race preparation. They look for clues, for example, such as the optimum heat range, air-fuel mix and ignition timing. Race crews typically tune their engines for maximum performance at wide-open throttle. If the racer is not running on the edge, he's likely not fast enough to win. His engine plugs only need to live long enough to cross the finish line.

Grounds managers, obviously, live in a different world. You want to start your mowers, rotary tillers, snowblowers and chain saws easily. And you want them to run smoothly, without stalling, sputtering or wasting gasoline.


Spark plugs are not silver bullets or magic wands. But, if you use their potential as a diagnostic tool, they can help make your life easier and your machinery longer lasting and less expensive to run.

Understand the difference between "hots" and "colds" You've heard it before: Read the directions. The operator's manual that came with your machine should recommend a spark plug with a specific heat range. Most plug manufacturers used to include the heat range in the type number stenciled on the plug. Higher numbers usually denoted hotter plugs. Because not all plug makers supply the heat range in this way anymore, it's safer to check the catalog in the store.

The heat range refers to a plug's relative ability to dissipate heat. A hot plug has a long insulator nose, which exposes more of the nose--the center electrode wrapped in a ceramic insulator--to the heat of combustion. A cold plug has a shorter nose and smaller surface area exposed to combustion heat. Plus, the cold-plug architecture provides a shorter path to conduct combustion heat from the cylinder, through the plug to the engine block and, if it has them, to the engine's cooling jacket and radiator.

The goal in heat-range selection is to find a spark plug that will retain enough heat in the tip to minimize deposit buildup and promote ignition of the mixture, yet stay cool enough to avoid engine-damaging pre-ignition. In most cases you can't go wrong using the heat range specified by the manufacturer, which knows its engines best. Even so, the engine manufacturer deals with typical uses and cannot predict all the types of service a product may see in the field.

In general, an engine operating for long periods at maximum power in high-heat situations may require a colder spark plug than one running for short periods at, or near, idle in cold weather. Manufacturers typically recommend heat ranges that are cold enough to keep engines safe in the worst conditions. The reason is simple: * If the plug is too hot, the tip can become hot enough to ignite the mix prematurely (pre-ignition). The result of this condition is often a hole in the piston--and a hefty repair bill. * If the plug is too cold, the result is premature plug fouling, an inexpensive fix requiring only a new spark plug.

Consider the symptoms Symptoms of a fouled plug are: * Hard starting * Failure to start * Misfiring when the engine is under load.

Why do these problems occur? Because products of combustion--mostly carbon--build up on the insulator tip and the electrodes. At some point, this carbon, which is an electrical conductor, bleeds spark energy across the ceramic insulator and away from the gap. The result, no spark.

Decide whether to clean it Some people still clean and file electrodes and re-gap spark plugs. Several companies sell spark-plug cleaners that blast the insulator with abrasive. But problems exist with this method. To remove deposits shielded by the sidewire, you either have to straighten the wire (stressing it at the bend) or leave deposits on the insulator shadowed by the sidewire. Also with abrasive cleaners, you risk introducing these unwanted elements into the engine. Today, most people simply replace the spark plug.

"Read" the plug Ideally, we want to match spark-plug heat range to the engine's operating conditions. Assuming proper engine tune and spark-plug installation, the heat range is correct when, after operating the engine normally, the insulator tip and electrodes are white or a light tan color (see Photo 1, page 76).

If the insulator is glazed or blistered, or shows other signs of excessive heat (see Photo 2, page 76), install a colder plug.

One sign of heat used by some professionals is electrode color. Many--but not all--spark-plug electrodes have an alloy composition that will oxidize green when subjected to high temperatures (see Photo 2, again). This nickel oxide on the center wire and sidewire would show up first at the tip of the electrode--the hottest part--and progress from there, depending on temperature. Excessive green oxide may signal it's time to change to a colder plug. Several causes of high spark-plug temperature are described in the boxed information at left. The main thing to keep in mind here is that not all electrodes oxidize (turn green) at the same rate, if at all. Therefore, your best bet is to turn to a technician with experience reading plugs.

If the plug looks dark or sooty around the insulator tip (see Photo 3, page 76), the plug is not running hot enough to burn off carbon deposits. The engine may have been idled excessively, it may be running too cool, the air/fuel mixture may be too rich, or you may need a hotter plug.

If the plug is wet, you may have flooded the engine, or you may need a hotter plug. If the plug is wet but clean, it will probably run if dried off. If the insulator is black, it is probably fouled and needs replacing.

If the plug is brown or black and wet, you may have a serious oil-control problem--which may be telling you something you already know.

Gap the gap The gap in a spark plug--the distance between the center and sidewire (typically marked with a - or + sign respectively)--is one of those good-news/bad-news dilemmas. Less voltage is needed to jump a narrow gap. That makes a tight gap appropriate--even critical--for small engines that get their electrical power from a magneto (which has low available voltage at cranking speeds) rather than a storage battery.

On the downside, that tight gap exposes less spark to the mixture and is easier to short out, or bridge, with carbon deposits.

Most plugs are sold with pre-set gaps. The plug manufacturer sets the gap for the most common application for that type/number of plug. Some engine manufacturers may require a different gap. It is always best to check the manual and re-gap if necessary.

Several types of gap gauges are available today. Most experts agree the round or wire gauge is the most accurate. To properly gap the plug, select the proper size gauge and slide it in the gap between the center wire and sidewire. If it slides through with just slight resistance, the gap is correct. If not, use the sidewire bending tool (usually on the gauge) to slightly bend the sidewire to the correct gap.

Some premium spark-plug designs have electrode configurations designed for optimum performance with the gap as provided. Several, for example, are designed to work best directly out of the box as gapped by the manufacturer. Again, read and follow the instructions on the package.

Recognize the "cold" truth Cold weather often poses special problems for starting small engines. Small engines, especially those used infrequently, may contain old gasoline, which does not vaporize well. This, combined with cold weather, results in insufficient vapor and too much liquid fuel in the combustion chamber. Remember the engine needs fuel vapor to run. Without it, the engine will not start.

If you encounter a starting problem in this type of situation, first try removing the plug. If the firing end is wet with fuel, try drying or changing the spark plug. If that doesn't work, dry or change the plug again and warm the engine to promote fuel vaporization. A note of caution: Fuel vapor and heat can be an explosive combination. Avoid enclosed areas where vapors can accumulate, and use a safe source of heat. Some seasoned mechanics in northern climates carefully pour warm water on the intake manifold to get an engine started.

Whether the season is hot or cold, if an engine refuses to start, check the tip of the spark plug. If it's wet, you know you are getting fuel to the combustion chamber. As long as you have the plug out, you might as well check to see if it is delivering a spark. Remember, an engine needs both fuel and ignition to run.

Reconnect the spark-plug wire, then place the metal shell of the plug against the engine to complete the electrical circuit and provide a ground. Do not hold onto the plug. Remember, the ignition system delivers high voltage, and you don't want to become the ground path! Finally, crank the engine to see if you get a spark. (As always, keep all combustible vapors away from the spark.) If you do not have spark, the spark plug you are using is either fouled or shorted. Or your ignition system has a problem. Most ignition problems are minor, and some are so obvious it's almost embarrassing. For example, is the ignition or safety switch turned on? Is the spark plug wire hooked to the spark plug?

Therefore, consider all the options when servicing your equipment's spark-plug needs.

Jerry Okenka is motor sports manager and field support engineer for Delphi Energy & Engine Management Systems (Flint, Mich.). He supplies track-side analysis and consultation to NASCAR racing crews who use ACDelco spark plugs.

The most common causes for spark plugs that run too hot are: * Advanced ignition timing * Excessively lean air/fuel mix * An improper heat transfer between the plug and the cylinder head * Improperly torqued (loose) spark plugs * An overheating engine * Loose threads in the cylinder head * An improper thread repair in the head * Incorrect fuel octane * Spark plug protrudes too far into the combustion chamber * Insufficient coolant flow in the spark-plug area of the cylinder head.

REASONS FOR COLD PLUGS The most common (in order, from most to least common) causes of cold plugs are: * The equipment is being run at low speed or in low-load driving * The air/fuel mix is too rich * The spark plug has the wrong (too cold) heat range.

* What if...you run your engine with a light load much, or most, of the time. Would it pay to try a hotter plug, which might run longer without fouling?

Most experts don't recommend it unless your current plug life is unacceptable. First, try adjusting your carburetor to run as lean as practical.

* What if...you try to "burn off the carbon" on plugs by running the engine at a heavy load?

If the deposits have not been glazed onto the insulator, you might be able to burn them off. If the engine has seen a lot of low-speed/low-temperature running time, you may be able to burn off some wet, or recent, carbon deposits. But you probably won't get all of them. And, if the plug already is shorted by deposits, you will never get it to run steadily enough to raise temperatures high enough for the plug to clean itself.

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© 2014 Penton Media Inc.

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