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Spark Plug Technology
Small, metal, ceramic. That just about sums up what most people know about spark plugs. Those in the parts business
may know a little more: they come in little rectangular boxes and are usually packaged 4, 6 or 8 in a larger box
or blister pack. They come in different sizes and brands, and cost anywhere from less than a buck up to several
bucks apiece.
If that sums up your knowledge of spark plugs, keep reading because you have a lot to learn; there’s more to spark
plugs than meet the eye. Spark plugs are one of the most important components under the hood because of their effect
on some critical engine systems - engine performance, fuel economy and emissions.
PLUG PROBLEMS.
If an engine cranks but won’t start, one of the first things a technician will check is spark.
Without a spark, the fuel won’t burn and the engine won’t run. No spark at any of the plugs usually indicates an
ignition problem that requires further investigation (a bad coil, ignition module, distributor pickup, crank sensor,
etc.).
If the engine runs but misfires, one or more spark plugs may be worn or fouled, or there may be one or more bad
spark plug wires. To diagnose this kind of problem, a technician may connect an oscilloscope to the ignition system
so he can observe the firing pattern for each cylinder. A higher-than-normal firing voltage in any one cylinder
may indicate excessive resistance in a plug wire, a loose plug wire or a badly worn or misgapped spark plug (too
wide). A lower-than-normal firing voltage in any one cylinder may indicate a shorted plug wire, or a fouled or
damaged spark plug.
Fouling is the number-one reason why spark plugs have to be replaced. Plugs also have to be replaced for preventive
maintenance because the electrodes wear as the plugs age. This increases the distance between the electrodes, which
in turn leads to a gradual increase in the firing voltage required to jump the gap. How gradual? The gap grows
about 0.00063 to 0.000126 in. for every 1,000 miles of normal driving, which means the firing voltage requirements
creep up about 500 volts for every 10,000 to 15,000 miles of driving! Eventually the plug’s firing voltage requirements
under load exceed the ignition system’s output resulting in a misfire. But most plugs foul out long before they’re
worn out.
Driving with a single fouled spark plug is like riding a horse with a broken leg, killing up to 25 percent of a
four cylinder engine’s power output. A fouled plug will also cause a big increase in fuel consumption and emissions
(more than enough to cause an emissions failure and/or the check engine light to come on if the vehicle has an
OBD-II system).
Plug fouling can occur if fuel or oil deposits build up on the plug’s electrodes. The ceramic insulator around
the center electrode prevents voltage from finding a shortcut to the steel plug shell and ground. Deposits here
may form a conductive path for the voltage to bleed off to ground, preventing it from jumping the gap and making
a spark. Deposits around the outer ground electrode or between the electrodes may form a barrier or bridge that
also prevents a spark from occurring.
"Reading" the old plugs can reveal not only the cause of a fouling problem, but other types of engine
problems, such as preignition and detonation. A melted or badly eroded electrode may be the result of over-advanced
ignition timing, engine overheating, low octane fuel or too hot a plug for the application.
HEAT RANGE.
The "heat range" of a spark plug determines how hot the plug runs during normal operation.
If the heat range is correctly matched to the engine application, the plug will run hot enough under normal driving
conditions to burn off fouling deposits before they can cause problems. Likewise, the plug won’t get too hot and
become a source of ignition itself, causing engine-damaging preignition and detonation. On the other hand, if the
plug’s heat range is too cool for the application, though, fouling deposits may build up faster than they’re burned
off.
For this reason, always follow the vehicle manufacturer or plug supplier’s heat range recommendations when selecting
a spark plug for a particular application. Two spark plugs may appear identical on the outside but may have entirely
different heat ranges.
Many of today’s spark plugs have a very broad heat range because the plug manufacturer uses a copper core in the
center electrode. Copper is an excellent conductor of heat, so the insulator can be designed to run hotter and
burn off fouling deposits without it getting too hot under increased load to cause preignition or detonation.
PLUG REPLACEMENT OPTIONS.
The recommended replacement interval for standard spark plugs has typically been every 30,000
to 45,000 miles. But most of today’s extended-life plugs have special wear-resistant electrodes made of platinum,
gold-palladium, nickel yttrium or other exotic alloys that minimize electrode erosion. Such plugs can usually go
60,000 to 100,000 miles plus with little or no electrode wear. Even so, they may still be vulnerable to fouling
if an engine has an oil consumption problem or spends excessive amounts of time idling.
Extended life spark plugs are a good upgrade for many engines, but may not be the best choice for an older engine
that uses oil - or even some performance engines. According to one plug manufacturer, platinum electrodes run hotter
than standard electrodes increasing the risk of preignition and detonation in some turbocharged and high-performance
engines. For such applications, a standard plug with a colder heat range would be a better choice.
There are also a wide variety of electrode configurations from which to choose today. Each manufacturer claims
certain performance benefits for their particular design. It may be reduced electrode wear, improved ignition reliability,
or both. Such plugs are often marketed as "premium" or "performance" plugs, and may command
a price of up to $6 or $7 apiece.
Some of these plugs (as well as standard plugs) also have multiple electrodes (two, three or four ground electrodes).
A spark plugs with more than one ground electrode will still only produce one spark per ignition cycle. But with
multiple paths from which to choose, the likelihood of getting a good spark to at least one of the ground electrodes
is multiplied for improved ignition reliability. Having more than one ground electrode also distributes the wear
to minimize electrode erosion and growth of the spark gap over time.
Are premium plugs worth the extra money? They are if they provide extended plug life or reduce the need for maintenance.
The plugs in many full-size vans with V8 engines, as well as those in front-wheel drive cars and minivans with
V6 engines, are very difficult to replace.
Installing extended-life plugs can almost eliminate the plug change hassle for good. Likewise, performance plugs
that reduce misfires can enhance performance for a smoother running, cleaner, more fuel-efficient engine. No spark
plug can create power out of thin air, but improved ignition reliability can minimize any horsepower loss due to
misfire.
When the plugs are changed, they should not be removed on engines with aluminum cylinder heads until the engine
has cooled. This will minimize the risk of damaging the threads in the head when the plugs come out.
Most threads on spark plugs designed for aluminum head applications are precoated to reduce the risk of thread
damage. Even so, some technicians apply a drop of antiseize compound to the plug threads before they go in for
added insurance. General Motors, however, does not recommend this practice because antiseize acts like a lubricant
and may allow the plugs to be overtightened - which can damage the threads. GM’s advice is to reduce the tightening
torque on the plugs 40 percent if antiseize is used on the threads.
How much the plugs should be tightened depends on the size of the plugs and the type of plug seat. Spark plugs
with gasket-style seats require more torque than those with taper seats.
Always follow the vehicle manufacturer’s torque recommendations, but as a general rule 14 mm plugs with a gasket-style
seat should be tightened to 26 to 30 ft. lbs. in cast-iron heads, but only 18 to 22 ft. lbs. in aluminum heads.
Likewise, 18 mm plugs with gasket-style seats should be tightened to 32 to 38 ft. lbs. in cast iron heads but only
28 to 34 ft. lbs. in aluminum heads. For taper seat spark plugs, 14 mm plugs should be tightened to 7 to 15 ft.lbs.
in both cast iron and aluminum, while 18 mm taper seat plugs should be tightened to 15 to 20 ft. lbs. in both types
of heads.
FORKLIFT
OPERATION AND SERVICE.
BE SURE TO VISIT THE HOME PAGE FOR INFORMATION ON LIFT TRUCKS REPAIR,
SERVICE, TROUBLESHOOTING, AND EVERYTHING YOU NEED TO KNOW ABOUT FORKLIFTS AND THEIR USE.
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