Question:How often should I replace the spark plugs in
my engine?
Answer:The spark plugs are the working end of the ignition
system. As such, they're responsible for reliable starting, low
emissions, good fuel economy and performance. Thanks to the elimination
of lead from gasoline that used to foul plugs in 10,000 to 15,000
miles, plug life today on unleaded gasoline is easily 30,000 miles
(the typical factory recommended replacement interval). Even
so, many plugs can go up to 45,000 miles or more in normal service
before they cause any noticeable problems and have to be changed.
The only reason spark plugs have to be changed at all today
is because of electrode wear -- unless the plugs have been oil
or carbon fouled, or damaged by abnormal combustion, in which
case they may have to be replaced sooner than usual.
Electrodes wear because the flow of electrons across the electrode
gap erodes away metal. Every time a plug fires, the high voltage
surge burns a few atoms of metal off the electrodes. After millions
of ignition cycles, the edges of the electrodes become rounded
and worn. This increases the voltage required to fire the plug
because of the increased distance across the electrode gap and
because a spark jumps more easily between sharp edged electrodes
than dull rounded ones. Consequently, the normal firing voltage
of 7K to 10K volts starts to creep up. It may rise to 12K, 15K,
20K or higher depending on how badly the plugs are worn and other
conditions that are present. Eventually the point is reached where
the plug starts to misfire because the ignition system can't provide
enough voltage to keep the fire lit under all conditions. The
engine may become hard to start, idle rough, use more fuel, lack
power, hesitate or stumble when accelerating, or it may fail an
emissions test.
Actually, all plugs (even new ones) misfire occasionally.
But usually the misfires are unnoticed because they happen so
infrequently. At 60 mph, the average plug is firing 22 times
a second. So an occasional hiccup usually isn't noticed -- or
least it hasn't caused much concern until now. But all 1996 cars
are now equipped with a new "OBDII" onboard diagnostic
system that monitors engine misfires. If enough misfires occur
over a given period of time, the OBDII diagnostic system will
set a fault code and possibly turn on the warning light depending
on the severity of the misfire. Therefore, ignition reliability
has become more important than ever before.
The electrodes in most standard spark plugs are made from
a high temperature nickel-chrome alloy called Inconel. It's a
good material, but for the above mentioned reasons doesn't last
forever. After 30,000 to 45,000 miles, Inconel electrodes wear
out. So most plug manufacturers now have a premium line of spark
plugs with electrodes made of special wear-resistant alloys.
LONG-LIFE PLUGS
Electrodes made with nickel-silver alloys wear at about one
third the rate of ordinary electrodes. But the two materials
that are used in the majority of long life plugs are platinum
(Bosch, AC & Autolite) and gold-palladium (Champion). Both
can significantly extend plug life by almost eliminating electrode
wear.
Platinum and gold-palladium plugs have the potential to go
60,000 to 100,000 miles or more in many applications. The plug
manufacturers do not make specific mileage claims for these plugs,
but the auto makers who use them as original equipment plugs in
certain new engines do. They say the recommended replacement
interval in most cases is 100,000 miles.
Cadillac was the first domestic auto maker to make the switch
to 100,000 mile plugs, which they did several years ago when the
new Northstar V8 was introduced. Since then, the list of new
cars with long-life plugs as original equipment has exploded as
a means of minimizing maintenance requirements and improving ignition
reliability. Many 1996 General Motors and Ford engines have the
new long-life plugs.
Will these long-life plugs really go 100,000 miles? Many
probably will. But as engines accumulate miles, a lot of things
can go wrong and prevent the plugs from ever seeing the 100,000
mile mark. Oil consumption and abnormal combustion can cut short
the life of any plug. If an engine develops a fouling problem,
experiences detonation or preignition, starts to use oil, runs
too rich, too lean, too hot or too cold for any of a variety of
reasons, it's going to affect the life of the plugs. So just
because the auto makers claim the plugs don't have to be replaced
for 100,000 miles in their advertising, it doesn't necessarily
make it so.
Should you switch to long life plugs when you change your
plugs? They don't cost that much more than standard plugs, and
the extra few bucks you spend on a set is usually more than offset
by the reduced need for replacement. So long life plugs are a
good replacement option for any engine regardless of what kind
of plugs they had in them as original equipment.
Long life plugs are recommended for any new engine that has
long life plugs as original equipment, but they can also be installed
in older engines, too. Long life plugs are a good substitute
for applications where plug access is difficult (vans, front-wheel
drive cars and minivans with V6 engines, etc.) or in vehicles
that tend to be neglected by their owners (hopefully, not yours!).
If an engine has a fouling or misfire problem, however, long
life plugs won't last any longer than standard plugs. The underlying
cause needs to be identified and repaired to eliminate the fouling
problem.
