You already know that spark
plugs wear out. Well, “burn up”
is more like it, because when a
spark jumps the gap between two electrodes,
it actually burns off (erodes)
minute amounts of metal from each one.
Over time, the gap grows to the point
where the spark can no longer make the
jump. That's when you get misfires, poor
gas mileage, lousy acceleration and, ultimately,
the dreaded “Check Engine” light.
To keep vehicles running at peak performance
for longer service intervals,
many car manufacturers install extended-life
spark plugs. Because their electrodes
are coated with precious metals that have
higher melting points, these plugs can
sometimes maintain a precise gap for up
to 100,000 miles. But even with higher
melting points, metals like yttrium (2,779
degrees F), platinum (3,222 degrees F)
and iridium (4,429 degrees F) can't stave
off erosion forever. The electrodes eventually
erode, increasing the gap, and, well,
you've already heard the rest of this story.
Replacing spark plugs
early makes sense
Unlike manufacturers' guidelines for oil
changes, which are overly cautious, the
recommendations for spark plug replacement
intervals tend to be overly optimistic.
For example, if you've already got
80,000 miles on a set of 100,000-mile
plugs, they're 80 percent worn and beginning
to take a toll
on engine performance
mileage. Worse yet, after that many miles, spark plugs
have a tendency to seize in the cylinder
head. Removing a seized plug can be a
costly job, especially if the threads in the
cylinder head are damaged in the process.
When you consider the gas mileage falloff
and the possibility of seized plugs,
early replacement makes sense.
Do it yourself
or take it to a pro?
The answer depends on the type of engine
in your vehicle. Some of the V-6 models
have very difficult spark-plug replacement
procedures that require removing portions of the intake manifold.
If you're not comfortable with that level of
disassembly, you should take your vehicle
to a pro. But if you have an engine with
easy access to the
rear bank, then
you can probably
do the job yourself. Just
be sure you gap the spark
plugs properly and use a torque
The tools shown are available at online suppliers and auto parts stores.