That depends on what you mean by "last." Is it possible for a spark plug to
function for 100,000 miles? Under ideal conditions, yes. Spark plugs made with platinum
or iridium coupled with today’s high output ignition systems may be able to create
a spark sufficient to fire the air/fuel mixture in the cylinder for 100,000 miles.
But, there may be some severe consequences to waiting for the 100,000-mile mark.
One is the additional burden placed on the ignition system by worn spark plugs.
An ignition system will only produce enough voltage to fire the spark plug, typically
5000 volts at idle to perhaps 15,000 volts under acceleration. Some modern ignition
systems such as DIS (Distributorless Ignition Systems) or COP (Coil-On-Plug) systems
can produce as much as 50,000 volts!
As a spark plug wears, the gap becomes wider and the electrodes more rounded.
Both conditions require more voltage to create a spark. So, if your worn spark plug
requires 40,000 volts to fire, the ignition system will do it. But producing that
kind of voltage will take its toll on the ignition system. The question becomes,
"Would you rather replace four, six or eight spark plugs at $5 each or four, six
or eight ignition coils at $90 each?"
There’s an even greater reason to replace spark plugs before 100,000 miles.
They have been known to seize in the cylinder head if left in that long. If that
happens, you could be looking at a $2000 repair bill to remove the heads and replace
the spark plugs. Will that happen to you? Maybe, maybe not. Are you willing to take
The 100,000-mile spark plug is nothing more than a 60,000-mile spark plug that
the carmaker’s marketing department calls a 100,000-mile plug. It sounds impressive
to say that their car doesn’t need a "tune-up" for 100,000 miles. It’s really a marketing
driven claim, not one based on sound engineering. Manufacturers often add stipulations
to the 100,000-mile interval that’s in the owner’s manual, but is often overlooked.
The most prudent thing to do is to replace standard spark plugs every 30,000
miles. Platinum and iridium plugs should be replaced every 60,000 miles.