Question:Should I replace the distributor cap and rotor
when I change the spark plugs?
Answer:Not unless the distributor cap is worn, cracked
or otherwise damaged. Some people will automatically replace
the cap when doing a tune-up as preventative maintenance. There's
nothing wrong with this approach, but it really isn't necessary
unless the cap needs changing.
The same logic applies to the rotor. If it's not worn and
there's nothing wrong with it, you don't have to replace it.
The distributor cap's role in the ignition system is to distribute
high voltage current from the coil to each of the spark plugs.
The revolving rotor inside the cap carries the current from the
center coil terminal to each of the plug wire terminals. The
spark jumps the small gap between the end of the rotor to complete
its journey to the plugs.
After zillions of such cycles, the dielectric material from
which the cap is made can start to break down. Small hairline
cracks or carbon tracks can form on the cap, creating new paths
for the high voltage to follow. This can lead to misfiring, hard
starting and rough running. Corrosion can also build up on the
underside of the plug wire terminals as well as inside the terminal
wells themselves adding additional resistance. This too, can
affect ignition performance and cause misfiring. Caps can also
be damaged by mishandling during installation and removal.
When the plugs are changed (or when an ignition problem is
suspected), the cap should be removed and carefully inspected.
The presence of any cracks, carbon tracks or damage would tell
you the cap needs to be replaced.
Many newer engines do not have distributors because a direct
ignition system is more compact and reliable.
