Question:I have a couple of bad plug wires on my engine.
Should I replace only the bad ones or the entire set?
Answer:It depends on your budget, how much longer you
plan to keep your vehicle, and how much time you want to spend
fixing your car. If one plug wire is bad, chances are you have
others that are getting bad, too. Replacing only the wire that's
bad will solve your immediate ignition problem and minimize your
repair expense. But sooner or later another wire will fail, and
another, and another. So you can wait until the next problem
develops to replace the next wire, and so on as the need arises,
or you can replace all of the wires at once and be done with it.
It's up to you.
DON'T IGNORE IT
How much of an effect can a bad plug wire have on performance?
One misfiring plug in a four cylinder engine can cause a 25%
drop in power and fuel economy, not to mention an enormous increase
in exhaust emissions.
There is also the possibility that a bad plug wire may damage
your catalytic converter. The converter's job is to reburn exhaust
pollutants, not raw fuel. But that's what happens when a plug
misfires because of a bad wire. With every other revolution of
the engine, a cylinderfull of unburned gasoline is dumped into
the exhaust. When the raw fuel hits the converter, temperatures
soar. This can overheat the converter and cause a meltdown of
the catalyst inside resulting in a partial or complete blockage
in your exhaust system!
ABOUT IGNITION WIRES
The spark plug wires or sables conduct high voltage current
from the distributor or ignition coil pack (distributorless ignition
systems) to fire the spark plugs. The cables are part of the
engine's "secondary" ignition system.
The spark plug wire's job is to provide a conductive path
that doesn't leak voltage. The earliest spark plug wires were
copper, aluminum or steel wire wrapped with insulation sufficient
to contain about 12,000 volts (12 KV) of electricity. But when
high voltage current surges through a low resistance wire, the
wire becomes a broadcast antenna and sends out radio waves. This
causes radio frequency interference (RFI) which disrupts radio
and television reception. In vehicles built during the past 20
years, RFI can play havoc with the electronic circuits in computers,
ignition and fuel injection modules. So the plugs' wires must
also "suppress" the frequencies that cause the troublesome
RFI. This is done by using "resistor" wires that create
enough resistance to suppress RFI, but not so much to interfere
with proper ignition.
Resistance in the spark plug cables can be created one of
two ways: by using graphite-impregnated fiberglass strands (commonly
called "carbon-core" wires) to carry the high voltage
current, or by using a special nickel alloy resistance wire called
"monel" wiring. The monel wire is wound spring-like
through the core of the cable. The spiral wound wire reduces RFI
with less resistance and better conductivity than standard carbon-core
cables.
Monel cables are considered to be "premium" grade
wiring. Because the wiring has less resistance than carbon-core
cables, the voltage required to fire the plugs is lessened, which
aids starting and reduces the odds of misfire. And unlike carbon-core
ignition cables, monel won't deteriorate over time.
Carbon-core cables have little particles of carbon impregnated
in strands of fiberglass. Over time, the carbon particles tend
to bunch up and separate. Heat and vibration contribute to the
aging process, as does twisting or jerking on the cables themselves.
Once the carbon begins to separate, resistance shoots up. This
raises the voltage needed to pull current through the wires and
fire the plugs. If the ignition system can't overcome the resistance,
the plug misfires. At this point, the plug wire needs to be replaced.
A bad plug wire with excessive resistance usually causes an
intermittent misfire rather than a steady miss. This happens
because the voltage required to fire the plugs changes with engine
speed and load. It is lowest at idle and under light load, but
rises sharply as the throttle opens or the load increases. Thus,
the engine may run fine most of the time, but then misfire or
stumble during acceleration. Unless the cables are replaced,
the carbon-core will continue to deteriorate. The symptoms will
worsen and eventually the occasional misfire will become a constant
misfire.
PLUG WIRE REPLACEMENT
Original equipment plug wires are supposedly engineered to
last the life of the engine (100,000 miles or more). But in real
life, many never make it. Heat, vibration and mishandling can
drastically shorten the life of the wires. After 50,000 miles
or more of carrying high-voltage current, the insulation may begin to break down and internal resistance may be inching
up.
Some experts suggest replacing 7mm carbon-core ignition cables
every 30,000 miles as preventative maintenance. Others say a
50,000 mile replacement interval is more realistic for today's
cars. In any event, the cables don't last forever. Eventually
they do have to be replaced -- but most people wait until they
start to experience a misfire before doing anything about it.
A professional mechanic can spot a bad plug wire with too
much resistance in it by watching for an unusually high firing
voltage on his scope. Resistance can also be checked with an
ordinary ohmmeter. For cables less than 25 inches in length,
resistance generally should not exceed 30,000 ohms. For those
over 25 inches in length, the maximum acceptable reading is 50,000
ohms.
If an ignition cableis chaffed, has age cracks, or burned spots
in the insulation, or is arcing (shorting out), the wire needs
to be replaced.
An easy way to check for arcing is to wait until dark, then
start the engine and open the hood. CAUTION: Keep your fingers
well away from the plug wires and moving parts under the hood!
If you see any sparks in the vicinity of the plug wires or plugs,
or hear a "snapping" sound, it means the plug wires
are arcing and need to be replaced.
Silicone insulation is superior to Hypalon insulation in terms
of heat resistance and longevity. A Hypalon cable touching against
a hot exhaust manifold will likely melt through while a silicone
cable usually will not. The thickness of the insulation is also
important. The thicker the cable, generally speaking, the more
voltage it can safely handle. That's why many late-model high-voltage electronic ignition systems have gone to 8mm, rather than
7mm, silicone ignition cables. Silicone costs more, but performance-wise
it's worth it.
REPLACEMENT TIPS
- Handle wires with care during installation. Do not jerk,
force, twist or sharply bend.
- Change one wire at a time to avoid mixing up the firing
order.
- Don't run long wires parallel to one another.
Separate them by several inches or crisscross them to avoid ignition
crossfire through induction.
- Make sure the terminals fit snugly into the distributor
cap or coil pack and that the terminals in the cap and coil are
clean, dry and corrosion-free.
- Wires should be supported by looms or brackets, and not
allowed to touch the exhaust manifold or rub against sharp objects.
- Ignition wires should be routed several inches away from
any wiring harnesses on vehicles with computerized engine controls.
