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Plug wires

2004 Sienna, 85,000miles. What is the genereal consensus of opinion on replacing plug wires? Live in Ariz. with all the accompaning heat, was thinking tha a good time to replace them would be when I have the timing belt replace, along with plugs. Have had absolutely no problems, missing/hard startin/ roug idle / etc that would indincate the wires are starting to go.

There aren’t any plug wires. You have a coil-on-plug system (a coil on top of every plug). If the coils are OK, you may need to replace the plugs, but no wires.

If you DID have plug wires…NEVER…Just clean them with a damp rage a couple times a year…I’ve had plug wires last the life of the vehicle (300k + miles) without so much as a hint of any problem.

Missing, hard starting, and rough idle in your case would probably mean you’re overdue for new spark plugs. You have platinum tipped plugs, and at 85,000 miles it’s very likely.

Also, you should be getting a Check Engine Light. Or Malfunction Indicator Light, whatever your nomenclature of preference. Youu should have stored codes to help send the tech in the right direction.

As already stated, you have no plug wires. On older engines, a single coil would send a high voltage discharge to a “distributor” which would send it through the “plug wires” to the proper spark plug, but on your car each individual spark plug has its own coil, leiminating the need to plug wires. The wires were a problem because the insulation would deteriorate and allow arcing to ground through other parts of the engine, causing erratic spark. Your engine has no such problem. But be sure you tell the tech of your operating erratic conditions so he/she can look into it. It may jus be the spark plugs, but there may be another cause.

Never having had a car without a distributor, maybe this is a dumb question: how does the electricity get to the individual coils? Are you saying the OP has coil wires rather than plug wires, or does it get there through some other method?

Am I right in thinking plug wires were once insulated tubes filled with carbon powder?

Mike, I Know, Right ? I Think It’s A Throw-Back To The Old Tune-Up Days. We’ll Get Some Flack And Reasons To Replace The Wires, But Hey, Works For Us.

When I worked at Volkswagen years ago, we sold lots of plug wires (and plugs and points). At the time it was fairly common for the plug wires to break down. The carbon stuff (not copper wire) inside would go to heck in a handbasket.

If word gets out to dispel the “spark plug wire myth,” entire plug wire companies would go out of business.

I have to believe that some people remove perfectly good, quality factory wires, and replace them with sometimes inferior aftermarket wires and then they need to repeat this procedure periodically.


Look At How Small The Wires Are That Supply Electricity Into A Conventional Coil Compared With The Large Wire That Feeds Out Electricity To The Distributor, That In Turn, Feeds The Large Spark Plug Wires.

Relatively small wires supply the electricity to coil-on-plug coils, too. Since the coil is right there, on the plug, the need for the large diameter plug wires is eliminated.


Plug wires will fail…but it can be prevented…Just wipe them down with a damp cloth a couple times a year…That way dirt won’t build up…and dirt will hold moisture. My 98 Pathfinder had the original wires when my daughter finally sold it with almost 400k miles on it.

The coil primary handles a pulse of less than about 400V, so the wire insulation can be thinner than wires designed for 20,000V or more.

Piter, the primary, which is the wire coil wound around the iron core, handles only the 12VDC that it takes from the car’s electrical system. That circuit is turned on and off. When it’s on, current travels through the wires creating a magnetic field around the wires and in the coil. When it’s shut off, the magnetic field collapses into the iron core and induces a momentary high voltage spike.

Voltage is potential. Voltage is the difference between the charge of one thing and the charge of a second thing. Load one side up with free electrons or depleate the other and and you have a charge. Free electrons will always want to move from the place with the surplus to the place(s) without. Only when there’s too much resistance between the two for the electrons to overcome do they do nothing and a charge remains.

As mentioned, the spark voltage (necessary to jump the spark plug gap) was high. The spark plug wires of old were designed to contain the high voltage and prevent any of it from discharging to the engine block or engine head components, both being “grounded”, connected through a resistance-free path directly to the battery.

Many years ago the coil primary was turned on and off by a set of “points”, which were basically just a cam operated set of switch contacts. The high voltage pulse to the plugs was routed through the “rotor” to the correct plug. In modern engines each individual spark plug is provided with its own coil. The coils are controlled by the engine’s ECU, using the engine speed and crank position along with other engine demand signals to determine the optimum spark timing. Since each plug has its own coil, they’re placed right at the plug rather than rout the high voltage signals through high voltage wires.

Spark plug wires or cables were once copper with heavy insulation. As radios became popular in cars, this type of wire produced too much static. The type of wire you described came next, and it was very trouble prone and short lived. Eventually, the core was made from a type of carbon known as aramid. It is somewhat like the stuff they make fighter jets from. It is very durable and can easily last the life of the vehicle. But now, even those are obsolete with the coil on plug technology.

True. As a matter of fact the really old cars from the early 1900s, the plug wires had open lug terminals and were attached with nuts (often wingnuts) to the threaded spark plug end, which was basically just the threaded other end of the electrode. Spark plugs today still have a threaded end with the clip connector screwed on. I wonder why that is???

TSM, I know you spent a lot of time on that reply, and thanks, but you didn’t get to my question: are there wires going to the individual coils? The electricity has to get from Point A to Point B and it’s not from free electrons zipping around the engine compartment. Where do the individual coils get their electricity? Wires, or some other means?

Maybe the attached pic will help. It’s from a Honda 3.0L engine. There are three small wires going to each “coil-on-plug” pack. One wire supplies 12 volts, one wire is ground, and the 3rd wire is the trigger wire.

All your coil-on-plug packs will have similar inputs. Though they may use different looking connectors on the outside. I don’t know if any designs incorporate the 12v feed with the trigger on the same wire.

Thanks. So there are wires, just different kinds of wires. I can understand the OP possibly referring to these as “plug wires”.

Thanks Joe for the help.

You’re right Piter, and I apologize. The major weakness of language is jargon. Your question was clear, my mind was just preprogrammed.

The good news is that the wires going to the coils are low violtage, low current, and don’t need replacing like the old high-tension spark plug wires did.

I do not agree that it is an issue of jargon here. A plug wire is a plug wire, period. Coil wires are a different thing here. The OP asked about plug wires and I’m sure that is what he or she meant. He or she probably was thinking about issues with previous vehicles that did have plug wires.

Now, about that “trigger wire”. Where did that term come from?

I personally have no problem accepting that the OP used the term “plug wires” to indicate whatever type of wires were associated with the sparkplugs. I think I’m fairly safe in suggesting that I too use terms inapprpriately in areas that I’m not knowledgable in. Sometimes I even use improper terms in areas that I AM knowledgable in. Sometimes I even MAKE UP terms! Knowing this helps me understand the question…usually, but not always. Sometimes I forget.

Pitre, those wires are called primary wires. Every coil has always had them. The coil wire and spark plug wires are only neccessary when there is a distributor involved. GM’s HEI system eliminated the coil wire a long time ago by putting the coil on top of the distributor cap.

The old ignition cables were called secondary wiring. Now, with the coils mounted right on top of each plug, all secondary wires are gone. It’s nice to have a better way to get spark. Now if only the coils were perfect.

Many “coil on plug” ignition systems have a replaceable rubber boot connecting the coil to the spark plug. It resides in a hot environment and should be replaced when installing spark plugs if it shows any signs of heat deterioration.