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Is spark plug replacement necessary with coilpacks?

I think I understand why spark plugs need to be changed on a vehicle with a distributor cap.

In a distributor cap system, voltage is held pretty much at a constant, and the spark fires when the rotor gets close enough to the pin that the breakdown voltage falls below the coil voltage.

Carbon deposits act like resistors effectively delaying the spark. A plug with worn electrodes would also fire late.

But on a coil pack driven car, like the Corolla, firing happens by computer and is completely independent of spark gap distance. (Thus the admonition not to adjust the gap distance.) So if this is the case, it seems to me that either the spark plug is firing and igniting the fuel, or it isn’t.

If it isn’t you’ll know because you’ll get misfires. If it is, then it seems a total waste to replace the plug.

Is there any reason to replace a functioning plug that I’m missing? Tt seems to me that regular replacement of spark plugs in a coil pack car is just wasting $$$…

Your theory about how the spark plug fires is incorrect. The computer only triggers the coils; it does not have anything to do with the discharge voltage of the coil.

Coils also work harder than they used to. The wider the plug gap the more voltage the coil has to put out to jump that plug gap. Add miles to the plug and the voltage has to be stepped up even more.
Many years ago plugs commonly had gaps of .025-.030 of an inch. Now .045-.065 is common. See what happens?

Replace plugs on a regular basis and it’s quite likely any coil pack problems will also disappear. Aged plugs are often the cause of the coil dying and yet another reason why plugs should never be left in place for 100k miles or eternity; whichever comes first.

The electrodes wear down in increase the gap to beyond tolerable. The spark will not jump the gap.

Spark plugs don’t care how they’re fired. They only know that they’re job is to create a high energy spark across a gap. And when you build up that much heat, somethings got to wear out.


As spark plugs age, the voltage required to fire them keeps increasing. At some point, the high voltage energy finds a new path to ground. The insulation that is part of the coil-pack breaks down allowing the energy to find a new path, destroying the coil-pack or worse, feeding high voltage back into the primary wires which lead back to the expensive ICM…

“Your theory about how the spark plug fires is incorrect. The computer only triggers the coils; it does not have anything to do with the discharge voltage of the coil.”

Just to be clear, I was talking about a distributor cap system, not a coilpack system at that point.

Basically what you appear to be saying is mostly what I expected: the older the plug, the wider the gap. The wider the gap, the more voltage required to jump it, but in a coilpack car the coilpack will make up the difference and still spark at the right time…

The one piece of information you are saying that I wasn’t aware of was that increased voltage could lead to coil pack failure. I take it what you are saying is that over time the breakdown voltage of the coil pack’s potting decreases, and if the spark plug requires too much voltage to fire then you can end up sparking though the potting of the coil pack… and once that happens even one time it’s all downhill from there.

So basically, it would seem, that the best thing to do is to simply visually inspect the plugs from time to time and make sure the electrode wear is not excessive. If it is, you can either bend the electrode down a little to return to a more reasonable distance (if you are really cheap), or just replace the plug.

Am I understanding you correctly?

Basically what I am taking from all this is that I am sort of half right. Worn spark plugs won’t effect the performance of the car at all in a coilpack car, at least right up to the point where the spark gap grows too large and then something “bad” happens, where bad could be anything from a misfire, to a destroyed coilpack, to a ruined ECU… Basically as long as the spark gap distance is not unreasonable, there is no problem.

I suppose what you’re saying is correct, but it’s the same whether you have a coilpack or a traditional coil.

The fact that you have a coilpack is not giving you any advantage with respect to your “worn spark plug” scenario.

  1. On a distributor car, the coil voltage is not held constant. It is triggered for each plug. That what the points (or later electronic pick up) were there for.
  2. On a COP (Coil on Plug) or coil pack car, the coil has a rated “breakdown voltage.” If the spark plug gap grows too large, then the voltage to fire the plug will exceed the breakdown voltage. When this happens, the voltage will arc inside the coil and eventually destroy it.
  3. COP = one coil mounted directly over each spark plug, usually no plug wires. Coil pack = one coil for every two spark plugs, plug wires always used, two or three coils in one pack.

You’re still incorrect as there is much more behind the coil firing than the plug gap itself.

It’s possible to have an engine that appears to run perfectly fine but every spark plug in it may have subtle misfires that will not be noticeable or set a computer code.
Every time one single subtle misfire occurs the coil voltage will spike. Enough of that and eventually the coil dies without you ever knowing a thing.

The coil voltage, no matter the system, is never constant and may not be even on the same cylinder.
If you ever get a chance to watch an engine firing pattern on an oscilloscope you will see what I mean. Pick a cylinder, watch the firing line, and you will see it fluctuating constantly.

Give you an example about plugs. I’m currently up at my son’s house here in northern Utah. We drove up here in his '07 Dodge Caliber (2.4 AWD with 46k miles).
I suggested a set of new plugs even though the car ran (apparently) great with no CEL on, etc.
Pulled the plugs out and my son says the plugs look fine but I say they’re garbage.
Anyhoo, this car got a steady 25 MPG before the plug change and on the 1100 mile trip here it got a steady 29-30 MPG and even flirted with 31 MPG at one time.

Spark plugs wear out from erosion. Anything that lights a torch will wear out after a while. We replace plugs at or about the time recommended by the manufacturer. If it’s at 100,000 miles, then that will be about when they will be worn down.

You’re still incorrect as there is much more behind the coil firing than the plug gap itself.

Right, not the least of which is the cylinder pressure. The higher the gas pressure, the more insulating the gas is and the higher the voltage required to arc across the same gap. Poor atomization of the fuel or incomplete mixing also contribute to fluctuating breakdown voltages.

Plugs are Plugs.

How the spark is generated means NOTHING. The plug gap still needs to be jumped. The COP (coil on plug) system it’s the basic same principle. The coil sends large voltage to the plug to ignite the compressed gas/air mix. The only difference is HOW the voltage is generated.

Plugs still need replacing. They wear out. Many manufacturers are using Platinum or Iridium plugs which last longer then conventional plugs…but they still need to be replaced. I replace the plugs on my 4runner once a year (about 40k miles). My wifes Lexus…every other year (about every 50k miles). You want to replace them BEFORE they NEED to be replaced.

I gonna attempt the best explanation I can muster. I’ll use a “point” system as a beginning example to simplify.

The way a coil works is thus:
when the points close, they complete a circuit that allows 12VDC from the battery to go through a coil of wire wound around an iron core. The current flow creates a magnetic field.

When the points open they break the circuit. The magnetic field in the coil collapses, inducing a voltage spike into the coil’s core. The voltage spike is high enough to creat and arc aross the spark plug electrodes to ignite the gas mix.

A coil pack is simply two or three coils in the same package, each assigned to two or three spark plugs.

Coil On Plug systems simply have an individual coil on each plug.

The only difference with modern systems is that rather than a set of points opening and closing the coil’s “low voltage” circuit (the windings), it’s done by a computer as the result of a group of sensor inputs driving a computer program. The actual process of the creating the spark voltage via a collapsing magnetic field remains the same as it alway has.

Spark plugs also work the same as they always have, except that the electrode materials have changed. The high voltage arc created by the coil, necessary to jump the plug’s gap, causes vaporization and erosion of the metal in the electrode. The better metals simply last longer. Irridium is 8 times as hard as platinum. Yes, spark plugs still need to be changed.

Oh, and the byproducts of the combustion process can still leave deposits on the plugs just as they always have, necessitating new plugs periodically just as always. The timeframes are longer because the combustion process in modern engines is far cleaner than it used to be, but deposits are still a fact of life.

I hope this helps explain it.

Just another data point here. I recently replaced the original plugs in my '06 Corolla at 100,000 miles. The recommended interval is 120,000 miles, and I just couldn’t believe they would last that long. The original plugs were clean and still had the specified gap of 0.043". The only visual change between old and new was a very slight transfer of metal from the center irridium electrode to the grounded electrode. The car ran perfectly before and after the plug change with no change in performance or gas mileage. Not your grandfather’s sparkplugs.

Outstanding. It’s amazing how far the technology has come and how clean today’s engines burn. We forget how much things have changed.

I have a Ford Explorer w/ the 4.0 SOHC V6. It is a coil pack engine with 3 coils and a wasted spark design, meaning each coil fires 2 cylinders at a time. I just changed the plugs again for the 3rd time at 188,000 miles. An interesting thing I noticed was the left side of the engine, fired on the positive side of the coil, were all worn on the platinum tips. The right side, fired on the negative side of the coil, were all worn on the steel ground electrodes.

But, after 60,000 miles, they were still visibly worn.

Right. The reason is that with wasted effort (oops, I mean wasted spark) designs the polarity is reversed on one plug.

It’s a dumb design.