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Should You Replace Your Spark Plugs?

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One of the leading causes of hard starting is fouled or worn spark plugs. When a fuel injected engine that normally starts quite easily has to be coaxed to life, it often means the plugs are overdue for a change. As the electrodes wear, the voltage required to jump the gap and ignite the fuel mixture goes up. At the same time, accumulated deposits on the insulator can drain off voltage before it even has a chance to form a spark. So the engine fails to start or starts only reluctantly after prolonged cranking.

Spark plug sales take off when cold weather arrives is because many motorists put off changing the plugs until they absolutely have to. The vehicle manufacturer's recommendations to change the plugs every 30,000 miles for preventative maintenance are ignored, so the plugs continue to rack up mile after mile until they've deteriorated to the point where they're causing noticeable driveability problems.

Emission checks will catch a lot of bad plugs and force motorists to change plugs that need to be replaced. But in areas where emission checks are not required, the only incentives for changing the plugs are the driveability problems created by the plugs themselves. So many motorists today think they're saving money on maintenance by putting off a plug change until it's obvious the engine needs new plugs. Then and only then will they begrudgingly spend any money on a new set of plugs.


What motorists need to be told today is why the plugs should be replaced according to their vehicle manufacturer's scheduled maintenance recommendations be it every 30,000 miles, or 60,000 to 100,000 miles in the case of long life plugs. So here are three good reasons why the plugs need to be changed:

REASON #1: New plugs maintain peak engine performance and efficiency. Every engine will misfire occasionally. But as the number of misfires per mile goes up over time, it increases exhaust emissions, wastes gas and reduces power. In the past, most motorist wouldn't notice the gradual decline in ignition performance until it reached a point where it created a steady miss, caused the engine to run rough, buck or stall, or made it hard to start. Not so today. All 1996 and newer vehicles have an OBD II onboard diagnostic system that tracks ignition misfires. When the rate of misfires exceeds a certain limit and causes emissions to increase 50% over baseline levels, it illuminates a warning light. Too bad every vehicle doesn't have this watchdog system onboard. Since they don't, the next best thing you can do is replace the plugs for preventive maintenance BEFORE they deteriorate to the point where misfires become a problem and create problems.

A new set of plugs isn't a cure-all for driveability and emissions problems, but in many cases a plug change can make a significant improvement. Changing the plugs can reduce hydrocarbon (HC) emissions up to several hundred parts per million, which may make the difference between failing and passing an emissions test.

REASON #2: New plugs improve cold starting. Bad plugs are often responsible for many cold weather "no start" service calls. Many times the battery has been run dead while cranking the engine because the plugs wouldn't light the fire. When the old plugs are removed and examined, they are often found to be worn or dirty. New plugs reduce the voltage requirements on the ignition system, which decreases the chance of misfire while leaving more amps for the starter and injectors.

Wet fouled plugs can also prevent an engine from starting, but in many instances the fouling problem has nothing to do with plug wear or neglect. If an engine is flooded with fuel while it is being cranked, gasoline can soak the plugs and bleed off the ignition voltage before it forms a spark. Wet fouling tends to be more common on older vehicles that have carburetors because pumping the gas pedal can easily flood the engine with too much fuel. Flooding can also occur if the choke sticks, the float is set too high or the needle valve leaks. On fuel injected engines, wet fouling is less of a problem but can happen if a cold start injector leaks or there's a fuel calibration problem that creates an overly rich startup mixture. The cure in all cases is to wait for the plugs to dry out, or to remove the plugs and clean or replace them.

REASON #3: New plugs minimize the risk of catalytic converter failure. A single misfiring plug can dump enough raw fuel into the exhaust to overheat and damage the converter. The presence of higher than normal quantities of unburned gasoline in the exhaust will cause the operating temperature of the converter to soar, which may lead to a partial of complete meltdown of the converter's substrate. This, in turn, may form a partial restriction or complete blockage in the exhaust that creates enormous backpressure and chokes off the engine's ability to exhale. The engine may lack power, especially at higher speeds, and deliver terrible fuel economy. Or, it may stall and refuse to run after it is first started. Replacing the converter will solve the restriction problem. But unless the plugs are replaced, the new converter may soon die from the same ailment.


As we've already said, the spark plugs are the business end of the ignition system. Whether an engine has a conventional distributor or a direct ignition (distributorless) system, a good set of plugs is absolutely essential for peak performance.

The typical spark plug needs anywhere from 5,000 to 25,000 volts from the ignition coil before it will fire. The exact firing voltage depends on:

  • Plug gap-the wider the gap, the higher the voltage required. The gap must be set to specs for good ignition performance.
  • Electrode condition-wear increase voltage requirements. Fouled electrodes may not fire at all!
  • Engine load-higher load increases voltage needed. If the plugs are worn or gapped to wide, they may misfire under load.
  • Resistance-electrical resistance in the plugs and wires increases voltage required. Replacing worn, damaged or loose fitting plug wires is recommended for improving ignition reliability.
  • Operating temperature-a cold plug requires more voltage to fire than a hot one.

Reliable ignition, therefore, requires a hot spark from the coil, good plug wires to carry the juice, and spark plugs that are clean, in good condition and gapped properly. If any of these criteria are not met, the spark may not reach it intended destination causing the engine to misfire.

One way to tell if the plugs need changing is to look at a vehicle's odometer. If it's been more than the recommended number of miles (usually 30,000 - 60,000) since the plugs were last changed, it's time for a new set.


Examining the tips of the spark plugs as they are removed can reveal a great deal about the health and performance of an engine. The appearance and color of the deposits can reveal other problems that may need fixing:

  • Normal deposits-light brown or tan colored.
  • Fuel fouled-black fluffy carbon deposits indicate an overly rich fuel mixture or possibly a weak spark. Check for such things as a stuck choke, a heavy or misadjusted carburetor float, a leaky needle valve in the carburetor, leaky injectors, low coil output or high resistance in the plug wires. Wet plug-a wet plug means the plug has not been firing. If not due to engine flooding, the problem may be a bad ignition cable (excessive resistance, shorted or arcing). But wet fouling can also be caused by dirt or moisture on the outside of the plug that provides a conductive path to ground, or by an internal crack in the ceramic insulator that shorts the plug to ground.
  • Oil fouled-heavy black deposits with an oily appearance. These are the result of oil entering in the combustion chamber, probably past worn valve guides, guide seals or rings. Switching to a hotter plug may help prolong plug life somewhat, but no plug will survive long under such conditions. The only permanent cure to this condition is to fix the oil consumption problem.
  • Glazed plug-yellowish melted appearing deposits on the insulator tip that result from high temperature operation. The engine may be running too hot (check for cooling problems), the EGR valve may be inoperative and/or the heat range of the plug may be too hot for the application. Switching to a cooler plug may be necessary if no other problems are found. Damaged plug-if the electrodes have been smashed flat or broken, somebody put the wrong plug in the engine. A plug that protrudes too far into the combustion chamber may hit the piston or a valve. Always follow the plug manufacturer's application recommendations when selecting replacement plugs to prevent this kind of problem.
  • Overheating-if the insulator is blistered, white and free from deposits, something is making the plug run too hot. If the plug's heat range is not too hot for the application, check for cooling problems, incorrect ignition timing or a lean fuel mixture.
  • Melted electrode-a symptom of severe preignition. The plug has been running too hot for a long time (see overheating above). This can be very damaging and may burn a hole through the top of a piston!
  • Detonation-if the insulator is split or chipped, detonation (spark knock) may be occurring in the engine. The underlying cause here might be an inoperative EGR valve, overadvanced ignition timing, excessive compression due to accumulated deposits in the combustion chamber, or engine overheating.

When spark plugs are replaced, you might want to upgrade to a premium long-life platinum or gold-palladium plug. These plugs cost a little more initially, but can actually save you money in the long run because they don't have to be replaced as often. Many of these plugs can go 60,000 to 100,000 miles. Such plugs would be a good choice for any vehicle where plug access is a problem. Another option is upgrading to a "performance" spark plug. These plugs typically have unique electrode configurations that increase spark exposure to the air/fuel mixture and have multiple edges to reduce the chance of misfire. Performance plugs are usually more expensive than standard or even long-life plugs, but may be a good alternative if you want the ultimate in ignition performance.

There are also spark plugs today that are specially designed for truck engines. Such plugs have increased fouling resistance and oversized electrodes for longer service life.


We have more articles on spark plugs, including details on the right way to replace them on a modern car or truck. Check out our technical library.