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Why Spark Plugs Need To Be Replaced

by Larry Carley copyright

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 spark 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.

One of the reasons why spark plug sales take off when cold weather arrives is because many motorists put off changing the plugs until they absolutely have to. The spark plugs continue to rack up mile after mile until they have deteriorated to the point where they are causing noticeable starting and driveability problems.

Emission checks will catch a lot of bad spark 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 spark plugs are the driveability problems created by the plugs themselves. So many motorists today think they are saving money on maintenance by putting off a spark plug change until it is obvious the engine needs new spark plugs. Then and only then will they begrudgingly spend any money on a new set of spark plugs.


What motorists need to know is that spark plugs do NOT last forever, even the long-life 100,000 mile plugs. All spark plugs need to be changed sooner or later. Here's why:

REASON #1 Your Spark Plugs May Need to be Replaced: Ignition Misfires Caused by Worn or Fouled Spark Plugs

Good spark plugs are necessary to 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 would not 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.

Since 1996, all passenger cars and light trucks 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. So to maintain good engine performance, spark plugs should be replaced BEFORE they cause misfires.

WARNING: Ignition misfire will cause your vehicle to fail an emissions test (plug-in or tailpipe). Ignition misfire will also cause a noticeable drop in fuel economy as well as a huge increase in hydrocarbon emissions which can damage your catalytic converter over time and cause it to fail!

Recommended Spark Plug Replacement Intervals for Different Types of Spark Plugs

Always Refer to your owners manual or maintenance schedule for the recommended spark plug replacement interval for your vehicle.

For STANDARD spark plugs (not platinum or iridium), the recommended service interval for most vehicles is typically every 45,000 miles. These type of spark plugs were used as original equipment in older vehicles (1990s and older) before long life spark plugs were developed. Standard spark plugs may still be used in some late model economy cars with four cylinder non-turbocharged engines. Or a previous owner of your vehicle may have installed the less expensive standard spark plugs if they replaced the original equipment spark plugs.

The recommended replacement interval for LONG LIFE Patinum and Iridium spark plugs, which are used in most late model turbocharged engines as well as most V6 and V8 engines (turbo and nonturbo), is 100,000 to 120,000 miles.

Long life platinum and iridium spark plugs experience very little electrode wear over their service life, so wear even well beyond 100,000 plus miles is seldom an issue. However, long life spark plugs like standard plugs may become fouled with carbon deposits if an engine is only used for short trip light load driving around town, or if the engine is burning oil due to worn valve guides, worn valve guide seals and/or worn piston rings.

REASON #2 to Replace Spark Plugs: To Improve Cold Starting and Performance

Worn or fouled spark plugs can cause cold starting problems, especially during subzero weather. Many times the battery has been run dead while cranking the engine because the plugs would not light the fire. For more info on this subject, see Spark Plug Fouling.

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.

As the electrode gap at the end of the spark plug wears, it requries more and more voltage to create a reliable spark. If the electrode gap is badly worn, the ignition coil may be be capable of supplying enough voltage to create a good, hot spark under all driving conditions, resulting in ignition misfire.

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 was more of a problem on older vehicles that had carburetors because pumping the gas pedal can easily flood the engine with too much fuel. Flooding can also occur on an older carbureted engine 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 is 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.

fouled spark plug

REASON #3 to Replace Spark Plugs: New plugs minimize the risk of premature catalytic converter failure

A single misfiring plug can dump enough raw fuel into the exhaust to overheat and damage the catalytic 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 substrate. This, in turn, may form a partial restriction or complete blockage in the exhaust that creates enormous backpressure and chokes off the engines 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 spark plugs are replaced, the new converter may soon die from the same ailment.

Really Technical Stuff About Spark Plugs

The spark plugs are the business end of the ignition system. So if you want a deep dive into all of the things that can affect ignition performance, keep reading.

The typical spark plug needs anywhere from 5,000 to 25,000 volts or more to create a spark that is hot enough to ignite the air/fuel mixture in the engine's cylinders. The required firing voltage will vary depending on engine speed, load and operating conditions. At idle, the spark plugs may only need 4000 to 5000 volts to fire. But at full throttle, the required firing voltage may increase to 25,000 volts or more. Variables that affect firing voltage include:

Fouled electrodes may not fire at all!

Reliable ignition, therefore, requires a hot spark from the coil, good plug wires to carry the voltage (if spark plug wires are used), or a good electrical connection between a coil that sits atop a spark plug and the plug itself. Grease, dirt or moisture on the spark plug, plug boot or coil boot can short circuit the spark and prevent the spark plug from firing.

One way to tell if the plugs need changing is to check your odometer. If it has been more than the recommended number of miles (typically 100,000 miles on late model engines with long life plugs), it may be time to change the plugs. If the Check Engine light is on or flashing, and there are codes that indicate cylinder misfires, new plugs may be needed.

Although almost nobody uses a scope anymore to check ignition performance, observing the ignition waveform can reveal a lot about the health of the ignition system. A bad plug that is misfiring will cause the coil firing voltage to shoot up to the maximum output of the coil. Badly worn plugs or plugs that have been misgapped too wide will also increase the firing voltage dramatically (as can a bad rotor and/or ignition cables on older engines with distributors). If the scope pattern shows initial secondary spikes approaching the upper voltage limits of the coil, it is a sure sign that the spark plugs (and/or plug wires if used) likely need to be replaced.

A fouled spark plug (or shored ignition cable), will show an unusually low firing voltage on an ignition scope pattern.

Firing voltages should not vary by more than 3 kV cylinder to cylinder. A cylinder that shows an abnormally low firing voltage probably has a grounded spark plug (deposits bridging the electrode gap), or a shorted ignition cable. A cylinder that shows an abnormally high firing voltage compared to the others likely has an open ignition cable or a plug with a wide gap.

The plug firing time (spark firing line) portion of the secondary ignition display shows the duration of the spark in milliseconds (thousandths of a second). The average spark duration with the engine idling should be about 1.5 milliseconds.

A duration of less than 0.8 milliseconds would mean there either is not enough voltage to keep the spark going (low coil output), or the voltage is having trouble reaching its destination (excessive resistance in the plug wires).

A longer than normal spark (1.8 milliseconds or more) is an indication that the firing voltage is experiencing little resistance because a plug is fouled or grounded (or a plug wire is shorted) probably due to accumulated carbon deposits. Fouling can be a problem if a plug's heat range is too cold for the application (which can be solved by installing hotter plugs). But it may also be the result of excessive oil consumption due to worn valve guides or seals, worn rings, or even short trip stop-and-go driving.

Intermittent misfires can be caused by a variety of ignition, fuel or mechanical problems. Lean misfire occurs when there is too much air and not enough fuel, so the engine should be checked for air or vacuum leaks, dirty injectors, carburetion problems or a leaky EGR valve. If the misfire appears to "jump around" from cylinder to cylinder, a manifold vacuum leak or a leaky EGR valve may be the cause. But if the misfire is isolated to a single cylinder, a worn or fouled spark plug (or bad plug wire) is the most likely cause.


spark plug diagnosis chart
Click on image above to view Spark Plug Diagnosis Chart.

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


When spark plugs are replaced, wqe would recommend upgrading to platinum or iridium long life plugs if your engine has standard plugs. Long life plugs cost maybe 2X to 4X times as much as standard plugs, but save you money and effort in the long run because they do not have to be replaced as often (or maybe never if you don't keep your vehicle forever). Long life plugs are a smart choice for any engine where access to the spark plugs is difficult (such as the back plugs on transverse-mounted V6 engines).

Another option is upgrading to "performance" spark plugs. 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 spark 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.

When you change your spark plugs, do it when the engine is COLD or at room temperature, never when it is HOT. This will reduce the risk of damaging the threads in aluminum cylinder heads when you take the plugs out. Applying a small dab of anti-seize compound to the threads on the new spark plugs will make them easier to remove in the future.

Although spark plugs come pregapped from the manufacturer, always check the electrode gap with a feeler gauge or spark plug gauge to make sure it is the correct gap for your engine. Sometimes the gap may require adjustment.

Be careful when tightening spark plugs in engines with aluminum cylinder heads. Aluminum is a much softer metal than cast iron, so the threads in the spark plug holes can be easily damaged if you overtighten the spark plugs. You're not tightening lug nuts or head bolts so go easy on the wrench. See the chart below for recommended torque specs.

spark plug torque spec recommendations

As a general rule 14 mm thread plugs (5/8 and 13/16 inch hex socket) with a gasket style seat in engines with CAST IRON should be tightened to approximately 18 to 25 lb-ft (24 - 34 N.m).

On engines with ALUMINUM cylinder heads, spark plugs with 14 mm threads and flat seats with gaskets (5/8 or 13/16 inch hex socket) should only be tightened to 18 - 21 lb-ft (24 - 28 N.m).

When installing spark plugs that have TAPERED seats with NO GASKETS, 14 mm thread plugs should be tightened to 7.5 to 14.5 lb-ft (10 - 20 N.m) in engines with ALUMINUM heads, and 11 - 18 lb-ft (15 - 24 N.m) in engines with CAST IRON heads.

If you are not sure how much to tighten the spark plugs, look up the specifications online or in a service manual. Torque specs will vary with plug thread diameter and application. Spark plugs with larger diameter threads (18 mm) typically require a little more torque while spark plugs with smaller diameter threads (10 and 12 mm) usually require less torque.

12mm spark plugs with flat seats and gaskets should be tightened to 11 to 18 lb-ft in CAST IRON heads, and 11 to 14.5 lb-ft in ALUMINUM heads.

12mm spark plugs with TAPER seats (no gaskets) should be tightened to 7 to 14 lb-ft in engines with ALUMINUM heads.

10mm spark plugs with TAPER seats (no gaskets) should be tightened to 7 to 9 lb-ft in engines with ALUMINUM heads, or 7 to 11 lb-ft in engines with CAST IRON heads.

how to replace spark plugs More Ignition Articles:

Spark Plug Technology

Ruthenium Spark Plugs

Spark Plug Fouling

Don't Neglect the Spark Plugs

Original Equipment Spark Plugs, Are They Best?

Ford Motorcraft Spark Plug Breakage Problem (2004 - 2008 Ford Trucks w/5.4L V8, & 2005 - 2008 Mustang GT with 4.6L V8)

Bosch Platinum +4 Spark Plugs

Don't Use Ordinary Spark Plugs with Waste Spark DIS Ignition Systems

Spark Plug Wires

Analyzing Ignition Misfires

Spark Plugs & Ignition Performance

Distributorless Ignition Systems

Coil-Over-Plug Ignition Systems

Diagnosing An Engine that Won't Crank or Start

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