Spark Plugs

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.

Usually motorists are put off by changing their spark plugs, preferring to put it on hold till there are notable changes in the performance of a car during cranking or while driving. The spark plugs continue to rack up considerable distances until they have deteriorated to the point where they are causing starting and drive ability problems.

Emission checks will catch a lot of bad spark plugs and force motorists to change plugs that need to be replaced. 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 they will begrudgingly spend any money on a new set of spark plug.

Reason 1:

What motorists need to know is that spark plugs do NOT last forever, even those long-life copper plugs. All spark plugs need to be changed sooner or later.

New plugs maintain peak engine performance and efficiency. Every engine will misfire occasionally. But as the numbers of misfires per kilometer go up over time, it increases exhaust emissions, wastes fuel and reduces power. In the past, most motorists 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.

Most 1996 and newer vehicles have an OBD II on board 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 on older vehicles, replacing the spark plugs at the recommended service intervals for preventive maintenance will reduce the risk of misfires and maintain peak engine performance. For standard spark plugs, the service interval is typically every 72000km. For platinum spark plugs, it is 160,000km.

A new set of plugs is not a cure-all for drivability 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 would not 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 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.

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 substrate.

This, in turn, may form a partial restriction or complete blockage in the exhaust that creates enormous back pressure 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.

The spark plugs are the business end of the ignition system. Whether an engine has a conventional distributor or a direct ignition 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 has been more than the recommended number of kilometers (usually 50,000) since the spark plugs were last changed, it is time for a new set.

Another way to tell is to observe the secondary ignition pattern on an oscilloscope. If there is an open plug or wire, the plug will not fire causing the firing voltage to shoot up to the maximum output of the coil. Badly worn plugs or plugs that have been misshaped too wide will also increase the firing voltage dramatically (as can a bad rotor and/or ignition cables with excessive resistance).

If the required voltage exceeds the maximum output of the system, the plugs may not fire. If the pattern shows initial secondary spikes approaching the upper voltage limits of the system, therefore, it is a sure sign that the plugs (and/or cap, rotor and cables) need attention.

A fouled plug (or shored ignition cable), on the other hand, will show an unusually low firing voltage. 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