Ignition misfire is a condition nobody wants because it kills power, increases emissions and gobbles gasoline. All cylinders will hiccup occasionally, perhaps a couple of times in every thousand compression strokes. Such misfires are considered normal and cause no noticeable performance or emissions problems. But when the rate of misfire increases or becomes a chronic condition, it means trouble.

Every time a spark plug fails to do its job, a whole cylinder full of unburned gasoline is dumped into the exhaust system. When the unburned fuel hits the catalytic converter, it ignites sending the converter's temperature soaring. If it gets hot enough, the ceramic substrate that supports the catalyst can literally melt, causing a partial or complete obstruction in the exhaust system. So anytime you find a plugged converter, chances are the underlying cause is either ignition misfire or a leaky exhaust valve.

Ignition misfire can be caused by any number of things. A fuel mixture that's too lean because of dirty fuel injectors, low fuel pressure or a vacuum leak can cause lean misfire. A different type of misfire can occur if the mixture is too rich because of leaky injectors, excessive fuel pressure, a sticky choke or flooded carburetor.

Misfire can also occur if the primary or secondary voltage is too low due to a low battery, low charging voltage, weak coil, worn distributor cap or rotor, or excessive resistance in the plug wires. Then there's misfire that results from loss of spark due to shorted or open plug wires, cracked distributor caps and rotors.

Yet another common cause of misfiring is plug wire boots that fit loosely around the spark plugs. A heavy accumulation of grease or oil on the plugs can also provide a shortcut to ground for the secondary voltage, preventing the plugs from firing reliably.


FACTORS THAT AFFECT IGNITION

The typical spark plug needs anywhere from 5,000 to 25,000 volts from the ignition coil to ignite the fuel mixture. The exact firing voltage depends on:

Plug gap - the wider the gap, the higher the voltage required. Wider gaps are also needed for lean fuel mixtures. But the gap must be within specifications for reliable ignition performance.
Electrode condition - wear increases voltage requirements. Fouled electrodes may not fire at all!
Engine load - higher load increases the amount of voltage needed.
Resistance - electrical resistance in the plugs and wires increases voltage required.
Operating temperature - a cold plug requires more voltage to fire than a hot one.
Reliable ignition, therefore, requires a hot spark and a set of spark plugs that are properly gapped and in good condition. If the ignition system can't provide enough voltage to fire the plugs, the engine will misfire. And if the plugs aren't in good shape, the spark may not reach its intended destination.


DIAGNOSING IGNITION MISFIRE

How do you find bad plugs that need replacing? Observing the secondary ignition pattern on an oscilloscope is a good place to start.

In the case of an open plug or wire, the plug won't fire so the firing voltage will shoot up to the maximum output of the coil. Badly worn plugs, or plugs that have been gapped 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 will misfire. So if the pattern shows initial secondary spikes approaching the upper voltage limits of the system, it's a sure sign that the plugs (or the cap, rotor or cables) need attention.

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

Firing voltages should not vary by more than three 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 will show you 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 tell you that there either isn't enough voltage to keep the spark going (low coil output), or that 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. You'll probably find a heavy accumulation of carbon deposits on the electrode.

Fouling deposits 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 (worn valve guides and/or rings) or a lot of 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's too much air and not enough fuel, so look 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, then suspect a manifold vacuum leak or a leaky EGR valve. But if the misfire is isolated to a single cylinder, then a worn or fouled spark plug (or bad plug wire) is probably the cause.


READING THE PLUGS

Examining the tips of the spark plugs as they are removed can reveal a great deal about the health and performance of an engine. But wait until the engine has cooled if it has aluminum heads before you attempt to extract the plugs. Hot plugs can sometimes strip the relatively soft threads in an aluminum head.

The appearance of the plugs as well as the color and kind of deposits can alert you to various kinds of problems.

Normal worn plug - the electrodes will be worn down and the insulator will have light brown or tan-colored deposits.
Fuel-fouled - black fluffy carbon deposits indicate an overly rich fuel mixture and/or a weak spark. Check for such things as a stuck choke, a heavy carburetor float, a fuel leak in the carburetor, a leaky injector, low coil output, high resistance in plug wires, a sluggish or dead oxygen sensor, or even a defective coolant sensor that prevents the engine's computer system from going into closed loop.
Wet plug - a wet plug means the plug has not been firing. The problem is often due to a bad ignition cable (excessive resistance, shorted or arcing), but it can also be caused by dirt or moisture on the outside of the plug which provides a conductive path for the voltage to bypass the electrodes, or by an internal crack in the ceramic insulator that shorts the plug to ground.
Oil-fouled - heavy black deposits, often oily in appearance, indicate oil burning. Oil drawn past the valve guides and/or the rings is entering the combustion chamber and fouling the plugs. Switching to a hotter plug may help prolong the life of the replacement plug somewhat, but such an engine is in need of major repairs.
No plug is going to survive long under such conditions. Oil leaking past the valve guide seals and down the guides can also form heavy deposits on the backs of the intake valves. The deposits act like a sponge and absorb fuel which can create a momentary hesitation problem when accelerating.
Glazed plug - yellowish melted-looking deposits on the insulator tip form when an engine normally used for stop-and-go driving is subjected to high temperature operation. A plug with a broader heat range may be recommended.
Damaged plug - if the electrodes have been smashed flat or broken, somebody put the wrong plug in the engine. If a plug protrudes too far into the combustion chamber, it may hit the piston or a valve. A foreign object in the combustion chamber may also cause such damage.
Overheating - if the insulator is blistered, white and free from deposits, something is making the plug run too hot. Check to see if the plug is the correct heat range. Also check for cooling problems, over-advanced ignition timing, lean fuel mixtures, or an air leak or other conditions that may be causing it to run hot.
Melted electrode - a symptom of severe preignition. The plug has been running too hot for a long time (see overheating above).
Detonation - if the insulator is split or chipped, detonation may be occurring in the engine. Check for over-advanced ignition timing, a defective knock sensor, excessive compression due to accumulated deposits in the combustion chamber, or an inoperative EGR valve. Switching to a higher octane fuel may be recommended.

REPLACEMENT TIPS

When the spark plugs need replacing, either for preventive maintenance (every 30,000 miles is recommended) or to solve a misfire problem, long life platinum or gold-palladium plugs are a good upgrade from standard plugs. These plugs cost a little more, but can go up to 100,000 miles without having to be changed or regapped - which is triple the life of most standard spark plugs. The precious metal electrodes virtually eliminate electrode wear, thereby improving ignition performance and fouling resistance.

Some spark plugs have specially configured electrodes that are grooved, split, fluted or pointed. Such designs provide a better edge from which the spark can jump and also expose more of the spark to the fuel mixture to increase ignition reliability and reduce the chance of misfire.

Some import engines require plugs with multi-electrode or surface gap electrodes because of their design (the Mazda rotary and some European engines). Make sure you install the type of plug recommended by the vehicle manufacturer because using ordinary plugs in such applications may adversely affect performance, fuel economy and/or emissions.

When installing plugs in an aluminum head, it's a good idea to put a dab of anti-seize on the plug threads before they go in. Also, use a torque wrench if possible when tightening the plugs because too much muscle on the wrench can easily strip the threads in the head.

Ignition misfire can occur when cylinder pressures increase under load. Opening the throttle wide draws more air and fuel into the engine's cylinders. The increase in-cylinder pressure raises the voltage required for the spark to jump across the electrode gap and ignite the air/fuel mixture. At the same time, increasing engine speed reduces the time between each plug firing. At higher rpms, there's less and less time for the ignition coil to reach full saturation. The combination of factors can add up to misfire under high load and/or engine speed.

Electrodes with sharp edges and points make it easier for the spark to jump the gap than ones with dull or rounded surfaces. This is what lowers the firing voltage requirements for improved ignition reliability.

Electrodes with an "open" design also exposes more of the spark to the air/fuel mixture. This reduces the risk of misfire by allowing the flame kernel to grow and expand without being extinguished. Some say an open electrode design also helps promote turbulence in the combustion chamber for better, cleaner combustion, too.