Most ignition problems fall into two categories: either the ignition doesn’t trigger at the right time (a timing issue), or something is preventing the spark from having a full charge (a resistance or failure of the system).
In the old days, it wasn’t possible to detect ignition problems until the car began running poorly. In a modern engine, the computer can sense ignition problems. It uses oxygen sensors to evaluate the air-to-fuel ratio of the engine output. Some engines also have a sensor to detect if the engine is firing too soon (anti-knock sensor), and to tell if the engine is misfiring by checking the motion of the crankshaft.
Timing issues occur when the spark plug is firing too soon or too late. Modern engines use the computer to trigger the timing based on the sensors checking the crankshaft and camshaft. If the timing in a new engine gets out of sync, it is usually due to the belt or chain wearing out or slipping. A dirty or failing crankshaft or camshaft sensor can also cause timing issues.
On older engines, the distributor and ignition need to be synced, or timed, to the position of the crankshaft
and camshaft. This can be done with a strobe light, and by rotating the distributor.
Knocking occurs due to pre-ignition—when the fuel and air in the cylinders ignites before the engine has compressed it to its maximum. It sounds like someone tapping the inside the engine with a hammer and it usually happens under acceleration or when the spark is advancing to give more power.
Pre-ignition is often caused by using gasoline with an improper octane rating, but it can also be due to deposits in the engine heating up and causing the fuel to ignite before the plug fires.
Knocking can be extremely damaging to an engine. Switching to a better fuel or a fuel that can clean out deposits might help eliminate pre-ignition. If not, check your computer codes and plugs for signs of problems.
Spark Not Advancing
When a car accelerates, the ignition “advances” the spark, which means it makes the spark turn on sooner to allow the fuel more time to burn as the engine spins faster. Before electronic ignition, vacuum and mechanical weights were used to change the position of the spark. The computer now adjusts the advance as it senses the engine speeding up. Problems with advance in a new engine are the same as timing problems: the computer can’t sync properly with the crankshaft and camshaft, or one of the other sensors is giving it wrong information.
The computer can’t tell you why the engine is misfiring, but it can tell you that something is wrong with one or more cylinders. When a cylinder misfires, the computer will sense a change in the speed of the crankshaft. It knows which cylinder was supposed to spark, so it can send an error code to tell you which one is misfiring (or if there is a random misfire through all the cylinders). When the computer reports a misfire, the best thing to do is to remove the spark plug (or plugs) and inspect them for problems.
SPARK PROBLEMS OR FAILURE
If the spark plugs don’t get a full charge of electricity, they may not trigger a spark big enough to get a good burn of the fuel, or they may not fire at all. The spark plug needs to make a good, clean electrical connection from the coil, through the wire (if used), and ground to the cylinder head. There are lots of things that can prevent proper spark from occurring.
Worn spark plug wires or signal wires can cause “arcing,” which is when the electricity used to fire the spark plugs is dissipated through an arc point. The spark plugs are carrying tens of thousands of volts and it is not hard for a small rub on a wire to cause a voltage arc. An arc, or short, can also ignite fumes outside the engine, so they can be dangerous. If you suspect a faulty wire, look for points where it may be touching the engine, or look for cuts or blackened outer areas where it might be arcing.
Ignition Coil Not Firing
The ignition coils generate the voltage that triggers the spark plugs. The number of coils your car has may determine the severity of your problem. If a single coil fails, the engine stops running. A coil pack may have one or all of its coils fail, and coil-on-plug only affects one spark plug.
If the spark plug is not making good connection to the cylinder head, it can’t complete the circuit, and won’t fire. If any part of the system loses its connection to ground, the plugs won’t fire. Ground issues can be maddening to locate and fix.
There are a lot of things that can contaminate the spark plug and prevent it from firing: oil from an engine that is seeping into the cylinder, coolant entering the cylinder, deposits from poor gas or poor combustion. Any foreign material that gets between the two points on the plug can keep it from firing normally. Keeping your engine maintained properly will prevent most problems.
Spark plugs will wear down under normal use. The two points that create the spark will slowly move away from each other, and the “gap” will increase, making it harder to create the proper spark. A plug that is damaged will not create a proper spark.
Spark plugs are designed to operate in a specific heat range, and engines are designed to use a specific plug. In general, you shouldn’t need to change heat ranges on your plugs.
Pulling and inspecting the spark plugs is the best way to find out what is happening inside your engine. They can tell you if there is a problem in one or all of the cylinders of your engine.
The spark plugs in your engine should look like this one: a light brown or light black color. The points, or electrodes, should be complete and not worn, and the base should not be coated or cratered. All the plugs should come out looking the same.
If you find a plug coated with residue or with burnt, broken, or worn parts, you need to check it against a spark
There are dozens of possible spark plug problems. A spark plug chart can help you identify your specific issue.
Shop manuals have small charts that show some of the more common spark plug failures, but online charts provided by the spark plug manufacturer or engine builder will usually be more comprehensive.
If your engine starts to run poorly, or if an error code tells you the engine is misfiring or has poor combustion, pull the spark plugs and check them. It is the best way to see what is going on inside the engine.