How to diagnose a weak ignition system
At one time or another, we have all been stymied by a gas engine with either a no-start or a hard-start condition, or another performance issue that seems to elude logic. The engine has fuel, spark, and compression. It should either run or not have the performance issue that it does. Often that simple logic does not apply in the real world. Something else is going on, and there is a very strong possibility it is with the ignition.
The ignition system is considered to be passive. The spark plug only takes from the ignition coil what it needs to create an arc across the plug’s electrodes. It must be recognized that ignition demand is determined by the following:
- Cylinder pressure
- Engine speed
- Air-fuel ratio
- Ignition timing
- Spark plug gap
Cylinder pressure is not meant to be confused with compression ratio. Cylinder pressure is the cumulative result of the load on the engine in relation to the volumetric efficiency and mixture strength. Compression ratio is a design function of the difference in the volume of the bore with the piston at bottom dead center, compared to when it is at top dead center. Cylinder pressure and ignition demand are constantly changing with engine load.
At idle, the required energy to arc the spark plug is very low since the cylinder pressure is minimal. This is due to the throttle being almost closed, the engine rpm being low, and the fact that there is no load on the engine.
If the engine was still idled and a load was gently applied (engaging a PTO, for example), the voltage required to arc the plug would increase dramatically. Due to this, the engine may run fine under a given operating state and then buck, pop, and misfire when faced with different operating conditions.
In like fashion, during throttle movement (transient operation), ignition demand spikes just as the throttle plate is evoked. To diagnose a performance issue expediently, you need to remember that electrical demand is linked to load.
What is a misfire?
An engine is considered to misfire when there is not enough electrical energy going to the spark plug to keep it arcing or there is a path- way in the engine for the energy to take that bypasses the plug’s electrodes. Electricity is lazy. It will always take the path of least resistance. If it is easier to go to ground through the insulation on the ignition wire instead of arcing the gap of the plug under high cylinder pressure, it will. When this occurs, that cylinder will not be contributing full power to the crankshaft. The engine will be weak, the unburned fuel will foul the spark plug, and the exhaust will pop. If the mixture is excessively lean, the engine will misfire since the fuel molecules in the intake air are too far apart. When the mixture is lean, the flame cannot expand across the cylinder bore since it requires both fuel and oxygen to accomplish this.
In most instances, the flame does begin since the air-fuel mixture by design is concentrated around the spark plug electrode. Once the flame expands away from that region, it dies out, and the cylinder is no longer contributing any power. The demand on the ignition will increase, and the energy will look for an easier path.
how a plug fires
A spark plug on most engines fires from the center electrode to the side electrode. That side electrode is grounded via the plug threads to the cylinder head. If the primary leads to the coil were accidently connected in reverse, the amount of energy the coil could produce would be greatly diminished. Most times, the engine would idle fine, but as soon as any load was applied, it would misfire.
The proper connection for a coil is that the voltage from the ignition switch goes to coil (+). Then the wire from either the breaker points or the ignition module goes to coil (-). Many modern gasoline engines do not employ a traditional distributor but, instead, have either a coil on each spark plug or a coil pack that fires two cylinders. When an engine has no distributor, it will use a sensor on the crankshaft to identify each firing event; the sensor also recognizes the number-one cylinder. The control unit is programmed with the correct firing order. This style ignition system allows for longer arcing time of the spark plug when measured in degrees of crankshaft rotation past top dead center. When a coil pack is used, it fires differently than a conventional ignition in terms of polarity. It will fire one cylinder from the spark plug center electrode to the side electrode (normal path) and the companion cylinder from the side electrode to the center electrode.
Commonly overlooked ignition problems
The following issues can present in different ways or operating states, but the most prevalent are listed below.
- Cracked spark plug porcelain. This causes misfiring or bucking under load. The engine may run fine at idle and light load.
- Fouled spark plug. This situation causes misfire (no defined operating state) and hard starting.
- Weak ignition wire insulation. In this case, the engine runs fine until engine temperatures rise and there is an increase in workload. The wire insulation then breaks down, and the engine bucks or misfires. However, the engine runs smoothly in cooler temperatures or under less workload. When the insulation fails due to heat, it’s because the molecules have moved farther apart, making an alternative path for the electricity to take if that path is easier than jumping the gap of the spark plug.
- Weak ignition coil. The engine idles fine but breaks up under load. The issue here is a coil that has worn out from the constant charging and discharging cycles. The proper method to check an ignition coil is using an oscilloscope, but an ohm meter works well, too. With the meter, you can check for an internal open in the primary windings or excessive resistance. You will need to check between the primary terminals with the meter leads.
If you have a shop manual for the engine, it usually provides a primary resistance check specification. This is important on pull-type start engines or other small-engine applications.
Keep in mind that it’s possible for the coil secondary to burn open internally. When this happens, the engine will run under light load but will misfire under other conditions. The spark is actually bridging the internal break in the secondary windings, leaving less energy for the spark plug. Older-style ignition coils were oil-filled as a coolant. Later designs were made with epoxy (identified as e-core coils). If you remove the secondary lead from an oil-filled coil and it is wet, the coil needs to be replaced.
Check a no-start
The proper method to check for spark is with a spark tester. It resembles a spark plug but has a sunken center electrode that simulates a load on the coil.
Holding the plug wire and seeing if a spark jumps to ground is inconclusive. The spark is arcing in atmospheric pressure. This is substantially less pressure than the energy required to arc the plug electrode under load.
For an engine to start and run properly, the energy must arc the plug and keep that arc going until all of the combustible mixture is consumed. Anything less and the engine will be weak, run rough, stall, and misfire.
Weak ignition is often confused with a fuel problem since the two systems are codependent. Many times, especially with small engines, the carburetor and the fuel are unjustly demonized for a problem when the ignition system is the real culprit. Think spark first then fuel, not the other way around.