Ignition Diagnostics & Repair

The purpose of the ignition system is to create a spark that will ignite the fuel-air mixture in the cylinder of an engine.  It must do this at exactly the right instant and do it at the rate of up to several thousand times per minute for each cylinder in the engine.  If the timing of that spark is off by a small fraction of a second, the engine will run poorly or not run at all.  The ignition system contains an ignition coil, a distributor, a distributor cap, a rotor, plug wires and spark plugs. Older ignition systems use what is called a "points-and-condenser system" in the distributor. Current ignition systems like the one your car most likely contains uses an ECU. An ECU or an engine control unit controls the spark and ignition timing.


Ignition Coil
An ignition coil that is in the first stages of failing will usually produce a steady but low voltage spark. This situation will introduce more unburned fuel into the exhaust system. This will be recognized by the computer in response to the free oxygen within the exhaust system signaled by the oxygen sensor. In response to this signal being out of parameters, the computer will lean the mixture to prevent damage to the converter and illuminate the check engine light.

Fuel economy will suffer and black smoke and fuel smell will be ejected from the exhaust. Generally the idle will suffer and the engine will become harder to start. This will also tend to foul the spark plugs, which will rapidly induce a misfire.

When the ignition coil has a complete failure, there will be no spark sent to the spark plugs; therefore, the engine will turn over, but not start.

Spark Plugs
Fouling is the number one reason why spark plugs have to be replaced. Plugs also have to be replaced for preventive maintenance because the electrodes wear as the plugs age. This increases the distance between the electrodes which in turn leads to a gradual increase in the firing voltage required to jump the gap.

A single fouled spark plug is bad news because it can kill up to 25% of a four power output. It is like riding a horse with a broken leg. A fouled plug will also cause a big increase in emissions.

Fouling can occur if fuel or oil deposits build upon the plug electrodes. The ceramic insulator around the center electrode prevents voltage from finding a shortcut to the steel plug shell and ground. Deposits here may form a conductive path for the voltage to bleed off to ground, preventing it from jumping the gap and making a spark. Deposits around the outer ground electrode or between the electrodes may form a barrier or bridge that also prevents a spark from occurring.

Fouling can be a problem if an engine uses oil. Worn valve guide seals and guides can allow oil to be sucked down the guides and into the combustion chamber. A heavy buildup of thick black deposits on the plug and intake valve would indicate such a problem. Worn or broken rings, or damage to the cylinder wall can also allow oil to enter the combustion chamber and form ash deposits on the plugs.

Extensive idling and/or short trip stop and go driving can also lead to a rapid buildup of normal fuel deposits. This occurs because the plugs never get hot enough to burn off the deposits, something which plugs are designed to do.

Powdery black deposits on the plugs can occur from "carbon fouling." The underlying cause here is a rich fuel mixture. On an older carbureted engine, the problem might be a broken or stuck choke. On a fuel injected engine, the problem might be a leaky injector, or a dead oxygen sensor or coolant sensor that prevents the engine control system from going into closed loop and leaning out the fuel mixture.

Ignition Control Module
An ignition module is a device in your car's ignition system that triggers the ignition coil, creating the spark that transfers to the spark plugs via the spark plug wire. Ignition modules lead a hard life and must survive millions of high-voltage shocks before giving out.

Perhaps the most telling symptom of ignition module failure is temperature-related stalling. Temperature failures most often happen when the module has an internal short or one of its relays stick closed. As current flows through the module, its circuits will overload and refuse to function until they're left to cool. Most often, the car will start and run fine, but will stop running anywhere from seconds to minutes later. The engine may not restart immediately but will do if left to cool for about 10 minutes.

At some point, you'll start to notice hesitation/misfire under acceleration. Consider this symptom separate from high-RPM misfire since it occurs no matter what the RPM and gets worse with throttle application. The vehicle may accelerate under normal conditions without too much drama, but will start to buck and surge when you really lay on the gas. Eventually, a bad module will result in a failure to start under any condition.

Engine Control Module/Power Control Module (ECM/PCM)
This is the "brains" of the operation. When this goes, everything will either stop working or will show erratic behaviors. Our ASE Certified technicians will be able to tell you if your ECM is bad.