The terms valve clatter, pinging, pre-ignition and detonation describe the strange rattle or knock coming from an engine. This occurs on acceleration with the engine under a load. Each term describes a slightly different condition, but the result is usually the same with each, engine damage.
What is pre-ignition or valve clatter?
With pre-ignition, the fuel/air mixture in the cylinder spontaneously explodes, slightly before the spark plug ignites it. In proper operation, the piston moves down the cylinder and draws the fuel/air mixture in. The following cycle compresses the mixture, making it explosive. As the piston moves higher into the bore, we more tightly compress the fuel and air. Just before the piston reaches the top, a spark ignites the mixture, which expands violently. This expansion and rise in pressure pushes the piston down the bore and drives the engine.
The fuel/air mixture takes a split second to burn fully, so ignition should occur a few degrees before the piston rises fully to the top of the cylinder. We call this ignition timing and the engine computer precisely controls the instant at which the spark occurs. With pre-ignition, spontaneous combustion of the fuel/air mixture, before the spark ignites it, causes a more violent explosion. This is where the term pre-ignition originates. Ignition of the fuel and air occurs before the ignition system lights it.
When the fuel/air mixture explodes prematurely, the force created tries to drive the piston down the cylinder, while the crankshaft is forcing it up. A characteristic "pinging" is often heard, due to the forces in the engine. A pinging noise on acceleration is a symptom of pre-ignition. Left untreated pre-ignition can severely damage an engine. The excessive pressure developed can damage pistons and cause head gasket failure. Many things cause pre-ignition and correction always depends on finding and correcting the cause(s).
Several other terms, such as a valve clatter, spark-knock and rattle, relate more to the sound produced but is roughly the same condition.
What is detonation?
People often use the term detonation to describe pre-ignition, though the condition is different. With normal combustion, the ignition spark lights the fuel/air mixture. The flame travels across the combustion chamber, compressing and heating the unburned gasses ahead of it. This burning continues until we consume the mixture and this produces a smooth rise in pressure. Detonation is an explosion at the end gasses, after combustion has begun. With detonation, the spark ignites the fuel/air mixture, as it should. Detonation occurs after ignition and causes an explosion of the mixture, rather than a smooth burn. Detonation is different, but creates the same problems as pre-ignition. The symptom is also similar, a pinging noise on acceleration.
What causes pre-ignition and detonation?
To enhance performance, engine designers raise the compression ratio of the engine. Compression ratio is the volume of the cylinder, compared to that of the combustion chamber. For instance, if the cylinder holds 80 cubic inches and the chamber size is ten cubic inches the compression ratio is eight to one. Squeezing the fuel and air more tightly produces more power but also increases the tendency for pre-ignition and detonation. This is why high performance engines require higher (89 to 93) octane fuel. The octane in fuel raises the point of spontaneous combustion, and helps to control pre-ignition.
Vehicle manufacturers consider many factors when specifying an octane requirement. For instance, engine compression, valve and ignition timing are all factors. The proper octane helps to prevent engine damage, by slowing the burn rate of the fuel. It also allows further advance of the timing which increases performance. Pinging can also occur, even with the correct octane. Vehicle manufacturers provide several systems that help control pre-ignition and detonation.
Exhaust-gas-recycle (EGR) is one system that helps to control pre-ignition and reduce emissions. EGR operates when conditions exist that cause pinging or valve clatter. A valve opens and circulates exhaust gas into the intake manifold to cool the combustion process. Exhaust gas is very low in oxygen. Adding exhaust gas to the fuel/air mixture, helps lower cylinder temperature. A lower temperature can help control pre-ignition and detonation. It does this, by raising the spontaneous combustion point of the fuel/air mixture in the cylinder. EGR also reduces oxides of nitrogen produced by pre-ignition. This makes it important as an emissions control device as well.
The knock sensor
A second system for reducing pre-ignition is the knock sensor(s). The knock sensor informs the computer that when knocking, such as pinging or valve clatter occurs. The computer modifies ignition and in some cases valve timing, to lower cylinder temperature. This helps the fuel/air mixture to ignite later, compared with piston position.
When either the EGR or knock sensor malfunctions, damaging pinging and valve clatter [pre-ignition] can occur. Vehicles built after 1996 have an onboard system to detect such problems. Normally a check-engine light will alert the driver of failure in these systems. Even with all systems working and the proper fuel, vehicles can suffer from pinging, pre-ignition [valve clatter] if outside issues or wrong.
Conditions that also contribute to pinging and pre-ignition
Wrong or gimmick spark plugs
Installing the wrong spark plug of an improper heat range or design can cause problems. The tip of a spark plug can become hot enough to ignite the fuel/air mixture prematurely. Aftermarket spark plugs may not be the same as the original. Gimmick spark plugs, with multiple electrodes do nothing to enhance performance or mileage and may not meet OEM standards. Improperly tightening or coating the threads a spark plug with never-seize can modify the heat characteristics and contribute to pre-ignition and ping.
Improperly installed spark plug wires
On engines with spark plug wires, improper routing of the wires can have a similar effect. The high voltage in one wire may induce a spark in another. To reduce this, manufacturers have a very specific routing pattern for spark plug wires. They often cross the wires, on consecutively firing cylinders, at a 90-degree angle. This helps prevent a spark being induced from one wire to another. Always check for wires improperly routed or worse, bundled together.
Vacuum leaks and bad sensors
A vacuum leak, improperly reading mass air flow (MAF) sensor, wrong or malfunctioning oxygen (O2) sensors can also cause cylinder temperature to rise. The MAF meter tells the engine computer how much air enters the engine. An improper reading from the MAF meter or the O2 sensors can cause the engine to run lean, meaning not enough fuel for the air. Lean mixtures can increase combustion chamber temperature and cause damage. A bad fuel pressure sensor may lower fuel pressure and create a similar situation. Low fuel pressure may cause a lean fuel air mixture.
An overheating engine is far more prone to pinging than an engine operating at normal temperature. Air trapped in the cooling system, from improper bleeding can cause hot spots in the engine. With the chamber temperature too high, the spark still ignites the mixture, but a secondary explosion may result before the fuel burns completely. This detonation of the unburned fuel creates a bad knock. Overheating, combined with detonation, can destroy an engine very quickly and often causes head gasket failure. Stuck, inoperative and thermostats left out of an engine can also cause hot spots and result in detonation.
An excessive carbon buildup can also cause pre-ignition or detonation. This can occur because of poor quality fuel or using the improper viscosity engine oil. Carbon in the combustion chamber or on valves can glow red hot and set off the fuel air mixture, before it is time. A carbon buildup may also plug EGR passages, causing the system to fail.
Engine software problems
Problems with the software that controls ignition timing may also cause problems. These issues may not show up for many miles and the vehicle may be out of the warranty. A careful search of technical service bulletins (TSB) for the vehicle may reveal an update to the engine computer. On newer vehicles merely updating the software, or "flashing" the computer, often accomplishes this. Some older models require replacement of the programmable-read-only-memory (PROM) chip or even replacement of the computer.
Allowing pinging, pre-ignition or detonation to continue will cause severe engine problems. The solution always involves a proper diagnosis of the cause(s).