Valve clatter, pinging, pre-ignition and detonation. These are all terms for the strange rattle or knock an engine can make when accelerating. Regardless of the name used, the cause is the same. The fuel/air mixture in the cylinder is exploding slightly before the spark plug would normally ignite it. This is where the term pre-ignition originates. The other terms such as valve clatter, spark knock and pinging relate more to the sound produced.
When the fuel/air mixture explodes prematurely, the force tries to drive the piston down the cylinder, while the crankshaft is forcing it up. Left untreated pre-ignition can severely damage an engine. There are a large number of things that can cause pre-ignition and correction always depends on finding and correcting the cause(s).
Octane in fuel is designed to raise the point of spontaneous combustion and help control pre-ignition. This is why high performance engines require higher (89 to 93) octane fuel. The vehicle manufacturer takes many factors into consideration when specifying an octane rating. For instance, engine compression and ignition timing. The proper octane must always be used to prevent engine damage.
Pre-ignition can also occur, even with the correct octane. Vehicle manufacturers provide several systems that help control pre-ignition. Exhaust gas recycle (EGR) is one such system. EGR senses 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. Since exhaust gas is very low in oxygen, it lowers cylinder temperatures and can help control pre-ignition. It does this by raising the spontaneous combustion point of the fuel/air mixture in the cylinder.
A second system is the knock sensor(s). The knock sensor(s) detects the knocking or valve clatter and retards ignition timing. This helps the fuel/air mixture to ignite later, in relation to piston position. When either the EGR or knock sensor malfunctions, damaging valve clatter [pre-ignition] can occur. Vehicle built after 1996 have on-board system to detect problems and normally a check-engine light will accompany failure in these systems.
Even with these system working and the proper fuel, vehicles can suffer from pre-ignition [valve clatter.] Some of the other things that cause or contribute to valve clatter include:
The wrong spark plugs or improperly routed spark plug wires. Manufacturers have very specific routing pattern for spark plug wires. Consecutively firing cylinders are often routed at 90' angle to each other. This helps prevent a spark being induced from one wire to another. Always check for wires improperly routed or worse, bundled together.
A vacuum leak, improperly reading mass air flow (MAF) meter, wrong or malfunctioning oxygen (O2) sensors. The MAF meter tells the engine how much air there should be to mix with fuel. An improper reading from the MAF meter or the O2 sensors can cause the engine to run lean, not enough fuel for the air. Lean mixtures can increase combustion chamber temperature and increase spark knock.
Excessive carbon buildup can also cause pre-ignition. Carbon in the chamber or on valves can glow red hot and set off the fuel air mixture, before it is time. Finally, there can also be problems in the software that controls ignition timing. Sometimes the problem may not show up for many miles and the vehicle may be out of warranty. Often a careful search of technical service bulletins (TSB) will reveal an update to the engine computer. On newer vehicles this is often accomplished by merely updating the software or "flashing" the computer. Some older models require replacement of the programable-read-only-memory (PROM) chip or even replacement of the computer.
In every case, allowing pre-ignition to continue will cause engine problems. Also in every case, the solution involves a proper diagnosis of the cause(s).
