* Faulty IAC Valve: The most likely culprit. A dirty or malfunctioning IAC valve can cause the engine to idle too high. This wouldn't directly cause piston damage, but a consistently high idle could lead to overheating if other issues are present.
* Vacuum Leaks: A leak in the vacuum system can affect the idle speed. Again, not a direct cause of piston damage but an indirect contributor to potential problems.
* Throttle Position Sensor (TPS) Issues: A faulty TPS can send incorrect signals to the computer, leading to an incorrect idle speed. This is another indirect issue.
* Mass Airflow Sensor (MAF) Problems: A dirty or faulty MAF sensor can cause the engine to run rich or lean, which can eventually damage the engine, including the pistons. This is a more direct contributor to potential piston damage.
* Computer Problems: A malfunctioning PCM (Powertrain Control Module) can also cause a variety of problems, including incorrect idle speed and potential misfires that damage pistons.
How these lead to piston problems:
The fast idle itself won't destroy pistons. However, the underlying problems causing the fast idle – particularly a faulty MAF sensor leading to a lean or rich condition, vacuum leaks leading to inconsistent air-fuel mixture, or a computer problem causing misfires – can cause significant engine damage.
* Lean Condition (too much air, not enough fuel): This causes the engine to run hot, potentially leading to piston damage due to overheating, detonation (pre-ignition), or even melting.
* Rich Condition (too much fuel, not enough air): This can lead to excessive carbon buildup, fouling spark plugs, and potentially washing down cylinder walls, reducing lubrication and leading to piston scoring or damage.
* Misfires: Unburnt fuel in the cylinder can create excessive pressure, potentially damaging pistons, rings, or other components.
In short: the fast idle was a warning sign. The actual piston damage likely stems from a more serious underlying problem related to the fuel/air mixture, ignition system, or engine management system that was not addressed. You'll need a mechanic to diagnose the root cause of the piston damage to determine exactly what failed and how.
