I Have A 1998 Nissan Maxima What Does A Knock Sensor Do

There are 4 Answers

1. Jensen says:

   It prevents the engine from pinging due to low octane gasoline or carbon deposits, etc. Severe and/or extended pinging can damage your engine.
2. brascappe says:

   A sensor that signals the engine-control computer when detonation is detected, momentarily retarding ignition timing until detonation ceases.
3. Cecil says:

   “It allows the engine to run with the ignition timing as far advanced as possible. The computer will continue to advance the timing until the knock sensor detects pinging. At that point the computer retards the ignition timing just enough for the pinging to stop.

   A knock sensor assures that you’re getting as much power and fuel economy as is possible from your engine.”
4. Country Boy says:

   Vicky, Modern cars which use unleaded fuel have knock sensors. They are an electrical devise coupled with the electronic ignition system to retard ignition timing if and when the sensor reads combustion chamber spark knock or pre-ignition.

   If you’re not familiar with new electronic ignitions I’ll run this by you to see if it makes sense. While the motor is idling the spark might fire @ 8 degrees of crankshaft rotation before the piston reaches top dead center in the compression stroke. As you travel down the road the spark could happen as much as 30 degrees of crankshaft rotation before the piston reaches top dead center. All of a sudden you encounter a hill or pass another car and the engine is under load. The low octane fuel of today will make the compressed air and fuel ignite in the combustion before the spark actually happens. This is called spark knock or pre-ignition. Picture this: In the far side of the combustion chamber a tiny piece of carbon is glowing red and causes the air fuel mixture to explode a millisecond before the spark plug fires. All of a sudden you have two burning shock-waves in the combustion chamber. When the two shock-waves meet you hear a sound of hollow glass milk bottles banging together.

   The *knock sensor pulls back or retards the ignition timing to prevent the shock-waves and pre-ignition. The higher the octane of the fuel the cooler and slower it burns which also prevents pre-ignition.

   I didn’t mean to be long winded but you asked a great question that deserved a truthful answer.