I was told it has a bad knock sensor. What exactly is spark knock.

I had no idea computers did that. Is there anything else I should know about KNOCK?

Spark Knock is really a misnomer because it has nothing to do with spark; it’s actually divided into two categories: 1. Knock. 2. Pre-Ignition or Pre-Detonation. Knock is caused by fuel igniting outside the normal flame capsule. Pre-ignition is the creation of a flame- front prior to the time needed for proper combustion. Both cause combustion to occur earlier than needed and create a resistance to piston movement that’s detrimental to engine health. Knock is usually caused by carbon deposits inside a cylinder that retain heat and cause fuel to ignite at those points. Pre-ignition is usually caused by an engine running hotter than it should or a fuel grade that has an Octant rating that’s too low for the engine’s compression ratio, so fuel ignition occurs randomly throughout the combustion chamber and earlier than needed….. By design, an internal combustion cylinder fires just before the piston reaches top-dead-center; the spark (or Diesel compression/heat) ignites fuel and the heat generated causes air in the cylinder to begin to expand so that maximum pressure in the cylinder occurs just after the piston crosses over top-dead-center and the piston has begun downward movement for its power stroke. When fuel ignites before this optimal time, before the spark plug fires or before a Diesel reaches proper compression/heat, it causes air to expand earlier, so that instead of expansion occurring at the optimal time for performance, it causes expansion to resist the upward movement of the piston, and this causes a jolt inside the cylinder that produces shock waves. In minor cases, you hear a slight ping, but as it worsens, it grows louder and is heard as a loud knocking sound. A knock sensor detects the shock wave early and signals the car’s computer to retard spark timing to reduce power and lower heat to palliate knock. With this retarded spark timing, you lose some engine efficiency/power, but it will save the engine from related damage.

Over the past hundred years, internal combustion engines have evolved with regard to engine timing, knock, and pre-ignition….. Long ago, before computers ran engines, we could modify fuel types to achieve cost savings in higher performance engines; so you could effectively run lower grade gasoline to save money. An engine that requires high Octane fuel only requires it when the engine is pushed to its limits, so an engine still performs optimally in every-day driving using lower Octane gasoline. With computers & sensors, this choice became more difficult to do, or even impossible to do, while retaining proper engine performance during normal street driving. I’ll explain this in three parts….. 1. Before computers, cars had no automatic control over how an engine worked. You could run lower Octane fuel during normal street driving without any degradation in performance. The only time you needed high Octane was when you drove in excessive heat, up steep grades, or were racing; if the car required 91 Octane or higher, you could use 87 or lower during all other operation to lower fuel cost. I did this, and if I ever heard a ping, I’d just ease up on the accelerator to stop it…… 2. With computer knock sensors, an engine addresses potential knock problems before they happen. In conditions that can lead up to pre-ignition, the knock sensor signals the computer to retard ignition timing and you lose some efficiency. You can still use lower Octane fuel, but the car won’t perform quite as well and you’ll see slightly lower fuel mileage….. 3. In many modern cars, in addition to knock sensors, you have fuel sensors that identify the type of fuel in the car. When lower Octane fuel is used, the computer modifies performance (retards ignition timing) every time the engine is run, so there’s no advantage to gain by using lower Octane fuels.