A knock sensor does exactly what its name says: It senses knocks in a running engine. The knock sensor does not diagnose the problem; it simply tells the vehicle’s electronic systems monitoring equipment that a problem exists. Any time an engine is performing less than optimally, logic suggests its gas mileage will be negatively affected. In fact, the typical problem identified after a knock sensor alert is one that has a conspicuous effect on fuel consumption.
Knock sensors are most commonly fastened to an engine’s block, although numerous manufacturers also locate them one to each cylinder head. When the sensor senses an untoward knock in the engine, it sends a signal to the vehicle’s interpretative computer. In some vehicles this is called a power-train control module, or PCM, in others an electronic control module, or ECM. Typically the control module informs the driver of the problem by illuminating the “Check Engine” malfunction indicator light, and a technician then uses a scanner to isolate the issue.
Severe Engine Knock
Severe knock is audible to the human ear and invariably indicates an issue of some consequence that should be immediately addressed. Pistons, connecting rods and valves are all components that can cause severe knock when malfunctioning, as can carbon deposits in the cylinder.
Mild Engine Knock
Mild knock sounds like the pinging noise that can occur when a metal ball bearing is bounced off sheet metal. Mild knock may go unnoticed by the driver but can nonetheless be indicative of engine problems that will become progressively worse. When audible, the noise is usually most noticeable under heavy acceleration. The most common cause is ignition of the fuel-air mixture before the optimum instant, just before the piston reaches top dead center. The top of the piston is still traveling upward as the prematurely ignited flame front is traveling down, and the collision rattles the piston and makes the noise. Because this is symptomatic of poor fuel ignition, the fault goes hand in hand with decreasing gas mileage.
“Scanner” is industry-speak for what is properly referred to as an onboard diagnostic tool, an OBD. The name is somewhat deceiving, because the scanner is not an onboard component but a device for interpreting signals from components that are a part of the vehicle. Scanners display the information they glean as codes, and the codes relevant to knock sensors are P0325 through P0334. Both vehicle service manuals and the scanner’s operator manual should give fault-specific translations of the codes.
Knock Sensors and Gas Mileage
PCMs are equipped to deal with mild symptoms on their own; they can retard the timing to resolve the problem. They do this before the knocking or pinging noise becomes audible to the human ear, thereby saving the day without the driver's even knowing. A bad knock sensor cannot let the PCM know it needs to do this, and reduced fuel economy is a direct result. Further, some PCMs are equipped to notice when a knock sensor is not functioning, and the unit may automatically retard the ignition just in case there's a knock it is not being told about. This also negatively affects gas consumption. A bad knock sensor can indeed affect gas mileage.
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