Have you ever driven a vehicle that pings or knocks going up hill at 75% or full throttle, or a vehicle that continues to run after you turn off the ignition? If you've ever spent much time working on a vehicle, then you probably have heard the terms "engine knock", "knocking", or "pinging" in reference to a vehicle's engine. You may also know they are just different words for engine detonation, and based on the sound the engine makes when detonation occurs. However, if you've been lucky enough to avoid owning a vehicle with combustion issues, you've may have heard engine detonation, misfire, pre-ignition, or dieseling used interchangeably depending on where you get your automotive answers.
During a conversation with a knowledgeable colleague about his recent classic car find, I took the opportunity to ask how the vehicle was running. Turns out his new rescue project was running fine, but was dieseling when he turned the engine off. What followed was a detailed explanation about the differences between pre-ignition, misfires, and engine detonation, the potential causes, like low quality fuel, lean fuel mixtures, or a bad EGR valve, and possible fixes for the issue.Though all of these combustion issues could result in catastrophic damage to your engine, they are different from engine knock. By clarifying engine knock, and what it may indicate about your engine, hopefully it will help you understand engine detonation, and guide you toward appropriate steps to extend the life of your engine. Let's begin.
What is Engine Knock?
Engine knock or engine detonation occurs before the spark plug ignites the air-fuel mixture in the engine's cylinder. When the pressure or temperature in the cylinder's combustion chamber is too high, pockets of fuel and air may combust before the spark plug fires. The result is multiple points of combustion of the air-fuel mixture before the proper point in the combustion cycle, and when these forces collide it produces a high-frequency pressure wave in the cylinder. This is what causes the metallic pinging or knocking sound in the engine.
Pre-ignition or dieseling refers to combustion occurring without a spark from the spark plug, and may cause your engine to continue to run for a few seconds after you shut off the ignition. This is commonly caused by carbon deposits creating hot spots in the cylinder, or a leaking fuel injector. It's possible for pre-ignition or dieseling to be followed by, or contribute to, engine knock.
When an engine misfires it's because there's an incorrectly timed spark or lack of spark from the spark plug in the combustion cycle, and could indicate a number of issues such as a bad spark plug, an overly lean or rich fuel mixture, slipping timing belt, or an air or fuel leak from the engine.
Is Engine Knock bad for your vehicle?
Yes, engine knock creates extreme pressure in the cylinder that can damage internal engine components like the pistons, bearings, connecting rods, or the cylinder wall. It also raises the temperature in the cylinder dramatically, which could lead to more instances of pre-ignition. Most modern vehicles come equipped with knock sensors, which signal the vehicle's computer to adjust the ignition timing and fuel mixture when knock is detected. Since knock sensors generally protect the engine in most cases, a failing knock sensor means your engine could be suffering from pre-ignition without you (or the computer) noticing. So, by the time you actually hear engine knocking or pinging, it may have a failing knock sensor, or the engine knock may be severe enough that your vehicle's computer can't correct it on its own. This usually means there is another problem in the cylinder that needs to be repaired as soon as possible to prevent engine damage.
What causes Engine Knock?
Low Octane Fuel - This is important to mention because you may have heard of, or been advised to use, a higher octane fuel to prevent engine detonation. This is because the higher the octane, the more heat is required for combustion. However, this typically only applies to high-performance engines, or modified engine builds with high compression. Most vehicles are designed to run on regular lower-octane unleaded fuel, so unless your vehicle requires a higher octane rating than you are using, a higher octane fuel may reduce engine knock, but will not fix the source of the issue.
Lean Air-Fuel Mixture - A lean mixture is basically an air-heavy mixture, so either you are getting too much air or too little fuel in your combustion chamber. This can happen in a few different ways. If you are pulling too much air into your engine, you may have a vacuum leak or a failing sensor that helps regulate the air mixture, such as an oxygen or mass air flow (MAF) sensor. If you are not getting enough fuel, you may have a clogged fuel injector or a weak or failing fuel pump. Fuel has a cooling effect on combustion temperatures, so when too little fuel is being delivered to the engine, combustion temperatures can go up.
Spark Plugs - Spark plugs have different heat ranges, and if your vehicle is using spark plugs hotter than those recommended by the manufacturer, the electrode on the spark plug may stay hot for longer periods of time, raising the temperature inside the combustion chamber. Worn or improperly-gapped spark plugs can also stay hot longer and cause engine knock.
Over-advanced Ignition Timing - This really only applies to older vehicles with distributors or customized engine builds where timing can be adjusted manually, but it's possible that your spark plug is igniting when the piston is too close to the spark plug. This can cause too much pressure in the combustion chamber and lead to detonation. On newer vehicles, this is normally corrected with the help of the knock sensor. However, if your knock sensor is failing, it may not be able to correct the ignition timing.
Bad Knock Sensor - If your knock sensor is failing then your vehicle's computer will not be able to adjust the ignition timing appropriately to prevent knock. This usually results in a loss of power, especially while accelerating or travelling at higher speeds. You may also notice a difference in fuel efficiency or a Check Engine light. If your Check Engine light is on, O'Reilly Auto Parts can scan your stored trouble codes for free, so stop by any local store and see if you need a knock sensor replacement.
Carbon Buildup - The more carbon deposits you have, the less space available in the combustion chamber, which increases the compression ratio and leads to increased pressure, heat, and hot spots during combustion. It's also possible for carbon embers to break off in the cylinder, igniting the air-fuel mixture.
Bad EGR Valve or Component - The exhaust gas recirculation system reroutes some of the exhaust gases into the combustion chamber to reduce emissions, but it also reduces combustion temperature. If your EGR system is failing, or if the valve is not opening or has a leak, the exhaust gases will not make it to the combustion chamber, raising temperatures, and causing engine detonation.
Engine Overheating - If your vehicle is low on coolant or has a failing water pump, it can quickly cause your engine temperatures to increase and the engine may overheat. The increased engine temperatures may also cause the air-fuel mixture to detonate.
We hope this has helped you with understanding engine detonation, and at least provided you with some additional knowledge about engine knock that may help you diagnose your own engine problems and make your repairs. If you decide the repairs to your car are more than you're comfortable tackling, we'd be happy to refer you to a local shop that can help fully diagnose and repair your engine detonation issues.
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