We have yet to meet a vehicle owner who doesn’t get bothered by a loud knocking, rattling, or pinging noise coming from under the hood. Known as engine spark knock, this phenomenon describes a very erratic form of combustion. It occurs because of excessive compression and heat in the combustion chamber. The controlled explosion of the fuel-air mixture no longer occurs at the precise moment in the four-stroke cycle. Instead of the explosion occurring within the normal envelope of the combustion front, it occurs outside of it. It sends shock waves through the cylinder, creating the characteristic knocking or pinging noise. The shock waves also increase cylinder pressure. Over time, and if not remedied, this can lead to head gasket failure, piston ring damage, and other serious engine problems. So, what causes spark knock?
Faulty EGR Valve
In normal operation, the EGR (exhaust gas recirculation) valve reroutes some of the exhaust gases back into the combustion chambers. This reduces emissions while also lowering the temperature inside the combustion chambers. During acceleration or running under load, the EGR valve remains open. This allows the rerouting of the exhaust to dilute the fuel-air mixture. If there is something wrong with the EGR valve, then it will not be able to reroute exhaust gases. This can lead to an increase in the temperatures inside the combustion chamber, resulting in the characteristic engine knocking sound. Most of the time, excessive carbon deposits can cause the EGR to not open the way it should. It is best to have this checked and replaced if needed.
Bad Knock Sensor
The modern engine has a failsafe mechanism that allows it to adjust the ignition timing in case of the occurrence of detonation. The knock sensor communicates with the car’s computer, providing information about the occurrence of a spark knock. This leads the computer to retard the timing ignition so that it coincides with the combustion. Doing so can also reduce the power that the engine generates. This is protective, nonetheless, saving your engine from irreparable damage. Unfortunately, if the knock sensor is not functioning properly, you will hear that engine pinging noise whenever you accelerate or run under load. There are some tests you can run to determine if it is a faulty knock sensor that is causing the engine to knock or ping.
Excessive Carbon Buildup
An issue with older, high-mileage vehicles, pinging or knocking can occur because of excessive carbon buildup in certain parts of the engine. For example, carbon buildup on the top of each piston or in the combustion chambers themselves can increase the compression forces in the combustion chamber. This is also true for vehicles that never get to warm up in an efficient manner or those vehicles that people drive only for very short distances. There are commercially-available fuel additives to help clean the combustion chambers. There are also products you can use to clean the tops of pistons. More often than not, a trip to your trusted mechanic will do the trick of cleaning your car’s system.
Low Octane Gas
One potential cause of engine knocking or pinging is using fuel with a lower octane rating than what the car manufacturer recommends. Many of these low octane fuel come with low quality characteristics. Aside from performance issues, low octane fuel can increase cylinder pressures and combustion chamber temperatures. These will lead to the pinging or knocking noise that we now associate with erratic combustion. The good news with this is that it is very easy to fix. All you need is to refuel with premium-quality, high-octane gas to get rid of the annoying engine noise.
Increased Pressure from the Exhaust System
It makes perfect sense that increased pressure within the exhaust system can lead to higher combustion pressures and temperatures. The exhaust gases that come out of the tailpipe are the result of the combustion process. If this “exit” gets plugged or blocked, then the combustion gases cannot get out. They also cannot move back through the intake manifold. Hence, pressure buildup in the exhaust system can also lead to engine pinging or knocking. For example, a clogged muffler, catalytic converter, or exhaust pipe can cause pressure to build up within the combustion chamber. Having these components inspected and evaluated should help you address the issue of engine detonation.
Too High Compression Ratio
This is often the case in rebuilt engines. It is also possible that the cylinders are oversized, which can increase the static compression ratio of the engine. In some cases, resurfacing of the cylinder head can lead to a reduction in the overall volume of the combustion chamber. In a way, this also increases static compression ratio. While higher compression ratios will result in greater engine power, it can also lead to engine knocking or pinging. This is more pronounced if the car uses the same low octane fuel it has always been using. Turbocharged or supercharged engines are at a higher risk of developing pinging or knocking of the engine. This is because of increased compression secondary to the forced-air induction system in place.
Lean Fuel Mixture
Each combustion chamber requires the correct mixture of fuel and air in the right amounts. Unfortunately, if there is more air than fuel, this can increase combustion chamber temperatures. In normal fuel-air mixture, the combustion chambers are able to maintain a near-optimal temperatures because there is adequate fuel to evaporate. In a lean fuel scenario, there is less liquid (fuel) to evaporate. This can lead to the unusual buildup of heat, causing the combustion chamber to heat up. As mentioned, engine pinging or knocking is due to excessive heat and compression within the combustion chambers. Vacuum leaks, dirty fuel injectors, low pressure fuel, or faulty MAF sensor can contribute to the development of a lean fuel mixture. Addressing the problem means looking into these components.
There are other potential causes of spark knock. However, the ones we mentioned herein are the most common ones. If you’re not sure what’s causing the problem in your car, a mechanic can help you isolate the issue and find a more appropriate remedy.