(Updated on January 4, 2021)
Picture this: you’re sitting at a stop light on a hot day, getting ready to merge onto the highway. As you pull out and step on the gas, you hear an awful noise that sounds like shaking a bunch of marbles in a can.
The car hesitates, stumbles, then proceeds to accelerate slower than normal. What just happened? You’ve probably just experienced engine knock.
What is Engine Knock?
Engine knock, pre-ignition, and pre-detonation are all names for explosions in the combustion chamber that happen when they’re not supposed to.
A combustion engine requires a carefully balanced mix of air and fuel. This air fuel mixture must be ignited at precisely the right moment. That moment is usually just before the cylinder reaches top dead center (TDC) on the compression stroke.
Virtually every engine knocks to some extent. There are countless variables in the environment that can’t always be controlled for, such as carbon buildup, fuel quality, and quick changes in the position of the throttle plate such as stabbing the gas.
Engine knock is usually only harmful when it happens at high loads, such as when you’re at full throttle. Cruising knock happens while driving at a constant speed. Cruising knock is very common and much less concerning.
An engine knock sensor is designed to detect these unplanned explosions and send a voltage signal to the ECU or powertrain control module. From there, the module will delay the ignition’s timing and prevent engine detonation.
Retarding (delaying) the ignition timing means the spark plugs fire a bit later than they used to. The piston will be a little higher in the cylinder and the combustion event will not make as much power on the combustion stroke.
Lowering the power output by retarding the ignition timing will decrease cylinder pressures, making the combustion event safer for the engine. The main downside is that your car will be slower until the ignition timing returns to normal.
This will help prevent damage to the engine as it goes through the pre-ignition process. If your knock sensor were to malfunction or go bad in your vehicle, there would be some noticeable symptoms that would present themselves.
Low Speed Pre-Ignition
On newer vehicles that have both direct injection and a turbocharger, there is an increased probability to experience pre-ignition when the driver requests a lot of torque at a low low engine speed.
Automotive engineers aren’t quite sure why this phenomenon occurs, but it is thought to be caused by the higher cylinder pressures typically seen on direct injected turbocharged motors with a relatively high compression ratio.
Supposedly there are engine oils that can mitigate this phenomenon, but nothing can prevent it entirely. If you drive a manual transmission vehicle that is both direct injected and turbocharged, consider downshifting before you floor it. Automatic transmissions will likely handle this for you.
What Causes Engine Knock?
Engine knock can be caused by a number of factors, especially when unfavorable conditions are combined.
1) High Intake Temperatures
Ever notice how your car is slower when it’s hot outside? Not only does heat make the intake air less dense, it will also increase the probability of knock.
On hot days, the temperature of the combustion gases will be higher. This increases the risk of combustion before the cylinder is ready to ignite the mixture.
To combat this, most engines will retard ignition timing in hot temperatures to make combustion safer.
2) Poor Quality Fuel
Ever put a lower octane fuel in your gas tank than what the manufacturer called for? An octane rating is the fuel’s resistance to detonation via compression. A higher octane rating has a higher resistance to compression ignition.
Using the wrong fuel will likely cause engine knock. Note that some vehicles are designed to run multiple octanes. In these situations, the engine will adjust accordingly.
3) Carbon Buildup
As a vehicle ages, carbon deposits build up on the valves, cylinder walls, and pistons. These carbon deposits can create hot spots that ignite fuel unevenly.
There are ways to mitigate or remove carbon buildup. Some examples are walnut blasting the valves, Seafoam treatment, or running a catch can. A catch can and an air oil separator (AOS) are two devices that clean the oily air mixture that is ventilated from the positive crankcase ventilation (PCV) system.
See Also: Best Fuel Injector Cleaners
4) Bad ECU Calibration
Sometimes an ECU is programmed to run the engine too lean or with very advanced ignition timing. A lean air fuel ratio maximizes fuel economy.
Advanced timing burns the mixture earlier in the combustion cycle, which maximizes power. Both of these situations can cause engine knock when taken to an extreme.
ECUs often run on the leaner side from the factory for emissions purposes. This is usually fine, but occasionally a manufacturer will take it too far.
When this happens, the manufacturer will issue a recall or technical service bulletin (TSB) to flash the ECU. The updated engine management software is designed to give the engine a safer calibration.
Top 5 Bad Knock Sensor Symptoms
Here are some of the most common signs of a faulty knock sensor that you should look out for.
1) Engine Warning Light
One of the first symptoms you might notice is the engine warning light illuminating on your dashboard. You should take this early warning sign seriously and have your vehicle inspected before the problem gets worse.
Of course, there are many reasons why an engine warning light could turn on and a bad knock sensor is one of them. Regardless of the reason, you shouldn’t take the chance of ignoring it for too long or it could have devastating effects on your engine.
2) Loud Sounds
When the knock sensor starts to malfunction, you will hear loud noises coming from the engine that almost resembles thumping sounds. The longer you go without fixing this problem, the louder those sounds are going to get.
The reason this noise occurs is due to the ignition of the air and fuel mixture inside of the cylinder. Normally, the mixture would reach the combustion point instead. Therefore, sounds like this should motivate you to take your vehicle to the mechanic promptly.
3) Bad Fuel Mileage
If you notice that you’re getting fewer miles per gallon than you normally do, then a bad knock sensor could be contributing to that. Again, there are many reasons for why you might get bad fuel mileage.
But if you notice any of these other symptoms in conjunction with bad fuel mileage, then that’s even more reason to believe it is because of a bad knock sensor.
4) Poor Acceleration
When you step on the gas pedal to accelerate your vehicle and it doesn’t accelerate fast, then a bad knock sensor is probably preventing the acceleration from being effective. You can be sure of this if you already have the previous three symptoms occurring.
5) Poor Engine Performance
The worst symptoms from a bad knock sensor will occur when internal engine components become damaged. If you’ve let this problem escalate without replacing a faulty knock sensor, then your vehicle will begin to increasingly drag and jerk around.
There may even be a burning smell coming from the engine and getting into the cabin of your vehicle. Any further use of the vehicle in this condition could result in the entire engine being destroyed.
Then, you’re looking at many thousands of dollars to replace your entire engine. Don’t wait for that to happen. It is much cheaper to just replace the knock sensor.
Knock Sensor Replacement Cost
The good news is that it won’t cost you an arm and a leg to replace a bad knock sensor. If you have a standard economy vehicle, you can expect to pay anywhere from $120 to $500 for replacing your knock sensor.
The parts cost will be anywhere from $65 to $200, while the labor costs will be anywhere from $50 to $350.
In most cases, the parts costs and labor costs will be about the same. If you can find a good deal on the labor from an independent mechanic, then maybe you can lower your overall costs considerably.