What is engine knock?
Let's start at the beginning or the rest of this article may seem a bit cryptic. What is engine knock? The first thing you should know about it is that it is not always audible. You may hear severe knock from inside the cabin but by the time you do, it may be too late already!
Check here first if you need some background
Engine knock is an abnormal combustion event resulting in high frequency vibrations which propagate in all directions from inside 1 or more of your cylinders. Normally, just before the "power stroke" in a 4-cycle engine, the spark plug fires and creates a tiny but rapidly expanding fireball that eventually hits the piston and drives it back down. The pressure in the cylinder can be graphed and it would look smooth and predictable under normal circumstances. When the engine knocks, the pressure rises higher than it should due to a number of possible reasons and causes the temperature to spike in accordance with the laws of thermodynamics. When this happens, random spots within the air-gas mixture (the part that hasn't been burned yet) begin to spontaneously ignite and the shock fronts collide with one another creating ripples or vibrations similar to what you'd see if you threw a handful of rocks in a still pond. These vibrations are deadly to the moving parts of an engine and, left unchecked, can destroy rods, crack pistons or worse. Cars are designed to prevent such catastrophes by employing a small piezoelectric microphone (a knock sensor) which "listens" to the vibrational pattern of an engine and when it hears the telltale sound of a knock event, it signals the computer to take action. The most typical countermeasure is to pull spark timing but there are other ways as well. What is engine knock? It's bad news! But it doesn't have to be. Let's now learn how to diagnose an engine knock event and what could be causing it.
This is what knock sounds like...
What causes engine knocking?
There are many specific causes of knock but they all lead down the same path. Above I said that there is a dangerous temperature spike in the air-gas mixture that causes knock. In fact for any given fuel there is a certain temperature at which the fuel will ignite all by itself with no source of ignition. This is known as the "auto-ignition" temperature. When the temperature in the cylinder rises above the auto-ignition temperature for any number of reasons, the fuel will suddenly become unstable and want to blow up like a bomb because every location in the fuel is now equally prone to begin combusting. When there's already a flame front expanding from the spark plug and this flame front gets interrupted by many other flame fronts coming from the opposite direction, you get mini explosions hitting each other, creating that high frequency noise that's so dangerous to the engine. Ultimately we need to ask ourselves, what's causing the temperature to go so high?
1) Low Octane
The first thing you should ask yourself is whether you're using the right fuel in the first place. Gasoline has what's known as an octane value. This metric is tied directly to the auto-ignition temperature of the fuel. The higher the octane, the higher the auto-ignition temperature and the less likely you will ever reach it and produce knock. Some engines, by design, will knock on low octane fuel because they are supposed to compress the fuel a bit higher than normal for better efficiency or power. Filling up with higher octane fuel (like 89, 91, 94, etc.) will raise the fuel's inherent resistance to auto-ignite. There doesn't have to be a problem with heat in order to see knock on a low octane fuel but hot outdoor temps or engine temps will make it even worse so check your owner's manual to see what type of octane you are supposed to be buying. If you're not buying the right type, get to a gas station and see if the right gas fixes the problem.
2) Too Much Timing
As the pistons move up towards top dead center and just before the power stroke, the spark plug will fire. It goes off early because it anticipates the speed of the flame reaching the piston by the time the piston is descending. This is factory calibrated but over time, the condition of your engine may degrade and for various reasons the factory timing will need to be "retarded" or reduced so that the spark plug goes off later. Here's why:
Imagine running point blank into a brick wall. Aside from being a stupid thing to do it'll hurt quite a bit. Now imagine doing the same thing again but this time, the wall is on wheels and moving towards you at high speed. That's going to hurt A TON. Now suppose you're running at the wall again but just as you get close to it, it stops and starts moving backwards. When you do hit it, the blow will be greatly reduced because it's moving away from you, reducing the relative impact speed. Now, imagine the flame front coming from a spark plug as you. Imagine the brick wall as the piston. If the piston is travelling upwards when the flame front hits it, the resulting pressure and temperature will be very high. Back up a few milliseconds and you will see that even before impact, the air-fuel mixture is compressing very rapidly because there's a hot flame front racing towards the rest of the unburnt fuel. What if the spark plug went off later though? If it went off later, the flame front would be smaller and further away from the piston so the pressure wouldn't rise as quickly and when the flame finally does reach the piston, it may already be on its way downward, just like the brick wall that reverses direction. This prevents the pressure and temperature from causing auto-ignition. That's why retarding the spark advance can prevent knock by staggering the piston synchronization with the approaching flame front and causing impact well into the down stroke.
Changing the timing in a modern vehicle requires a personal OBDII tuner (not a scanner) or a mechanic with a Tech II. If you want to buy one I highly recommend HPTuners, as it gives you complete control over every parameter in the car that you would ever want to change. Anyway, by retarding the spark slightly you can prevent knock and in fact the computer does this automatically (albeit after already detecting knock) as a means of saving your engine from knock. That's why you should rarely ever "hear" a knocking engine. Unless the knock is really bad, the computer is already on top of it.
3) Environmental Conditions
The environment in which you drive your car can induce knock. It makes a big difference whether you're driving in Arizona or Vancouver. This is because the properties of the air set the initial conditions that are then amplified greatly by being compressed inside the cylinders. If the air is hotter to begin with, it'll be at least that much hotter after compression. The difference in 2 different climates or between summer and winter can easily push the air-fuel chage above the auto-ignition point for that octane level, especially in an older engine. Furthermore, dry air has a lower heat capacity than moist air does, so the temperature will rise quicker in response to being compressed, since there's nothing (like water) to soak up the heat. So your Arizona desert car is going to knock a lot more for no other reason than because it's sucking up hotter, dryer air.
Also keep in mind that the fuel resevoir (and the fuel inside) as well as the engine coolant (and the engine internals) will be hotter because the car's cooling systems don't work as well when the heat sink (the outside) isn't as "cold" compared to the gas or coolant or what have you. Hot gasoline and hot cylinder walls only add to the problem of pumping hot air into the engine.
Adding a larger radiator, a cold air intake or a colder thermostat (with reprogrammed fan settings) can all lead to lower combustion temperatures, which will help reduce knock.
4) Dirty Engine Deposits
Over time, your engine will get fouled up. Varnish and soot will accumulate around seals and on bearing surfaces, spark plugs will get dirty and sensors will fall out of calibration. Aside from throwing off your factory tune in general, deposits within the cylinders themselves can be responsible for a knocking engine. Think of a cylinder as a giant heat sink. It is metal after all so heat dissipates fairly evenly through the walls. However when you start to get localized deposits of carbon and other filth, you are basically interupting the natural flow of heat through the cylinder walls. The deposits themselves can come from low grade gasoline, poor air filtration, incorrect air-fuel ratio, dirty oil from the sump passing by the piston rings, things like that. Unlike metal surfaces, small hydrocarbon deposits tend to hold heat rather than dissipate it and they will gradually heat up until they are glowing red hot and are unable to cool down from one 4-stroke cycle to the next. When this happens, these hot spots can become initiation sites for knock by locally heating the air-fuel mix and bringing it to its auto-ignition point.
If this is the problem, it may take a while to fix if you aren't inclined to tear down the engine and clean it. You can switch to top tier gasoline and use injector cleaners, upper cylinder cleaners and other gas treatments to try to dissolve the deposits in your engine. Alternatively... and I'm not recommending this even though I've done it myself... you can buy a few 1L tins of acetone from the hardware store and add a tin to each tank of gas. Acetone is a powerful solvent which is harmless to metal components but can attack some polymers over time so use at your own risk! My car handled it just fine and the acetone really does a great job of cleaning the entire fuel line and the engine itself. It's also an octane booster and increases torque due to its volitility and oxygen content.
5) A Rich Air-Fuel Ratio
If you have a vacuum leak, malfunctioning MAF, MAP, IAT or O2 sensor you could be running rich, meaning there is more fuel than you need for a chemically stoichiometric mixture. You'll hear people all over the internet saying that running rich is always better or safer or makes more power than running lean. This is a blanket statement that is often untrue. The reason people believe running rich is safer is because extra fuel doesn't burn and has high heat capacity so it can soak up extra heat. This is very important when you're running a boosted or sprayed engine because the engine is making more power than it was rated for and is at greater risk of heat-induced detonation and knock. Actually that's why going lean is so dangerous when you're making a lot of power because the charge is too dry (not enough fuel) to soak up the tremendous heat. A little extra fuel helps keep temperatures lower but you still need to retard timing quite a bit under these high power conditions. For a regular commuter car or a lightly modded car (with only breathing mods for example), the compression of the engine doesn't change and you don't need to deal with excessive pressures or temperatures. Adding extra fuel WILL increase the velocity of the flame front during combustion, making it hit the piston earlier. As mentioned previously, this WILL cause knock with timing settings that were otherwise fine with a leaner mixture. The only reason to run rich is to get more power (to a point) during full throttle and then you should retard timing as well.
You can tell if you're running rich by way of a scan tool, by measuring the O2 voltage directly (0-1 volt sensor where anything over 0.45V is on the rich side), by using a wideband O2 sensor or emissions equipment at a shop. Check for leaks in the intake tract, examine the MAF sensor to see that it's clean and flowing properly (check the mesh for damage). Perform a road scan if you have a scan tool and check your STFT and LTFT fuel trims (negative numbers indicate a rich condition) and once again, check the fuel you're using. Gasoline has a ratio of 14.7:1 but gas with ethanol in it has a lower value and E85 is lower still. If your car's calibration is too far out of whack it may not be able to cope with large fluctuations in fuel composition and may start skewing lean or rich just by virtue of adjusting for the fuel alone.
6) Performance Mods and Tuning
Of course a knocking engine could be your own damn fault if you are tinkering with your car to get more power and you don't know what you're doing. Power adding mods like nitrous oxide, superchargers, turbochargers, multiple breathing mods (CAI, exhaust, headers, throttle body, intake runners) will all increase compression and combustion temperatures in one way or another. They all therefore require a combination of timing adjustment, high octane fuel and cooling provisions to compensate for the extra power the engine is now making. Don't forget the factory tunes each car to run well the way they built it. When you start pushing the limits of the car, you no longer have factory equipment anymore and you're eating into the safety margins set by the engineers who designed the car. You WILL get knock unless you compensate for the extra power and heat. Furthermore, knock becomes far more dangerous when you have a modded car because the internals are stronger than they need to be if your car is stock so the engine will tolerate some knock if it's making stock power. Once you are making 25% + power, you don't have that cushion of forgiveness. Your safety factor is gone and if you make a mistake it could be a $5000 mistake. The bottom line is, if you have a modded car and you have knock, hopefully you're reading it on a scan tool as a couple of degrees of KR and not hearing it inside the cabin. If you're getting KR, retard timing immediately and do a road scan to check your commanded AFR, O2, ECT, IAT, MAP, EGT and make sure nothing is out of range.
7) Exhaust Obstructions
In rare cases, things like clogged catalytic converters, mufflers, a malfunctioning turbocharger or turbo wastegate or any other physical blockage in the exhaust tract can cause knock. The mechanism of action here is that the blockage causes back pressure because the exhaust can't escape at the same rate that it's being produced. The pressure builds in the exhaust line and the engine tries to fight against it. Each time the cylinders complete a power cycle, the exhaust valve opens and the piston tries to drive the exhaust out but it can't because the old exhaust from the last cycle is all backed up and can't go anywhere so the cylinder remains somewhat pressurized even after the exhaust valve closes. Since engines have multiple pistons that fire out of sequence with each other, one piston may be about to go into its power stroke while another is fighting to expel exhaust gas. The piston expelling exhaust is fighting the rest of the pistons and slowing them down so the one going into its power stroke is sluggish. Guess what happens? The piston is too slow to reach top dead center before the flame front hits it and the pressure spikes, causing knock or even worse, the flame hits the piston before top dead center and tries to back-drive the piston. This is serious and you can't fix it with any simple or crude techniques. You have to get the car serviced right away.
Engine Knocking Theory
The Last Line of Defense
Bring your car to a mechanic if all else fails and you just can't figure out what's causing the knock. Sometimes it's a combination fo things and you're just unlucky. There's no shame in getting a professional to give you a second opinion. After all, it's your pride... or your engine... take your pick. If you have a scan tool like HPTuners, you can get an excellent head start on diagnosing the problem so you can give you mechanic some sort of clue as to what's going on when you bring the car in. Most of the things that cause knock are easily fixed once you know what the root-cause is. A proper diagnostic is more than half the battle. Hopefully through reading this article you're better equipped to protect your car from the onset of knock and if you do find your engine knocking, you can respond quickly before it does any permanent damage. Happy motoring!
Sondra r loper from Bowie on April 10, 2019:
Did i blow a seal
chriscamaro (author) from Ontario, Canada on March 04, 2014:
GM had a few bulletins for clogged cats so I just happened to have experienced that one myself. It usually changes the exhaust note a bit and you have a tremendous loss of power assosciated with the knock. Glad you found the article useful.
Christopher Wanamaker from Arizona on March 04, 2014:
Very thorough article. I enjoyed reading this and learned a few things as well. I had an old truck that was knocking badly and neither I nor the mechanic that looked at it could figure it out. Now I'm wondering if it wasn't a clogged catalytic converter. That's about the only thing that we didn't check. Thanks!