Detonation (also called "spark knock") is an erratic form of combustion that occurs when multiple flame fronts occur simultaneously inside your engine's combustion chambers. Instead of a single flame front expanding outward from the point of ignition, multiple flame fronts are generated spontaneously throughout the combustion chamber. As the multiple flame fronts collide, they produce the sharp metallic pinging or knocking noise that warns you nasty things are taking place.
If your engine has a detonation problem, you'll be most apt to hear it when accelerating under load, when giving the engine gas when you are in a high gear or when lugging the engine. Detonation occurs because the fuel's octane rating(a measure of its detonation resistance) can't handle the elevated heat and pressure when the engine comes under load. When that happens, the fuel mixture autoignite creating the destructive multiple flame fronts.
Mild detonation can occur in almost any engine and won't cause any harm. But prolonged heavy detonation is bad news because it hammers the pistons and rings. If the problem is not corrected, severe detonation may damage your engine. It can crack pistons and rings, cause the head gasket to fail, damage spark plugs and valves, and even flatten rod bearings.
Detonation also results in a loss of power because the rise in cylinder pressure occurs too rapidly for an efficient power stroke. Instead of building gradually, it peaks too quickly then drops off. The result is more like a sudden blow instead of a strong, steady push.
One way to prevent detonation is to use a higher octane fuel. The octane rating of a motor fuel is a measure of its detonation resistance. The octane that is posted on the filling station pump is "pump octane," which is an average of research and motor octane ratings. The method of determining a fuel's octane number varies depending on the technique used, but the higher the octane number the better able the fuel is to resist detonation. Fuel with an octane rating of 87 is less resistant to detonation than fuel with a rating of 89 or 91.
A gasoline's octane rating can be improved by additional refining to increase the proportion of heavier hydrocarbons in the fuel, by using a higher grade crude stock or by adding ethanol alcohol as an octane booster (all of which may increase the cost of the fuel).
Tetraethyl lead was long used as an anti-knock additive to improve gasoline octane. It was the most effective and least expensive additive that could be used for this purpose. But prolonged exposure to lead has been associated with numerous health risks. Leaded gasoline was phased out in the U.S. back in the 1970s, so increased refining (cracking, isomerization and other processes) are used to raise the octane rating of the base gasoline. Additional octane boosters such as MBTE, ethanol alcohol, aromatics and highly branched alkanes are now added to gasoline to meet octane requirements for adequate detonation resistance.
If you drive an older muscle car and can't find pump gas with a high enough octane to prevent detonation in your engine, and you don't want to detune your engine by retarding spark timing or reducing its compression ratio, you can add an aftermarket octane boosting fuel additive to the fuel tank. Some octane boosting additives also contain lead or lead substitutes to protect the exhaust valves in pre-1973 engines (which lack hardened valve seats) from premature wear. Such products can boost the octane rating of pump gas several points depending on the concentration used (always follow directions). But even this might not be enough to eliminate a persistent spark knock problem if your engine has a compression ratio over 10:1, or is supercharged or turbocharged.
Detonation can have multiple causes. Anything which increases combustion temperatures or pressure (such as turbocharging or supercharging), or increases the engine's operating temperature increases the risk of detonation. Overadvanced spark timing, or anything that causes the air/fuel mixture to run leaner than normal may also cause detonation.
Some engines require premium fuel (91 or higher octane) and may experience detonation if you fill the tank with mid-range or regular grade fuel. Under light throttle the engine may run fine on the less expensive fuel, but during hard acceleration or when lugging the engine under load, detonation may occur.
The knock sensor is supposed to detect the vibrations that signal detonation is occurring and retard spark timing temporarily until the detonation stops. Even so, it may not prevent detonation entirely. Our advice is to use the grade of gasoline recommended in the owners manual or printed on the fuel filler cap to minimize the risk of detonation.
Other causes of detonation may include any of the following:
Too much compression can cause detonation. An accumulation of carbon deposits in the combustion chambers, on piston tops and valves can increase compression to the point where it causes detonation. Carbon deposits can also cause "preignition" which is a condition where hot spots in the combustion chamber become ignition points, causing the fuel to ignite before the spark plug fires. Preignition is also what makes an engine run on after the ignition has been turned off.
How quickly the deposits accumulate depends on the type of driving done and the quality of the fuel burned. Carbon deposits gradually accumulate in a new engine for the first 5,000 to 15,000 miles, then level off. A state of equilibrium is reached where old deposits flake off at about the same rate as new deposits are formed. Infrequent driving, infrequent oil changes or internal engine problems such as worn valve guides, or worn, broken or improperly seated rings that allow oil burning can greatly accelerate the accumulation of deposits.
To get rid of the deposits, pour a can of "top cleaner" down the carburetor or through the throttle body while the engine is idling (follow the directions on the product). Allow the chemical to soak the recommended period of time, then restart the engine and blow out the crud (an oil change afterwards is recommended). Repeat as need if the first cleaning fails to eliminate the detonation problem.
If chemical cleaning fails to remove the carbon, there is always the "Italian Tuneup" method of blowing the carbon out of the engine. Take your vehicle somewhere where there is little or no traffic and you can safely accelerate at full throttle up to the posted speed limit (or beyond if you don't mind risking a speeding ticket). Repeat this several times, then cruise at highways speeds for at least 15 minutes to clean the carbon out of the combustion chambers.
If a high mileage engine is so badly carboned up that chemical cleaning and/or hard driving can't get the carbon out, another option is to use a "soft" blasting media such as crushed walnut shells to scour the combustion chambers clean. This job can be done with the cylinder head in place by removing the spark plug, blowing the media in through the plug hole to knock loose the carbon, then sucking out the debris with a shop vacuum.
If your engine has a static compression ratio that is higher than 10:1, the only way to totally eliminate a detonation problem on pump gas may be to rebuild the engine with lower compression pistons, or cylinder heads with larger combustion chambers, or to replace the stock head gasket with a thicker head gasket to reduce the compression ratio!
Over-advanced ignition timing can cause detonation. Too much spark advance causes cylinder pressure to rise too rapidly. On older vehicles with a mechanical distributor, rotating the distributor to retard timing several degrees and/or changing the spark advance springs so timing does not advance as quickly can reduce the risk of detonation, but it will also hurt performance. On newer vehicles with electronic spark timing, it may be possible to change the spark advance curve with a special tuner scan tool.
Engine overheating can cause detonation. A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Overheating can be caused by a low coolant level (check for coolant leaks), a defective fan clutch, an undersized fan or missing fan shroud, an electric cooling fan, fan relay or temperature sensor that is not working properly, a thermostat that is sticking shut, a bad water pump, a clogged radiator, or a severe restriction in the exhaust such as a clogged catalytic converter that is backing heat up into the engine. Poor heat conduction inside the engine due to rust or scale accumulation inside the engine's cooling jackets can also make an engine run hot. Check the operation of the cooling fan (electric fans should come on when the A/C is turned on), and check for coolant leaks. Check the condition of the coolant. If dirty, add a bottle of cooling system cleaner to the cooling system, run for the specified period of time, then drain and flush the cooling system.
Overheated air can cause detonation. On older vehicles with carburetors, the thermostatically controlled air cleaner provides hot air to aid fuel vaporization during engine warm-up. If the air control door sticks shut so that the carburetor continues to receive heated air after the engine is warm, the engine may experience detonation, especially during hot weather. Check the operation of the air flow control door in the air cleaner to see that it opens as the engine warms up. No movement may mean the vacuum motor or thermostat is defective.
If you have an open style air cleaner on an older engine with a carburetor, or a "cold air" intake on a newer fuel injected engine, the intake may be pulling in heated air from the engine compartment. To reduce the risk of detonation, you want cooler, denser air from outside the engine compartment or ahead of the radiator entering the intake system.
Lean fuel mixtures can cause detonation. Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor or throttle body gaskets, or intake manifold gaskets can allow extra air to enter the engine. Lean fuel mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter or a weak fuel pump.
If the fuel mixture becomes too lean, "lean misfire" may also occur as the load on the engine increases. This can cause hesitation, stumble and a rough idle condition.
The air/fuel ratio can also be affected by changes in altitude. As you go up in elevation, the air becomes less dense. A carburetor that is calibrated for high altitude driving will run too lean if driven at a lower elevation. Altitude changes are generally not a problem with late model feedback carburetors and electronic fuel injection because the oxygen and barometric pressure sensors compensate for changes in air density and fuel ratios.
The wrong spark plugs can cause detonation. Spark plugs with the wrong heat range (too hot) can cause detonation as well as preignition. Copper core spark plugs have a broader heat range than ordinary spark plugs, which lessens the danger of detonation.
Loss of EGR can cause detonation. Exhaust gas recirculation (EGR) has a cooling effect on combustion temperatures because it dilutes the incoming mixture with inert exhaust gas. This lowers combustion temperatures and reduces the formation of oxides of nitrogen (NOX). It also reduces the risk of detonation. So if the EGR valve is not working or someone has disconnected it or plugged the EGR vacuum hose, combustion temperatures will run much higher likely resulting in detonation when the engine is under load.
Excessive turbo boost can cause detonation. Controlling the amount of boost in a turbocharged engine is absolutely critical to prevent detonation. The turbo wastegate bleeds off boost pressure in response to rising intake manifold pressure. On most late model engines, a computer controlled solenoid helps regulate the operation of the wastegate. A malfunction with the manifold pressure sensor, the wastegate control solenoid, the wastegate itself or a leak in the vacuum connections between these components can allow the turbo to deliver too much boost, which will send the engine into early retirement if the condition is not corrected.
Improved intercooling can also help. The intercooler's job is to lower the incoming air temperature after it exits the turbo compressor. Adding an intercooler to a turbo motor that isn't intercooled can eliminate detonation worries while also allowing the engine to handle more boost. And if a factory turbo motor has been tweaked, replacing the stock intercooler with a larger, more efficient aftermarket intercooler may be necessary to prevent detonation.
A bad knock sensor can cause detonation. Many late model engines have a "knock sensor" on the engine that responds to the frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to momentarily retard ignition timing until the detonation stops. A knock sensor can usually be tested by rapping a wrench on the manifold or cylinder head near the sensor (never hit the sensor itself!) and watching for the timing to change while the engine is idling. If the timing fails to retard, the sensor may be defective or the problem may be within the electronic spark timing control circuitry of the computer itself.
Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.
Some late model turbocharged engines with Direct Fuel Injection can experience detonation at low RPM following a cold start or after prolonged idling. The problem seems to be due to gasoline mixing with residual motor oil on the cylinder walls in the upper cylinder area. Many motor oils contain large amounts of sodium as part of their detergent additive package. When the sodium mixes with fuel, it forms a compound that can easily detonate when the engine is lugging hard under load or accelerating. The cure is to switch to a motor oil that contains less detergent, or less sodium in its detergent additives.