Detonation (also called "spark knock") is an erratic form of combustion that can
cause head gasket failure as well as other engine damage. Detonation occurs when
excessive heat and pressure in the combustion chamber cause the air/fuel mixture
to auto ignite. This produces multiple flame fronts within the combustion chamber
instead of a single flame kernel. When these multiple flames collide, they do so
with explosive force that produces a sudden rise in cylinder pressure
accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock
waves created by detonation subject the head gasket, piston, rings, spark plug
and rod bearings to severe overloading.
Mild or occasional detonation can occur in almost any engine and usually causes
no harm. But prolonged or heavy detonation can be very damaging. So if you hear
knocking or pinging when accelerating or lugging your engine, you probably have
a detonation problem.
A DOZEN WAYS TO PREVENT DETONATION
1. Try a higher octane fuel. The octane rating of a given grade of gasoline is a
measure of its detonation resistance. The higher the octane number, the better
able the fuel is to resist detonation. Most engines in good condition will run
fine on regular grade 87 octane fuel. But engines with high compression ratios
(over 9:1), turbochargers, superchargers, or with accumulated carbon deposits in
the combustion chamber may require 89 or higher octane fuel.
How a vehicle is used can also affect its octane requirements. If a vehicle is
used for towing or some other application where the engine is forced to work
hard under load, a higher octane fuel may be necessary to prevent detonation.
If switching to a higher octane fuel fails to eliminate a persistent detonation
problem, it probably means something else is amiss. Anything that increases
normal combustion temperatures or pressures, leans out the air/fuel mixture, or
causes the engine to run hotter than normal can cause detonation.
2. Check for loss of EGR. The Exhaust Gas Recirculation (EGR) system is one of
the engine's primary emission controls. Its purpose is to reduce oxides of
nitrogen (NOX) pollution in the exhaust. It does this by "leaking" (recirculating)
small amounts of exhaust into the intake manifold through the EGR valve. Though
the gases are hot, they actually have a cooling effect on combustion
temperatures by diluting the air/fuel mixture slightly. Lowering the combustion
temperature reduces the formation of NOX as well as the octane requirements of
the engine.
If the EGR valve is not opening, either because the valve itself is defective or
because its vacuum supply is blocked (loose, plugged or misrouted vacuum hose
connections, or a defective vacuum control valve or solenoid), the cooling
effect is lost. The result will be higher combustion temperatures under load and
an increased chance of detonation.
Refer to a service manual for the configuration and hose routing of your
engine's EGR system, and the recommended procedure for checking the operation of
the EGR system.
3. Keep compression within reasonable limits. A static compression ratio of 9:1
is usually the recommended limit for most naturally aspirated street engines
(though some newer engines with knock sensors can handle higher compression
ratios).
Compression ratios over 10.5:1 may create a detonation problem even with 93
octane premium gasoline. So unless an engine is being built to run on racing
fuel, keep the compression ratio within a reasonable range for pump gasoline.
This, in turn, may require using lower compression pistons and/or cylinder heads
with larger combustion chambers. Another option would be to use a copper head
gasket shim with the stock head gasket to reduce compression.
Retarding the cam timing can also lower cylinder pressures to reduce detonation
at low RPM., but doing so hurts low speed torque which is not recommended for
street engines or cars with automatics.
For supercharged or turbocharged applications, a static compression ratio of 8:1
or less may be required depending on the amount of boost pressure.
Another point to keep in mind is that boring an engine's cylinders to accept
oversized pistons also increases the static compression ratio. So too does
milling the cylinder heads. If such modifications are necessary to compensate
for cylinder wear, head warpage or damage, you may have to use a thicker head
gasket if one is available for the application or a head gasket shim (a dead
soft copper spacer shim) to offset the increase in compression.
4. Check for over-advanced ignition timing. Too much spark advance can cause
cylinder pressures to rise too rapidly. If resetting the timing to stock
specifications doesn't help, retarding the timing a couple of degrees and/or
recalibrating the distributor advance curve may be necessary to keep detonation
under control.
5. Check for a defective knock sensor. 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.
If the "check engine" light is on, check the vehicle's onboard computer system
using the prescribed procedure for a "trouble code" that would correspond to a
bad knock sensor (code 42 or 43 for GM, code 25 for Ford, or code 17 for
Chrysler).
A knock sensor can usually be tested by rapping a wrench on the manifold near
the sensor (never hit the sensor itself!) and watching for the timing 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. To determine the cause, you'll have to refer
to the appropriate diagnostic chart in a service manual and follow the
step-by-step test procedures to isolate the cause.
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.
6. "Read" your spark plugs. The wrong heat range plug can cause detonation as
well as preignition. If the insulators around the electrodes on your plugs
appear yellowish or blistered, they may be too hot for the application. Try the
next heat range colder spark plug. Copper core spark plugs generally have a
broader heat range than ordinary plugs, which lessens the danger of detonation.
7. Check for engine overheating. 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, a slipping fan clutch, too small a fan, too hot a thermostat,
a bad water pump, or even a missing fan shroud. Poor heat conduction in the head
and water jackets can be caused by a buildup of lime deposits or steam pockets
(which can result from trapped air pockets).
8. Check the operation of the heated air intake system. The thermostatically
controlled air cleaner's job is to provide a carbureted engine with hot air when
the engine is cold started. This aids fuel vaporization during engine warm-up.
If the air control door sticks shut or is slow to open so that the carburetor
continues to receive heated air after the engine is warm, the added heat may be
enough to cause a detonation problem -- 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. Also, check the heat riser valve to make sure it's opening properly,
as it, too, can affect the air intake system.
9. Check for a lean fuel mixture. Rich fuel mixtures resist detonation while
lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor
gaskets or the induction plumbing downstream of a fuel injection throttle can
all admit extra air into the engine and lean out the fuel mixture. Lean 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 occur as the load on
the engine increases. This can cause a hesitation, stumble and/or rough idle
problem as well.
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's calibrated for high altitude driving will run too lean if
driven at a lower elevation. Altitude changes are generally not a problem with
engines that have electronic feedback carburetors or electronic fuel injection
because the oxygen and barometric pressure sensors compensate for changes in air
density and fuel ratios.
10. Remove carbon deposits. An accumulation of carbon deposits in the combustion
chamber and on the top of the pistons can increase compression to the point
where detonation becomes a problem. Carbon deposits are a common cause of
detonation in high-mileage engines, and can be especially thick if the engine
consumes oil because of worn valve guides and seals, worn or broken piston rings
and/or cylinder wear. Infrequent driving and not changing the oil often enough
can also accelerate the buildup of deposits.
In addition to increasing compression, carbon deposits also have an insulating
effect that slows the normal transfer of heat away from the combustion chamber
into the head. A thick layer of deposits can therefore raise combustion
temperatures and contribute to "preignition" as well as detonation.
Carbon deposits can often be removed from an engine that's still in service by
using a chemical "top cleaner." This type of product is poured into an idling
engine through the carburetor or throttle body. The engine is then shut off so
the solvent can soak into and loosen the deposits. When the engine is restarted
the deposits are blown out of the combustion chamber.
If chemical cleaning fails to remove the deposits, it may be necessary to pull
the cylinder head and scrape the deposits off with a wire brush or scraper (be
careful not to scratch the face of the cylinder head or engine deck!).
11. Check the boost pressure. 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
destroys the head gasket as well as the engine in short order if not corrected.
Improved intercooling can help reduce detonation under boost. 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 (or
installing a larger or more efficient intercooler) can eliminate detonation
worries while also allowing the engine to safely handle more boost.
12. Change your driving habits. Instead of lugging the engine, try downshifting
to a lower gear and/or accelerating more gradually. Keep in mind, too, that the
engine and drivetrain have to be matched to the application. If you're working
your engine too hard, perhaps you need a transmission with a wider gear ratio or
a higher final drive ratio in the differential.
PRE-IGNITION
Another condition that is sometimes confused with detonation is "preignition."
This occurs when a point within the combustion chamber becomes so hot that it
becomes a source of ignition and causes the fuel to ignite before the spark plug
fires. This, in turn, may contribute to or cause a detonation problem.
Instead of the fuel igniting at the right instant to give the crankshaft a
smooth kick in the right direction, the fuel ignites prematurely (early) causing
a momentarily backlash as the piston tries to turn the crank in the wrong
direction. This can be very damaging because of the stresses it creates. It can
also localize heat to such an extent that it can partially melt or burn a hole
through the top of a piston!
Preignition can also make itself known when a hot engine is shut off. The engine
may continue to run even though the ignition has been turned off because the
combustion chamber is hot enough for spontaneous ignition. The engine may
continue to run-on or "diesel" and chug erratically for several minutes.
To prevent this from happening, some engines have a "fuel cutoff solenoid" on
the carburetor to stop the flow of fuel to the engine once the ignition is
turned off. Others use an "idle stop solenoid" that closes the throttle
completely to shut of the engine's air supply. If either of these devices is
misadjusted or inoperative, run-on can be a problem. Engines with electronic
fuel injection don't have this problem because the injectors stop spraying fuel
as soon as the ignition is turned off.
CAUSES OF PRE-IGNITION
Carbon deposits form a heat barrier and can be a contributing factor to
preignition. Other causes include: An overheated spark plug (too hot a heat
range for the application). Glowing carbon deposits on a hot exhaust valve
(which may mean the valve is running too hot because of poor seating, a weak
valve spring or insufficient valve lash).
A sharp edge in the combustion chamber or on top of a piston (rounding sharp
edges with a grinder can eliminate this cause).
Sharp edges on valves that were reground improperly (not enough margin left on
the edges).
A lean fuel mixture.
Low coolant level, slipping fan clutch, inoperative electric cooling fan or
other cooling system problem that causes the engine to run hotter than normal.
http://www.misterfixit.com/deton.htm
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