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 autoignite. 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.
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.
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
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.
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
Retarding the cam timing can also lower cylinder pressures to
reduce detonation at low r.p.m., 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.
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.
6. "Read" your spark plugs. Get them
necessary. 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
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.
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.
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.
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
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.
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.
Message me at BobHewitt@Misterfixit.com
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