spark knock and how do you get rid of it
Drawing on left shows completion
of normal combustion. Cutaway on right shows a detonating
cylinder, where the last portion of the air/fuel mixture
self-ignites and collides with the normal combustion front.
Spark knock (detonation) is an erratic form of combustion
that occurs when multiple flame fronts occur simultaneously
inside a combustion chamber. Detonation occurs because fuel
is subjected to either too much pressure, too much heat or
both. It usually happens during acceleration when the engine
is heavily loaded and cylinder pressures are at their peak.
Instead of a single flame front growing outward smoothly
like an expanding balloon from the point of ignition,
multiple flame fronts are generated spontaneously throughout
the combustion chamber as the fuel automatically ignites
from heat and pressure. The multiple flame fronts collide,
creating shock waves that produce a sharp metallic pinging
or knocking noise.
Mild detonation can occur in almost any engine and will not
cause damage. Prolonged heavy detonation can crack pistons
and rings, blow out head gaskets, damage spark plugs and
valves, and flatten rod bearings.
Any of the following can cause detonation:
Too Much Compression: An accumulation of carbon deposits in
the combustion chambers, on piston tops and valves can
increase compression to the point where it exceeds fuel
octane rating. If a top cleaner fuel additive fails to
remove deposits, a new alternative is to blast the deposits
loose by blowing crushed walnut shells through the spark
plug hole. Otherwise, the head will have to be removed so
the deposits can be scraped off.
Overadvanced Ignition Timing: Too much spark advance causes
cylinder pressure to rise too rapidly. If resetting the
timing to stock specifications does not help, retarding
timing a couple of degrees may be necessary to eliminate
Engine Overheating: A hot engine is more likely to suffer
spark knock than one which runs at normal temperature.
Overheating can be caused by low coolant, a defective fan
clutch, too hot a thermostat, a bad water pump, etc. A
buildup of lime and rust deposits in the head and block can
also reduce heat transfer
Overheated Air: The thermostatically controlled air cleaner
provides the carburetor with 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, detonation may occur,
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 a loose
vacuum hose or a defective vacuum motor or thermostat.
Lean Fuel Mixture: Rich fuel mixtures resist detonation
while lean ones do not. Air leaks in vacuum lines, intake
manifold gaskets, carburetor gaskets or fuel injection
intake plumbing downstream of the 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.
The air/fuel ratio can also be affected by changes in
altitude. A carburetor calibrated for high altitude driving
will run too lean if driven at a lower elevation. Altitude
changes are generally compensated for on computer cars by
the barometric pressure sensor.
A lean fuel condition can be diagnosed by watching for lean
misfire on an ignition scope, or by using a four-gas
infrared analyzer and watching exhaust oxygen levels. A
reading over about 3% to 4% oxygen would indicate a lean
Spark Plug Too Hot: The wrong heat range plug can cause
detonation as well as pre-ignition. Copper core plugs are
less likely to cause detonation than standard spark plugs.
Loss of Exhaust Gas Recirculation (EGR): EGR keeps
combustion temperatures down, reducing the tendency to
detonate. If the EGR valve is inoperative or someone has
disconnected or plugged its vacuum hose, higher combustion
temperatures can cause pinging.
Low Octane Fuel: Burning cheap gas may be one way to save
pennies, but switching to a higher grade of fuel may be
necessary to eliminate a persistent knock problem.
Defective Knock Sensor: The knock sensor responds to
frequency vibrations produced by detonation (typically 6 - 8
kHz), and signals the computer to momentarily retard
ignition timing until detonation stops. A knock sensor can
usually be tested by rapping a wrench on the manifold near
the sensor (never hit the sensor itself). If there is no
timing retard, the sensor may be defective.
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