Although
diagnosing misfires can be a tough assignment for
any
import driveability technician, the diagnosis can be made much simpler if
he understands the basics of misfire diagnosis. For example, when the first OBD
II misfire monitor was first introduced in 1996, technicians were puzzled by
problems with false misfire diagnostic trouble codes (DTCs). Case in point,
vehicles being driven on washboard dirt roads began recording false misfires.
Some manual transmission models also recorded false misfires
because their misfire monitors were over-sensitive to torsional vibrations in
the clutch and drivetrain. In response, auto manufacturers began programming
their powertrain control modules (PCMs) to recognize conditions that would
record false misfire trouble codes. Although most misfire diagnostics is now
relatively simple, the presence of a misfire code often indicates problems
existing outside of the primary and secondary ignition systems.
THE MISFIRE MONITOR
In addition to referencing crankshaft angle, the crankshaft
position sensor also measures decelerations in the rotating crankshaft that are
caused by a misfire event. The misfire monitor runs continuously. When the
misfire monitor detects a misfire condition that might damage the engine’s
catalytic converter, fail a state emission test or cause exhaust emissions to
exceed 1.5 times the Federal Test Procedure standard, it stores a P0300-series
DTC in the PCM’s diagnostic memory. These codes are generally referred to as
Type “A” or “one-trip” misfires and will cause the Malfunction Indicator Light
(MIL) to immediately illuminate. A type “B” or two-trip misfire can be stored
in pending codes, but won’t turn on the MIL.
When a tech actually begins to solve application-specific
misfire trouble codes, he’ll likely discover that real-world criteria for
storing a P0300-series DTC can vary widely according to model, model year and
vehicle manufacturer. In addition, the misfire criteria change quite
dramatically in three-quarter ton and heavier trucks. With those issues in
mind, I’ll concentrate on mechanical failures that might cause those mysterious
P0300-series misfire DTCs.
CYLINDER-SPECIFIC MISFIRE CODES
Individual cylinder misfires are stored as P0300 codes. The
“P” indicates “Powertrain,” the 0 indicates a generic code, the 3 indicates
cylinder misfire, and the next two digits indicate which cylinder is misfiring.
If available, the current misfire data screen on your scan
tool is useful for determining which cylinders are currently misfiring. A badly
misfiring cylinder might also cause a secondary misfire condition on adjacent
cylinders and, if the misfires occur on the same cylinder bank, they can be
caused by a clogged primary catalytic converter in that bank’s exhaust system.
If available, a misfire history screen is most useful for
detecting intermittent or random misfire conditions. Here again, check the
misfire history for each cylinder in relation to its place in the firing order
and according to its cylinder bank location. Each bit of data will provide
valuable clues in determining the cause of the misfiring cylinders.
Individual cylinder misfires are usually caused by
insufficient spark, fuel or cylinder compression. A bad distributor cap,
ignition cable, spark plug or individual coil-on-plug assembly can cause
insufficient spark. Short-to-ground ignition misfires can also be caused by
perforated spark plug boots and carbon-tracking on the spark plug insulator and
boot. In most spark misfire cases, the spark plugs are worn out or have been
incorrectly installed by an untrained mechanic. Keep in mind that spark plug
wires and coil boots should be replaced with the spark plugs at the recommended
routine maintenance intervals. In addition, modern procedures should be adopted
for installing spark plugs (See photos below).
A faulty fuel injector or a vacuum leak into the intake port
of the cylinder in question can cause a lack of fuel. Insufficient valve lash,
broken piston ring or a leaking cylinder head gasket can cause a partial loss
of compression. Vacuum leaks on individual cylinders commonly occur around
intake port gaskets or vacuum hose connections.
If the spark plugs are due for replacement, carefully
inspect each external spark plug insulator for indications of carbon tracking
or flash-over. If carbon tracking is evident, the spark plug wire or boot
should be replaced. Also, the color of the internal insulator tip should be the
same for all spark plugs. If an insulator is wet, the cylinder might have a
leaking head gasket. If the insulator tip is dark tan or black, the cylinder is
low on compression or running rich. On some applications, a leaking fuel
pressure regulator can cause one or more cylinders to run rich.
If the spark plug is encrusted with oil ash, oil might be
leaking through worn valve stem seals or piston rings. If a stem light or
borescope inserted in the cylinder indicates that the piston head is washed
clean around the edges or across the dome, the oil is washing around worn or
stuck piston rings. If the piston is dry and the oil ash is primarily
accumulated on one side of the spark plug, the oil is likely entering the
cylinder through a worn valve stem seal or guide.
GENERAL MISFIRES
General misfires are those in which misfires are distributed
through a majority of the engine’s cylinders. Lean air/fuel ratios caused by
defective mass air flow sensors, worn fuel pumps, clogged fuel filters, clogged
fuel injectors, and leaking intake gaskets and O-rings can cause general
misfires on some vehicles. Rich air/fuel ratios caused by leaking fuel pressure
regulators and other fuel control issues can also cause general misfires on
other vehicles. Freeze-frame data obtained by a scan tool can be used to
determine the conditions under which a general misfire has occurred.
In these cases, short- and long-term fuel trim numbers can
be used to help diagnose a general misfire condition. If the PCM is adding fuel
at idle, or if the fuel trim is adding fuel to one cylinder bank, suspect a
vacuum leak. If the fuel trim is subtracting fuel, suspect a leaking fuel
pressure regulator, fuel-contaminated crankcase oil, or other component failure
that introduces excess fuel into the cylinders.
If, during a road-speed test, the fuel trim numbers exceed
10% positive, the PCM is compensating for a failure or set of failures that are
causing a lean fuel condition. Here again, any condition or failure that causes
a loss of fuel pressure or incorrect metering of engine intake air can also
cause a general misfire condition. Keep in mind that a contaminated hot-wire
MAF sensor often causes an engine to run rich at idle and lean at high speeds.
UNUSUAL MISFIRE PROBLEMS
When diagnosing a misfire problem, remember that the PCM is
actually looking for a momentary deceleration in the rotating crankshaft
assembly. To illustrate, I’ve had a sunflower seed shell caught in a MAF sensor
cause a general misfire. If the shell turned edgewise, the engine ran great. If
the vehicle hit a bump, the shell would turn sidewise and block the airflow
through the MAF sensor and cause the air/fuel mixture to go lean, which created
multiple misfire codes.
In another case, I had a brand-new CKP sensor indicate
misfires at road speed as fast as the PCM could count them. Last, worn valve
seats and guides can create an intermittent misfire condition by causing an
intermittent loss of compression.
Other unusual issues that have been reported to me are heavy
lap splices in serpentine belts and vapor bubbles forming in single-line fuel
systems that eventually cause random fuel injector misfires. Last, but not
least, are the efforts of some amateur techs causing a misfire by replacing the
dual-electrode plug used on some Toyota engines with single-electrode spark
plugs.
In other cases, substituting a plated, long-life spark plug
with a non-plated, steel-shell spark plug will cause a misfire due to the less
efficient electrode configuration and considerably shorter service life of the
cheaper spark plug.
To prevent the MIL from illuminating due to a P0300-series
misfire, it’s always best to replace ignition parts at specified intervals with
OE or OE-equivalent parts. Always allow an engine to cool before removing the
spark plugs. When replacing a seized spark plug, soak the threads with
high-quality penetrant and alternately tighten and loosen the spark plug in
about 1/8-turn increments until it turns free. Thoroughly clean the spark plug
seating area.
Last, tighten the new spark plugs to the manufacturer’s
recommendation and apply a small amount of dielectric grease to the inside of
the coil or wire boot before installation.
Some misfire-related DTCs describe primary ignition and fuel injector failures.
Cardboard or plastic sleeves protect the electrodes and electrode air gaps on most long-life spark plugs.
Grasping a spark plug by its threads will prevent contaminating the insulator with greasy fingerprints.
Use a clean spark plug boot or dedicated holding tool to handle and install new spark plugs.
Improper handling can easily damage electrodes plated with precious metals. Always use the correct tools to adjust spark plug gap.
If the air gap must be increased, use a recommended tool to gently lift the ground electrode.
If the air gap must be reduced, gently tap the ground electrode with the gapping tool.
If recommended by the auto manufacturer, brush a very small amount of the recommended anti-seize compound on the threads directly behind the ground electrode.
Always inspect the spark plug boot for carbon tracking, oil saturation or perforation.
Cleanliness is always important on modern engines. Before removing spark plug, use compressed air to blow debris out of spark plug well. Clean the seating surface before installing the new spark plug. Don’t forget your safety glasses!
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