Diagnostic Solutions: Diagnosing Misfires

By Gary Goms. Although diagnosing misfires can be a tough assignment for any 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.

[sws_2_column title=””]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).

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. [/sws_2_column]

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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.