Diagnose Engine Misfire (Part 1)

Depending on the cause, a common driveability issue is a misfire that is not easily to diagnosed. It is hard to miss if a misfiring cylinder is in a four-cylinder engine. It’s like a horse that trying to run on three legs if the engine losing a power output about 25%. At idle engine might be shake and and the engine vibrations can be felt throughout the vehicle and in the steering wheel. Engine might stall at idle or hard to start, it depend on accessory that loaded into the car such electric rear defroster, air conditioning, or headlights.

Some disturbing issues such as emissions and idle quality, and performance suffers along with fuel economy when misfire occurs. Most vehicle will failed on emission test when misfiring occurs, because the exhaust hydrocarbons (HC) reading an unusually high levels.

ENGINE MISFIRE CODES

On 1996 and newer vehicles misfire monitoring by OBD II (The Onboard Diagnostic) systems, it will set a DTC (Diagnostic Trouble Code) if an emission increased and on certain value the misfire rate exceeded. When the machine is misfiring, the Check Engine light may flashing or illuminate because of possible severe misfire. Near steering column and under the dash you can find a vehicle’s diagnostic connector, plug the scan tool on it to read the fault code(s).

Misfiring cylinders can be indicated by the code reading result, the last two numbers of misfire code telling you the misfire cylinder. Following is the code that corresponds to the cylinder number.

CAUSES OF ENGINE MISFIRE

Why cylinder misfire occurs and what causes it. It’s three thing that you should know: Loss of compression; Loss of spark; and the air/fuel mixture is too far out of balance to ignite.

If the coil voltage does not jump over the electrode gap at the end of the spark plug, it means to engine losing the spark and that is a problem. It can be caused by a cracked distributor cap, bad spark plug wires, damaged or fouled spark plugs or worn. Not only a single cylinder is affected, but all cylinders will be affected if there is excessive rotor gas or weak coils inside the distributor.

Compression Misfire

Before cylinder can be ignited, and it has lost most of the air/fuel mixture, then it will lose of compression. Blown head gasket or exhaust valve leaks common cause loss compression occurs.

The thin area of the head gasket between the cylinders has likely blown out or burned, when two adjacent cylinders are misfiring. Bad head gasket will cause leaking of compression and coolant, and will cause the engine to losing coolant or overheating.

Intermittent Misfire

The worst misfire is Intermittent misfires, this misfire is not easy to diagnose because comes and goes of this misfire depends on the engine operating conditions or the engine load. Sometime for no apparent reason this misfire seem to occurs. When cold the engine misfire occur, however after the engine warms up then it goes smooth out. Or, it may idle and start fine, unfortunately when the engine comes under load, the engine become misfire or hesitate. Also, for no apparent reason the engine suddenly cut out or misfire even most of the time the engine is running fine. In situations like this usually occur due to ignition-related fault such as bad or erratic crank position sensor signal, bad connection of coil wiring or a bad coil, or shorting out of plug wire or spark plug.

Lean Misfire

When “Lean misfire” can happen when the air/fuel mixture is too lean (not enough gas in the mix) to burn off.
When the air/fuel mixture is too lean (in the mixture is not enough gasoline) to burn, then “lean misfire” can occurs. The occurrence of misfire depends on the type of fuel delivery system and compression ratio, the engine and when the ratio of air/fuel lean out about 18 to 1, the misfire will start to occur. Under certain driving conditions with a leaner air/fuel ratio without misfire, most engines can not handle it, but a newer engines with gasoline direct injection can do it.

Leaky fuel pressure regulator, weal pump that cause a low fuel pressure, inoperative, clogged, or dirty fuel injector, restricted fuel filter or air leaks can causes lean misfire.

Not just one cylinder that would be affected by low pressure issue, but all cylinders will be affected as well, as do most air leaks. Suspect the fuel delivery problem or low fuel pressure, if misfire code is found in some cylinders. However when you found the misfire coder just on one cylinder, suspect a compression and spark problem, or the injector on current cylinder.

EGR valve leaks also can cause lean misfire. The same effect as an air leak occur if EGR valves do not closing and cause exhaust leaks back into the intake manifold. Talk about fact, if the EGR P0401 code is found with other misfire codes in a vehicle, the problem is probably the buildup of carbon under the EGR valve.

USING SCAN TOOL TO DIAGNOSE MISFIRE

There is no misfiring code output that tell you why the misfiring occurs on current cylinder, when you check for the fault, the scan tool only can read the fault code and show you the cylinder that misfiring. It can be fuel, ignition or compression related. If there a code P0304 found, it tell you cylinder number 4 is misfire, and possibly you have a bad injector that causing a misfiring when misfire code P0204 found (the injectors-related read as code P0200 series).

Probably carbon buildup under the EGR valve that cause misfire when your scan tool read EGR codes P0401.

And may be it can be a dirty fuel injector when the code reader detect the codes P0171 or P0174 (lean codes).

Try to check the ignition components on the cylinder if there is no other code except the misfire code. It may caused by fouled spark plug or badly worn, weak or defective coil in a multi-coil distributor-less ignition system or carbon tracking or moisture inside the boot of a coil-on-plug ignition coil.

Misfire can randomly occurs and moving around from cylinder to cylinder, if you check it using the scan tool, you may find random misfire code P0300. It commonly happen because the engine’s air/fuel mixture that are interrupted by something, such faulty pressure regulator or weak pump (unusually low fuel pressure), a leaky EGR valve, or major vacuum leak.

If you sure the engine is misfiring and no codes found (random or single cylinder misfire code) when you check it out, then you can take a look at the raw misfire data that is being tabulated for each cylinder using a professional grade scan tool that can access Mode $06 data. Every cylinder normally should counts misfire close to zero or zero. If the actual misfire count has not exceeded about two percent for the given cylinder, OBD II system will not set a misfire code. So to able seeing abnormally high misfire rate on any cylinders, you need take a look at the actual Mode $06 misfire data.

Example case, when scan tool reading result with Mode $06 showing all cylinders except the cylinder 4 a zero or close to zero misfires (it counted 80 or more), it telling you if cylinders number 4 need to be diagnosed because it probably has compression, ignition or fuel problem.

Continue to Diagnose Engine Misfire (Part 2): Steady Misfire