Tuesday, July 23, 2024 Detailed Auto Topics
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Single or unique-misfires occur on an identifiable cylinder.  The power control module marks these with a suffix to the P0300 code.  P0307 or P0302 suggests misfires on cylinders seven and two respectively.  With such a symptom, the cause is unique to the cylinder named as opposed to multiple cylinder misfires.

Identifying a single-cylinder  misfire

With a factory-level scan tool and a digital lab-scope, many tests are simple.  Without special tooling, substitution is a very effective means of testing components.  This method is superior to replacing parts, as new components are NOT ‘known-good.’   A misfire on cylinder four means the cause lies with its components.  Some possible causes include, a bad spark plug, the ignition coil, fuel injector and low compression.  Move the spark plug from the missing cylinder to another that is running well, for instance, number-three.   Setting code, P0303 after substituting the spark plugs suggests a bad part.

component substitution to diagnose a misfire

A misfire remaining on the original cylinder says the suspected component is not the only cause.  Having multiple causes is possible, such as a bad plug and a failed coil.  Repeating the tests may be necessary here.  Physical evidence, by inspection, is also part of the process.  We replace a fouled sparkplug, knowing a bad ignition-coil, low compression or a stuck injector may cause it.

The order of tests

The order in which we swap components is a matter of personal preference.  We swap parts most likely to cause an issue first. Technicians often decide by experience.  Ford's engines are known for coil-on-plug failure.  We would swap coils before other tests.

checking injector pulse with a simple noid light

With more difficult replacement, other tests may take priority.  For instance, before moving a fuel injector, testing for a pulse from the PCM is wise.  A lack of a computer signal is less likely than a failed injector, but it is far easier to check.  This simple test may save additional work.  The same may be true with checking engine compression.  Lack of compression is less likely, but the time required could be far less than moving an injector.

Other codes can help

A misfire can cause other codes or result from other issues that leave codes.  P0175 suggests that bank one, shows to be too rich.  This means there is more fuel in the exhaust than there should be. Unburned fuel often results from a misfire.  With a P030# code present on that bank, we may investigate the misfire first.

P0171 suggests that bank one may be running lean. Too much air, or not enough fuel sets this condition.  A misfire on the same cylinder-bank could be the result of the lean condition, rather than the cause.  Here, things that cause such a problem are checked first.  A leaking intake manifold, allows un-metered air to enter the cylinder.  With too much air, in relation to the fuel, the cylinder will misfire.

Low fuel pressure also causes a lean condition.  This is not suspected initially, because the low-pressure effects all cylinders.  The same would be true for a bad air flow meter.  All cylinders use the meter, so creating a single-cylinder misfire is not likely.  We consider all codes in memory, to formulate our theory.  Tests are based on theory, and results drive further tests.

Diagnosis is not easy

Other examples are not so clear.  For instance, a technician does not generally relate an EGR code to a single-cylinder misfire.  Exhaust-gas-recycle normally affects the whole engine.  Designs vary, and with some Honda engines, plugged EGR passages can cause a misfire on one cylinder.  Familiarity with the type of system is necessary to find these type problems.

diagnosis is not easy, it is the most difficult part of the repair

The above tips give a logical approach to isolating misfires.  Clearly there may be multiple causes, overlapping symptoms and situations too difficult to figure out.  Technicians need a vast knowledge of how engines operate, and years of experience.  Forming a relationship with a high-quality repair shop is a great idea.  Paying a professional for guidance is far less expensive, when a situation exceeds our experience.


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