As a Motor Age reader, you are already in a select group of technicians: Those who know that knowledge is power in this business. As a young
tech, I couldn't wait for the next issue in the hope of learning something new I could use to become more proficient, and
make more money in the process. This article is geared to those newer techs, or those who feel less than competent when it
comes to driveability diagnosis.
Monitors are invaluable to making a correct diagnosis.
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Misfire codes are one of the most common causes for a customer's "My Check Engine light is on." And I know you have worked
with at least one tech whose answer to this problem was usually "Tune it up." Let's take a look first at exactly what a misfire
code is ,and how it is set to put a quick end to that overly simplistic statement.
With the advent of On Board Diagnostics 2 (OBD2), misfire monitors were needed to maintain a vehicle's emissions quality.
Typically, manufacturers detect the presence of misfires by monitoring the signal from the Crankshaft Position Sensor (CKP).
Variances in the RPM of this signal could indicate that a cylinder didn't add its share of power to the engine, temporarily
slowing things down. If this continues past a specific number of engine revolutions, a misfire code is generated. Severe,
catalyst-damaging misfires are generally referred to as "Type A" misfires, while those that only result in unacceptable increases
in emissions are referred to as "Type B" misfires.
Time to put your thinking caps on: What does this brief description of the monitor's strategy tell you as a diagnostic technician?
Testing Relative Compression
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The first thing that comes to mind is the fact that a misfire code can be set by any condition that affects the cylinder's
ability to provide its equal share of power to the engine. That could be low compression, poor airflow through the cylinder,
improper air/fuel mixture, or a weak ignition system. The second thing that comes to mind is the fact that, because the Engine
Control Module (ECM) is relying on the CKP, its signal integrity must be good and manufacturing variances in the sensor's
reluctor wheel must first be learned by the ECM.
And the third thing that comes to mind is the fact that, because misfire monitors are sometimes intentionally ignored by the
ECM when crank speed might be influenced by operating conditions, such as driving on a rough road, it is possible to have
a misfire you can feel without a code being set.
Ignition Waveform Primer
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Finding the offending cylinders is getting easier. Most aftermarket scan tools with enhanced data will allow you to access
misfire counters — historical, current (active) or both. Ford now has misfire PIDs (Parameter Identifications) to help provide
more accurate information on the conditions the vehicle was under when the misfire(s) occurred, in addition to their Mode
$06 misfire information.
Vehicles employing Control Area Network (CAN) protocol will also list these individual cylinder results in their Mode $06.
For more details on Ford's Mode $06 misfire tests, see Motor Age, November 2006 or visit http://www.autoservicetech.com/.
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