Diagnosing those maddening misfires

Motor, May 2000 by Masterman, Jeff

All OBD II systems are designed to flag misfires. But how each manufacturer goes about it can be maddening to the service technician. Here are some tips and procedures that'll help you diagnose misfires faster and more accurately.

With the introduction of OBD II, a new diagnostic "tool" was added to the PCM that promised to make the job of misfire detection easier. The idea that a PCM could tell the technician which cylinder was misfiring and how severe the misfire was sounded almost too good to be true. It sounded even more appealing when you consider that it could be accomplished from the comfort of the front seat! The purpose of this article is to investigate this new tool-the Misfire Monitor-and see why it can cause an affliction I call "misfire madness."

Fortunately some of this madness comes from a lack of understanding on our part. I say "fortunately" because this part of the malady can be cured rather easily through proper training and other resources. However, there are some misfire madness symptoms that don't have an easy solution.

Many techs make the mistake of assuming that OBD II is a static technology This is far from the case. Since it was first conceived, OBD II has been constantly evolving due to improvements in technoloy and the tightening of emissions limits. In addition, OBD II development has taken place in dif ferent manufacturers' engineering departments, so it has taken on a different flavor for each make, depending on the specific design criteria and philosophies that exist within each group.

Additional confusion occurs when a technician encounters a vehicle that's obviously misfiring, but finds the MIL off and no DTCs stored. In some cases the problem is self=induced, so specific service steps may be all that are needed to get the system operational again.

Always On Guard

The Misfire Monitor is really considered a continuous monitor, as opposed to those that function one or more times during a drive cycle. In other words, it constantly monitors fluctuations in the crankshaft sensor signal that might indicate a misfire. If a cylinder increases the speed of the crankshaft on its power stroke, the PCM considers it to be working properly A cylinder that doesn't fire, on the other hand, will slow the rotation of the crankshaft at the time when it should be accelerating.

This small fluctuation can then be filtered out of the signal by the PCM and identified as a misfire. If a misfire is consistent, the PCM uses the cam sensor to identify the offending cylinder. Note that a DTC for a single-cylinder misfire, such as P0304, indicates that cylinder No. 4 is misfiring, not the fourth cylinder in the firing order. If the misfire is inconsistent or occurs on multiple cylinders, the PCM sets DTC P0300.

The calculation the PCM uses to determine misfire is very precise, so the signal from the crankshaft sensor must be nearly perfect for the monitor to work. In addition, differences from one engine to another due to manufacturing tolerances must be accounted for. These differences are learned by the PCM so that the information from the crankshaft sensor can be calibrated to eliminate them.

With the exception of the Misfire Monitor and Comprehensive Component Monitor, all other OBD II monitors require the vehicle to be driven under specific operating conditions in order for the monitor to run. The Misfire Monitor has no such enabling conditions, but there are some conditions that will prevent it from operating.

In Search of the Perfect Signal

As stated earlier, the PCM learns the "perfect" crankshaft position sensor signal to use for misfire detection. That means that if a vehicle suffers from, say, a dead battery before a technician begins his misfire diagnosis, that tech will have to take certain steps for the crankshaft correction to be relearned. This must be done before the Misfire Monitor can function.

The correction factor is calculated while the engine is rotating but not under the influence of combustion. The best time to do this is during deceleration from a fairly high rpm because the PCM shuts the injectors down.

Chrysler vehicles require three deceleration periods before their crankshaft correction, or Adaptive Numerator, as it's called, can be learned. There:s a data PID (Parameter Identification Display) for this that shows "Adaptive Numerator Learned? Yes or No" on the scan tool display.

Note that simply erasing the DTCs will not erase the Adaptive Numerator. The battery cable must be disconnected or "All Adaptives" must be erased with a scanner. To accelerate the learning process, Chrysler transmissions can be placed in a low gear and the vehicle accelerated and decelerated without braking.

Ford calls its crankshaft correction the Profile Correction Factor, which requires three deceleration periods from 60 to 40 mph with no braking before it's learned. Again, the battery cable must be disconnected for the correction to be erased.

General Motors introduced a specif is procedure that's done with a scan tool in order to learn its correction factor, called Crank Learn. This procedure checks the ECT to be sure the engine is at operating temperature before it will allow the technician to continue. The PCM then temporarily increases the value of the rpm limiter that's designed into the software. The technician is then instructed to hold the accelerator pedal to the floor, then release it as soon as the engine cuts out. At the end of the deceleration period, the scan tool displays a message to indicate whether or not the exercise was successful. This procedure may sound a little scary, but engine rpm will be controlled by the PCM. Still, it's a good idea to check the oil level before beginning. As with Ford and Chrysler, clearing the DTCs on a GM vehicle will not erase the crankshaft correction factor.