Yes, even spark plugs have resistance you can measure. A higher than normal resistance requires higher than normal voltage
to fire, and can be a cause of misfire. (PHOTO: PETE MEIER)
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Misfires are a common drivability issue. If the misfire rate exceeds the programmed threshold, the Engine Control Module (ECM)
will record the problem and turn on the Malfunction Indicator Lamp (MIL). If the rate is severe enough to threaten the health
of the catalytic converter, the MIL will flash in an attempt to warn the driver that something is seriously wrong. Some misfires,
though, are intermittent enough to be noticeable but not trip a code, and these can be the toughest of all to repair.
Misfires occur when the combustion process is not as it should be. Any factor, therefore, that affects this process is suspect
when it comes to diagnosis.
Which Cylinder(s)?
Even if the MIL is not illuminated, it's a good idea to check for stored or pending codes to see if any cylinder specific
misfire codes are recorded. If the MIL is on, check the freeze frame records for the conditions the engine was under when
the code was set. This can help you narrow down the possible causes and duplicate the misfire while testing. 
1 The ACE Misfire Detective® is a software program designed for use with the PICO lab scope and FirstLook® sensor. It monitors
exhaust pulses to help identify misfiring cylinders.
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If no codes are recorded, there are a few other techniques that can help you isolate which cylinder(s) are not contributing
their fair share. Many OEM enhanced data modes have misfire history and current misfire counters in the data list. Pre-Controller
Area Network (CAN) Fords have the misfire monitor results listed in Mode $06, and all CAN-equipped vehicles have these results
recorded.
In addition to the scan tool, there are software programs available that, when coupled with your lab scope, use either the
oxygen sensor signal or a pressure transducer signal to help isolate the offending cylinders. If you have a clear shot at
the exhaust ports, an infrared temperature gun can be used to measure the temperature at the manifold. Misfiring cylinders
tend to run a little cooler than healthy ones.
Once you have the weak cylinders identified, it's time to figure out what element(s) of the combustion process is missing.
Step 1: Engine Health
2 This is a running cylinder pressure pattern, taken with a pressure transducer installed in place of the spark plug. It
can help pinpoint all manner of engine sealing and breathing issues, and is excellent for capturing intermittent sealing problems.
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The mechanical health of the engine is vital to the combustion process. It must be able to breathe in a full air/fuel charge,
seal the combustion chamber, compress that charge and then exhale all the remains.
The ability of the engine to seal and compress the air/fuel charge can be easily tested using a variety of methods. The first
I perform is a relative compression test using a Digital Storage Oscilloscope (DSO) and a high amp clamp, so I can measure
starter draw at a time base where I can see the impact each cylinder has on current draw. Weak cylinders take less effort
to turn than healthy ones, and that's apparent on the scope pattern.
If you don't have a scope, a manifold vacuum gauge can be used. Vacuum should be between 18-inch/Hg and 21-inch/Hg at idle,
and the needle should stay steady. A low reading also can indicate a restriction to flow through the engine.
3 This V-6 was recording misfires on every cylinder on the right bank of the engine. The painted end is typical of the spark
plugs installed at the factory. Needless to say, they were pretty worn, while the left bank plugs were fairly new. I guess
the right bank was too hard to change.
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If these preliminary tests show the possibility of a problem, additional testing is warranted. Traditional compression tests
using a compression gauge should be performed, followed by a cylinder leakdown test to isolate the lack of seal to a piston/ring
issue or valvetrain fault. If your first quick tests indicate the engine is OK, you can move on to the ignition and fuel systems.
However, I'm finding intermittent sealing issues that can't be uncovered using these traditional methods.
