From the Magazine: Misfire diagnosis

While misfire codes have certainly made our job easier, it is important to remember that misfires don’t always set code and that not all misfire codes are accurate for the source cylinder.

Misfire codes are based on crankshaft velocity changes. When a cylinder misfires, the crankshaft will slow down due to the non-contribution to rotational speed. Cylinder misidentification can sometimes occur, especially when the misfire is inconsistent. This is why it is important to verify the misfiring cylinder even with a DTC present, instead of just trusting the scan tool.

Sometimes the PCM is unable to determine which cylinder is causing the issue, so a P0300 random misfire code will be set. A P0300 really doesn’t tell us much about the problem; we know there’s a misfire and we can feel it.

The difficulty in finding the misfiring cylinder will depend on the type of vehicle you are working on and the tools you have at your disposal. Years ago, before the days of OBD2 and misfire codes, we had to determine the misfiring cylinder by using a variety of testing methods.

Cylinder balance testing has been a tried-and-true method of misfire detection for decades. With distributor-style ignition systems, it was simply a matter of pulling each spark plug wire off the spark plug and noting RPM change. The cylinder with little or no change was the culprit. When waste-spark ignition systems came out, shorting cylinders using a test light and short lengths of vacuum tubing between the wire and coil tower was also a common method.

Neither of these techniques however are very catalytic converter-friendly, as fuel is sprayed into the non-firing cylinders and will end up in the crankcase and the converter. Disabling fuel injectors is a much safer method of performing a cylinder balance test and some modern scan tools will still allow the technician to perform it using the active test or bi-directional control function. However, when it comes to intermittent or partial misfires, sometimes killing cylinders and noting RPM change can be inconclusive.

There are other methods of determining which cylinder is misfiring, which require the use of an oscilloscope. One method uses crankshaft sensor data and math calculations to determine the frequency change of the crankshaft when the misfire occurs. This method is limited to higher-end scopes that have this capability built in. That is a topic for another article. You can usually hear a misfire occurring when putting an ear next to the tailpipe. Pressure changes in the exhaust due to the misfire will make a popping sound.

An old method that I was taught early in my career is the paper test. This is an old-school trick that can give you an indication of a misfiring cylinder. You hold a slip of paper in front of the tailpipe and observe if it is being pushed away or pulled into the pipe. Pulled into the pipe indicates vacuum present. This rudimentary method will not allow you to pinpoint which cylinder is misfiring, though. For that, we will need an oscilloscope and a pressure sensor.

When a cylinder misfire occurs, a low pressure is created in the cylinder as the piston approaches BDC of the power stroke. When the exhaust valve opens, this low pressure is transferred to the exhaust stream. The pressure change can be observed on an oscilloscope with a Delta pressure sensor placed in the tailpipe (see Images 1 and 2 — click on all images to expand).

A Delta pressure sensor is an extremely sensitive piezoelectric device that detects a change in pressure. This type of sensor doesn’t show the amount of change as a measurement value, it simply reacts to pressure changes and creates a voltage when pressure changes occur. If pressure pulses remain constant, the waveform will also remain uniform. If the pressure increases or decreases as in the case of a misfire, the voltage will also change accordingly.

By synchronizing the scope on a second channel to the cylinder 1 ignition coil control wire and knowing the engine’s firing order, the misfiring cylinder is easily identified. It is important to remember when doing this type of waveform analysis that the exhaust stroke will occur 180 degrees after the coil firing event. Once this has been established, it is simply a matter of following the firing order.

There are different methods of establishing where the exhaust stroke is occurring on the waveform, which will depend on the type of scope you are using. Some scopes like the Pico, will have built-in phase rulers that can be set up showing 180-degree stroke intervals between them (Image 3). Other scopes will require the use of a stroke overlay software that will also allow phase rulers to be set up and added to the waveform after it is saved (Image 4). The technician will have to copy and paste the waveform into the overlay software. Once it is copied into the software, it can be manipulated using phase rulers, measurement cursors and a piston chart which is set up based on engine size, firing order and the cylinder used to synchronize the waveform. This stroke overlay software can be found online and downloaded free of charge.

Let’s do some testing

The test vehicle used for these images is a 2009 Suzuki Swift+ with a 1.6L Ecotec engine. This vehicle has no misfire issues and is simply being used to illustrate how simple and effective the exhaust pressure pulse method can be at determining which cylinder is misfiring.

A Delta pressure sensor is installed in the tailpipe and the engine is started and allowed to idle. The resulting waveform is captured on the oscilloscope so we have a “normal” or “known good” waveform for comparison (Image 5). The scope is set up with a very slow time base (1 second/division) in order to capture a large amount of data. The waveform in image 6 is the same waveform ‘zoomed-in’ which gives a clear image of each cylinder pressure pulse.

The engine was then given a misfire: A spark plug was removed and the gap was closed to .005”. The plug was reinstalled and the engine was started. The closed gap has resulted in an intermittent misfire. The waveform clearly indicates downward spikes occurring every time the cylinder misfires (Image 7). Again, the waveform is ‘zoomed-in’ in to get a better picture of what is occurring (Image 8).

In order to determine which cylinder the misfire is occurring on, a second channel is synced to the coil 1 control wire (Image 9). The image zoomed in gives us a clear indication of when the misfire is happening in relation to the coil 1 firing event (Image 10). This engine uses a “cassette” style of coil, which has a separate COP coil built into the cassette for each cylinder. These are “smart” coils that have the switching transistor built into them. This is why the waveform we get when back-probing the control wire is a square wave signal. It is simply a trigger signal from the PCM which turns on the base of the switching transistor in the coil. The width of the square wave is the charge period or “dwell” period of the coil.

With the Pico phase rulers applied to the diagram, the misfiring cylinder is easily identified (Image 11). As mentioned earlier, when there is no combustion in a cylinder, a low-pressure ‘vacuum’ is created in the cylinder as the piston moves down the cylinder bore with both valves closed. The exhaust valve opens about 40 degrees before BDC and the low pressure is transferred to the exhaust stream, creating the downward spike in the waveform.

Image 12 shows the same waveform copied and pasted into the online stroke overlay software which can be used with any scope capture, including the Pico scope. Sometimes the online software is more useful for diagnosis as the phase rulers offer more measurement markers than those offered in Pico scope. The ability to also add a piston chart to the waveform is a great diagnostic tool, especially when working on V6 or V8 engines.

This is just one of many different methods that can be used and it’s a very quick and easy method of determining which cylinder is misfiring. One of the great advantages of this test is that it’s completely non-intrusive and can be used on any vehicle. Obviously, further testing must be done to determine the cause of the misfire, this test is simply to correctly identify the misfiring cylinder.

John Post has been an automotive service technician in the Ontario aftermarket for more than 30 years. He has been teaching electrical, fuel systems and drivability diagnostics at Centennial College since 2015

This article originally appeared in the December 2023 issue of CARS magazine.