A friend once asked me about the difference between a mechanic and an automotive technician. After giving it some thought, the simplest explanation I could come up with was that a mechanic might spend 10 percent of the repair time diagnosing the problem and 90 percent of the time repairing the issue. Conversely, an automotive technician might spend 90 percent of the repair time diagnosing and 10 percent of the time actually repairing something. Most shops are a combination of both types of this work, and each tech/mechanic uses whichever description they self-identify with. My friend asked for an example of each. I provided that a brake noise caused by worn-out brake pads would be my impression of “mechanic” work and that misfire diagnostics would be more of a “technician” type job. After giving it some additional thought, I came to the conclusion that chasing down a misfire could be a little of both.
Misfire diagnosis can be fairly straightforward, but far too often one misses (pun intended) a crucial piece of information and ends up down the rabbit hole. This article’s purpose is to look at the different causes of misfires and how to identify or eliminate the potential causes to come to an accurate diagnosis. I believe a solid plan of action (POA) is required to stay on task and avoid wasting time and money by missing the misfire.
There are some things we need to take into account: pattern failures, Technical Service Bulletins (TSBs), vehicle history and customer descriptions. We need to examine the misfire conditions like load, humidity, ambient temperature and engine temperature. Is the misfire a single cylinder or multiple cylinder problem? Is the misfire on the same bank, on adjacent cylinders or on sister cylinders? All these are critical pieces of the misfire diagnostic process. The who, what, when, where and how questions need to start at the service counter. The “why” is our job as techs to identify.
Chasing down the “why”
Misfires occur when combustion does not happen or is incomplete. Combustion requires three things: compression, fuel (or correct air to fuel ratio) and ignition. Where do we test first?
The best POA involves gathering as much information with the least amount of effort. I believe this always starts with a scan tool. Modern scan tools have gotten so much better at aiding in the diagnosis of misfires. OBD II codes, freeze frames, mode $06, cylinder contribution and relative compression tests are all great improvements to modern scan tools, not to mention the ability to graph data and fuel trims.
The scan tool approach begins with checking for codes and freeze frames to hopefully identify the cylinder(s) and the conditions when the misfire occurred. Next, fuel trims, O2 sensors and other data Parameter Identifiers (PIDs) should be examined to help identify the root of the problem. From here, our goal is to form a hypothesis and design an experiment to test our theory. One of the major hairs to split with misfire is whether we are dealing with a fuel or ignition misfire issue. O2 sensor readings and fuel trims (FTs) give us some direction and help to differentiate between these two. An important note to remember about using FTs is that we should be in closed loop and should always check our loop status PID.
What can fuel trim tell you?
Let’s start with looking at a single cylinder fuel-related misfire. In this example, we had a bad poppet-style injector assembly on a 1996 Trailblazer with a 4.3 Liter V6 that coded P0300. The scan tool indicated a misfire on cylinder 6 and the FTs were +35 percent on Bank 2. Is this an ignition-related misfire or a fuel-related one?
What happens if we have a single cylinder ignition misfire? We know that if a cylinder misfires because a fuel injector does not open, the cylinder still pumps oxygen. The same could be said if a cylinder misfires because of an ignition misfire, correct? So shouldn’t the oxygen sensor report the same for an ignition misfire as it does for a fuel misfire? Let’s think about this a little more. When combustion occurs, are the HC (raw fuel) and O2 being consumed? Molecularly, what is really happening?
We know that the engine takes in HC + O2 + N2. Heat causes the atoms to disassociate and reform new molecules after combustion/oxidation occurs. With combustion taking place we ideally would expect to see the output of CO2 + H2O + N2. In all actuality, it would be more like CO + CO2 + O2 + HC + H20 + N2 + NOx. Regardless, the same number of atoms came out that went in…HC, O2 and N2. Or better yet: H, C, O and N. All of the atoms involved separated from one molecule and became a new molecule. The quantities of the individual hydrogen, carbon, oxygen and nitrogen atoms, however, stayed the same.
The amount of oxygen remains the same. The ignition misfire still produced the same exhaust output of 20 percent oxygen and 6 percent fuel. The fuel misfire produced different results – 20 percent oxygen and 4.5 percent fuel. Therefore, with the fuel misfire, the A/F ratio was lacking 25 percent of the fuel it should have had (or had 25 percent too much oxygen.) This would result in the oxygen sensor detecting a lean condition and would cause the fuel trims to go positive.
As a generalization, a single cylinder misfire caused by fuel usually manifests itself with high positive fuel trims. Conversely, a single cylinder misfire caused by ignition will generally have minimal positive fuel trim corrections. To prove this to yourself, design an experiment and record the results using the same vehicle while creating a single cylinder misfire by disabling ignition to a single cylinder and then by disabling fuel to a single cylinder.
What about mechanical causes?
Up to this point we have discussed the two more common causes of misfire; fuel and ignition. However, the mechanical misfire is another type of misfire that troubles techs and drives service advisors and owners crazy after ignition or fuel related parts/procedures have been recommended and an issue still persists. Obviously compression and cylinder Volumetric Efficiency (VE) have an effect on combustion in the cylinder and can result in a misfire issue as well. A colleague once told me that he trains his techs that driveability diagnostics is often times more of proving what something isn’t as much as it is proving what something is.
Mechanical misfire is a perfect example of this. I want to figure out very early in the diagnostic process that I can eliminate engine mechanical as a cause of the misfire rather than after ignition or fuel related parts/procedures have been recommended. I was taught a great technique to do this with a lab scope by John Thornton. It is called the “Relative Compression with Sync” test, and I use it every time I pull out a scope for a driveability issue and most certainly for misfire diagnostics. I also couple it with a cranking vacuum test with a transducer in the intake manifold to quickly eliminate engine mechanical as a cause of a misfire or a driveability problem. Let’s look at how to perform this quick, efficient and essential diagnostic test.
The test requires a lab scope and a couple of peripherals to complete it. A high current inductive probe is needed, as is the ability to capture an ignition sync of some sort. This could be an inductive sync probe if plug wires are present, a low current inductive probe, a scope lead and attenuator (if required) for a single cylinder’s primary voltage or to a scope lead attached to a COP trigger signal. The fuel system is disabled and a steady RPM crank/no start condition is required. The high current inductive probe is attached to the starter, the battery positive or the negative battery cable. This test uses the principle that the compression stroke in a cylinder is the stroke that requires the starter to work the hardest to overcome the pressure being built in each cylinder. It also recognizes that there are compression strokes occurring at different times. As the starter has to work harder to overcome each cylinder coming up on its compression stroke, the starter current raises and then falls as the piston passes Top Dead Center (TDC) of the compression stroke in each individual cylinder.
Next, an ignition sync should be added. The ignition sync can be secondary voltage, primary voltage, primary current or ignition trigger. The purpose of the ignition sync is twofold, to identify a specific cylinder in order to apply the firing order correctly and to determine if the ignition firing event is timed properly to piston position. If the ignition sync signal fires to the right of the TDC portion of the starter current on the relative compression waveform it can be an indication that the plug is being fired after TDC compression in the synced cylinder. This can be caused by the reluctor mechanically shifting. Examples of this could be a broken flywheel or a worn balancer key.
Adding a vacuum transducer to the intake manifold will enhance our engine mechanical testing by showing whether each cylinder has a distinct vacuum “pull” and whether they are relative to one another. Transducer testing is a whole other article in itself, but what I want to know quickly and early on is whether or not I can take engine mechanical off my list of possible suspects in my misfire investigation.
Here’s an example
The following vehicle is a 2000 Honda S2000 with 2.2 liter 4 cylinder with 156,000 miles. It had set PCM codes for P0300, P0301, P0302, P0303 and P0304. It had been recently tuned up and had the fuel injectors and upper intake tract cleaned. The shop also replaced a couple of ignition coils. The vehicle didn’t seem to run profoundly bad, however, the idle was slightly shaky. The FTs were slightly negative and the MAP voltage seemed a little high at 1.4 volts at idle. A scope was used to gain better diagnostic direction.
The relative compression with sync test will used first and then the cranking vacuum test will be added. Note how the compression peaks (TDC) in the current waveform are not relative to one another. The cranking vacuum test reveals that each individual cylinder’s vacuum “pulls” are not relative to one another either. Both of these results are indicative that the misfire is most likely mechanical in nature and that further engine mechanical testing will be required to complete the diagnostic. The one thing we do know is that there is a mechanical issue that needs to be corrected.
A compression test is performed and the results are compared. Cylinder 1 has 171 psi of compression and cylinder 3 has 141 psi.
The results of the misfire codes in multiple cylinders, the relative compression with sync test, the cranking vacuum test and the compression imbalance between cylinders, it appears that the issue lies in the valve train. Since the Honda uses a mechanically adjusted valve clearance (screw and tappet), the first logical place to look was the valve clearance. The valve adjustment is checked and the clearances are too tight.
Once the adjustment is completed, the compression is now equal in the cylinders; the cylinders in the relative compression test and cranking vacuum waveform are now “relative” to one another, the idle smooths out and the misfire codes no longer set. If the above scope testing had been performed earlier in the diagnostic, the technician wouldn’t have “missed the misfire.”
In summary, having a good POA, starting simple with a scan tool, using fuel trims and using the scope tests to eliminate mechanical issues will serve you well in misfire diagnostics.
