This month, we’ll look at how to use scan data to help identify a misfire. For an engine to run correctly, it needs three main things: compression, spark, and the correct air/fuel mixture. If any of these is missing, the result will be no combustion in the affected cylinder
There are a number of tests to help determine which of these items is at fault. You can test fuel injectors, scope the secondary ignition, inspect spark plugs, perform compression tests, and much more.
But performing all these tests takes time. To narrow your search, you can use scan data to choose a logical path and hopefully eliminate the superfluous tests.
You’ll want to look at these PIDs:
- fuel system status
- short term fuel trim
- long term fuel trim
- downstream oxygen sensor voltage
But before you can analyze that data, you must first understand what good looks like.
The fuel system or loop status PID indicates whether the vehicle is operating in open or closed loop. The vehicle needs to be in closed loop before fuel trim has meaning. When the vehicle operates in open loop, fuel trim has no real effect on vehicle operation, so you’d be missing one of the pieces of the puzzle.
The second piece of the puzzle is fuel trim. What you really want is total fuel trim, which probably isn’t something your scan tool displays. To calculate total fuel trim, add short term fuel trim and long tern fuel trim together. This represents the total amount of fuel the PCM is adding or subtracting at that moment. In general, plus or minus 5% is ideal, while plus or minus 10% is acceptable.
Finally, check the downstream oxygen sensor voltage. The downstream sensor has three main jobs: monitor catalyst efficiency, provide input for minor fuel trim corrections (on some applications), and cross check the upstream sensor.
When diagnosing a misfire, the downstream sensor will help you identify how well the engine was fueled. A good downstream oxygen sensor voltage could read anywhere from low to high. Usually the voltage will be relatively flat and float somewhere in the middle 800 millivolts. What’s important is that it should never be flatlined at the bottom or stuck near zero volts.
Look at the data from bank 2 on a V8 engine (figure 1). The vehicle is running correctly with no misfires. This is the baseline, or known-good data. The PCM is in closed loop, so the fuel trim numbers can be trusted. The total fuel trim correction is +4.7% (-3.1+7.8) which is well within acceptable limits. Finally, the downstream oxygen sensor voltage is hovering at an acceptable 785 millivolts.
Now look at the data from the same vehicle, but this time it’s misfiring on a single cylinder on bank 2 (figure 2). Again, the PCM is in closed loop so you can trust the fuel trim numbers. Total fuel trim correction is now +21.9% (3.9+18.) This positive correction means the PCM is adding fuel to compensate for a lean condition.
In this case, the four cylinders on the right side of the engine are responsible for 100% of the air fuel mixture that will be going past the oxygen sensor for that bank. So each cylinder is responsible for 25% (100 divided by 4) of the mixture for that side of the engine. Basically, if one fuel injector were to fail, that bank of the engine would be about 25% lean. In our case, a +21.9% correction is very close to that measurement, and is a strong indicator of a fuel injector-related, single-cylinder misfire.
You can apply this math to most engines: A V6 engine with two upstream oxygen sensors would have three cylinders exhausting past one oxygen sensor, so an injector would be responsible for 33% of the fuel on that bank. A V6 engine with a crossover pipe and only one upstream oxygen sensor would be about 17% if a single injector were to fail, and so on.
You can also examine the downstream oxygen sensor voltage. On this vehicle, it’s flatlined at 65 millivolts. The abundance of oxygen in the exhaust causes low sensor voltage and is an additional indicator of a fuel-related problem.
The next failure is on the same vehicle (figure 3). It has a single-cylinder misfire on bank 2, so we’ll observe the same PIDs. This time the misfire is ignition related. You might think that, if a cylinder fills with oxygen and fuel but combustion doesn’t occur, the unconverted oxygen will cause the oxygen sensor to drop lean. This lean condition should then cause a positive fuel trim correction. It doesn’t:
Again, the engine’s in closed loop. This time, total fuel trim correction is +11.7%, or just barely outside the acceptable range. This fuel trim isn’t nearly as positive the previous, injector-related misfire. Here’s why: Oxygen sensors don’t actually measure oxygen in the exhaust. They’re actually more of an oxidation sensor, creating a signal based on the oxidation of unburned or improperly burned fuel left in the exhaust. (For more on that, check Steve Bodofsky’s article in the March 2004 issue of GEARS.)
In the case of an ignition misfire, the engine still has enough fuel, so it shouldn’t require any drastic fuel trim corrections. For reference, the upstream air/fuel ratio sensors would yield the same result.
The downstream oxygen sensor voltage shows that the engine was fueled adequately. The data gathered during this misfire is 935 millivolts; very close to our known good readings. That’s because an ignition-related misfire leaves fuel trim levels and downstream oxygen sensor PIDs very close to normal.
This technique is a valuable way to improve diagnostic efficiency and reduce the time invested for diagnosis. For example, if the numbers indicate a fuel-related misfire, there’s no reason to waste time testing the ignition system.
Nothing in our industry is 100% and this technique is no different. There are two flaws with this process: First, it works about 95% of the time; occasionally a vehicle won’t behave as described here. When that happens, the fuel trim numbers are usually off, but you should still be able to trust the downstream oxygen sensor. In that case, the worst scenario is that the data suggests a fuel issue, the fuel system tests good, and you have to perform additional tests to rule out an ignition issue.
The second flaw is that this technique is great for choosing a fuel or ignition related diagnostic path, but does nothing for a mechanical misfire. That’s because how fuel trim numbers and downstream oxygen sensors behave for a compression-related issue depend on the type of mechanical failure.
For example, a leaking valve would yield different results than a stuck closed valve. And whether it’s the intake or exhaust valve could cloud the results even further. Again, worst case scenario would be having to perform additional testing.
Hopefully, a quick check of these data PIDs early in your misfire diagnosis will help streamline the process. Next time a misfire comes in the door, give it a shot.
