I’ve always found the term “misfire” interesting. It’s a term that often leads inexperienced techs down the wrong diagnostic road; one where they immediately fault the ignition system because of that word fire. We’ve all seen techs do it, and if we are honest with one another, I’m guessing we’d have to admit that we’ve all been guilty of throwing plugs and wires at a misfire complaint at least once.
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What Is A Misfire?
There is one common troubleshooting tip that we’ve stressed over and over. To properly diagnose any system on any car, it pays to first understand how that system works and how the Engine Control Module (ECM) performs its own tests of that system. After all, the ECM decides whether or not the Malfunction Indicator Lamp (MIL) is turned on and if your repair doesn’t meet its standards, that little light will come back on and your customer will just plain come back.
Take a moment to read over that rather generalized description of a misfire detection strategy. What did you learn about the troubleshooting process just from that paragraph?
Is it possible to have a misfire and no MIL on or code stored? According to what we learned in the description, it is. If the misfire is not occurring often enough to meet the test threshold, the ECM will not officially recognize it. Some manufacturers have some pretty high thresholds, as most of you already know.
Wait, mode what? Don’t get ahead of me; I’ll get there.
Who’s The Weakling?
Identifying the weak cylinders is normally easy enough. The MIL light is on, and misfire codes (P0300-P0312) are stored, making cylinder identification simple. But what if there is no MIL on or codes stored? It still pays to connect your scan tool, starting in Generic or Global OBDII mode.
Still no luck? There is one more place you can look using the Global OBDII mode of your scan tool, provided the car you’re working on is a Controller Area Network (CAN) vehicle.
Choose Mode $06 to access the latest test results for all the non-continuous monitors and misfire detection monitor. Scroll down the test results, looking for test identifications starting with A2. That’s the misfire results for the cylinder No. 1, and typically includes two results. The first is the average misfire count over the last ten tests, and the second is the misfire counts for the last test run. Remaining cylinders are in numerical order; cylinder No. 2 is A3, cylinder No. 3 is A4, and so on.
These are not active tests, so you can’t watch them change as you run the engine. But you can use them to see what cylinders have recorded misfires that may or may not have been enough to set a related code. If the vehicle is a pre-CAN Ford, you can also find misfire test results in Mode $06, but the test identification numbers and counting methods are a bit different (see the related link).
OOwn none of the above and still haven’t isolated the weak link? If you can consistently duplicate the misfire, try performing a manual power balance test. Start by disabling each cylinder one at a time with the misfire occurring and the engine held at a steady rpm. Note the corresponding rpm drop associated with each disabled hole. Rpm loss will be greater when a strong cylinder is lost than when the weak one(s) is lost. If the miss is at idle, you’ll have to bypass the idle control in order to see the rpm drop before the ECM does.
Isolating the Cause
Diagnosing the reason for the misfire becomes easier when you know what cylinder(s) is failing and when. If the problem is limited to one cylinder, it has to be caused by a fault that affects only that cylinder. If more than one cylinder is involved, consider what the cylinders have in common with one another. Are they all on the same bank? Do they share a coil? Are they side-by-side?
A word of caution when assessing the mechanical health of the engine. More and more, techs are running into intermittent valve sealing issues resulting in intermittent misfires. Valves rotate slightly as the engine runs. Because of this, a valve with heavy carbon deposits or even one that is mildly bent, can seal perfectly for a time and leak like a sieve the next. In these cases, it is very possible for a cylinder to pass a traditional compression/leak down test. If the misfire comes and goes with no apparent reasoning behind it, this may be your problem.
Dealing with a single cylinder miss? Parts swapping is a perfectly acceptable diagnostic process. If the engine uses a Coil On Plug (COP) ignition system, try switching coils and plugs to see if the misfire moves with the swap. Take a good look at the plug while its out. I’d be willing to wager the plug has a gap twice as large as it should be. Excessive gap creates excessive demand on the coil, and is a leading cause of COP coil failure. But don’t let that alone lead you to the replacement of parts. If the miss didn’t move, there is another culprit to be found. Recommend the plugs for maintenance, not for the repair.
Did youIf the single cylinder miss doesn’t go away, the only thing left is the fuel delivery to the cylinder. Swapping the injectors is little more time consuming on most engines. Instead, use your scan tool or manual injector tester to perform an injector balance test. This test records the amount of pressure drop in the fuel rail as each individual injector is activated. Higher than average drops can indicate a leaking injector and lower than average drops point to a restriction. You can perform the same test even faster using a pressure transducer or sensor mounted in place of your fuel gauge, and a scope.
Dealing with multiple cylinder misfires will involve testing of these same basic systems, only in a different light. Fuel delivery issues (low pressure/volume) will impact all the cylinders, for example. Leaking head gaskets typically affects two cylinders at a time. Cam timing problems can be across the board or only on one bank, depending on the engine design.
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