Weeding out the weak

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.

Like this article? Sign up for our enews blasts here.

But when that didn’t fix the problem, we had a choice. We could either learn how to troubleshoot misfire complaints, or continue to throw parts at the problem until the problem, or the customer, went away. I prefer to focus on the first, don’t you?

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.

In the case of misfire detection, the most common method in use is the monitoring of engine rpm via the Crankshaft Position sensor (CKP) signal to the ECM. When a cylinder fails to produce its fair share of power, the engine momentarily slows down. The ECM monitors these speed fluctuations, and records them over a number of engine revolutions as a percentage. If this percentage of misfires exceeds the programmed thresholds, the ECM will flag the event officially, logging the rpm and load conditions present at the time and storing the information as a pending code.

If the ECM sees the same thing happen, under the same conditions and on the next consecutive test run; it will mature the code, store the freeze frame information and turn on the MIL. If the percentage is sufficiently high enough to pose an immediate threat to the catalytic converter, the ECM will flash the MIL light, bypass the need for a confirmation and immediately set a hard code.

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?

The most obvious lesson is that detection is based on variations in engine speed. So as a professional diagnostic technician, you’d ask yourself, “What can cause that variation?” Certainly, any factor that impacts the ability of a cylinder to produce its share of power would be suspect, including a strong spark from the ignition system, the correct fuel amount as supplied by the fuel system and the full compression and retention of the air/fuel charge in the cylinder. All this has to happen in the right amount and the right time. Is that all? Could a faulty CKP signal trigger misfire detection? Sure it could, and while not common, it should be included in your original list of suspects as a possibility.

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.

Does that mean there is no useful information in the ECM? Not at all. For a mild misfire to turn on the MIL, it has to occur under the same rpm and load conditions as the first detection, and it must occur on the next consecutive test to turn on the MIL. This is the standard definition of a two trip code. And while the code may not be mature, it may be stored as a pending code and may even have related Mode $06 information that can be helpful in our troubleshooting.

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.

Your first stop is Mode $03, or currently stored codes. Even if the MIL light is off, it is possible to find stored trouble codes still alive in the ECM’s memory. The ECM will turn off the MIL all by itself if the problem it originally recorded goes away for at least

three consecutive runs. The code, however, will still be listed for quite a while longer. If there are no codes stored here, move over to Mode $07. Likely listed on your scan tool as pending codes, this is a list of test results for the three continuously monitored systems under the ECM’s control; misfire detection, fuel trim and those components included in the comprehensive component monitor. If the misfire is intermittent, you just might find it here.

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).

f course, if your scan tool has enhanced capabilities, check the data stream for misfire Parameter Identifiers (PIDs) and/or built-in tests you can use to isolate the weaklings. Own a scope? The Pico scope has an available diagnostic software package that can help isolate weak cylinders. There are also aftermarket products available that can be used for misfire isolation and troubleshooting, like the ACE Misfire Detective (for the Pico) and the EMissFire Detector (for the ATS EScope).

Own 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?

If you have access to a scope, the very first test you should perform is a relative compression test. This test can be done in a few different ways, but the most common uses starter current draw as the means to quickly check for compression/sealing issues in the engine. A strong cylinder requires more current to turn through than a weak one, and this can be seen on the scope pattern. No scope? Grab your manifold vacuum gauge. If you see the needle bouncing or get a lower than normal vacuum reading, you may be dealing with a mechanical cause. Check the misfiring cylinder(s) compression and leak down rate.

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 you

know you could verify a weak Distributorless Ignition System (DIS) coil by swapping the companion cylinders? A DIS coil fires both cylinders at the same time but if the coil is weak, the positive side cylinder won’t fire under compression. Swap the secondary leads for the companion cylinders at the coil. If the misfire changes places, the coil is no good.

If 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.

The key is to take a step-by-step, logical approach to the problem at hand and testing to confirm your observations. Understand how the systems work and interact, take the time to learn how the ECM is pinpointing a problem, and use that information to plan your diagnostic attack. Above all, verify your finished repair and your customer will come back because he wants to, not because he has to.

Have articles like this sent to you weekly by signing up for our enews blasts here.