In theory, diagnosing a misfire should be simple and straightforward – just figure out which cylinder isn’t operating properly and then figure out why.
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In reality, diagnosing misfires can be really challenging. Unlike no-starts or other problems that constantly are present or really obvious, misfires can be intermittent problems that show up only
One unfortunate thing about European vehicles is that they’re often high mileage units full of aging parts that could be potentially causing the problem – not to mention that many vehicle owners claim to be flat broke and just want the bare minimum fixed.
Misfires happen when the combustion process doesn’t occur as it should on a single cylinder, across multiple cylinders — regularly or intermittently. The force from each cylinder’s power stroke can be felt at the crankshaft and when that force from the cylinder isn’t detected the crankshaft sensor detects the slight slow-down in the crankshaft and reports a misfire to the controller. This may or may not turn on the Check Engine Light, depending upon how serious the problem is.
While misfires are usually easy to feel on four-cylinder engines, on larger engines it’s sometimes difficult to pinpoint exactly what’s going on since the misfire can only be felt as a bit of a stumble as opposed to the huge loss of power felt on a smaller engine.
Misfires also can be difficult to reproduce, and a bit of a challenge to fix, but throwing parts at the vehicle until it, or the customer, goes away just isn’t a responsible option. It’s important to be able to find problems fast and fix them correctly – and for that you need to develop a plan of attack, verify your suspicions, and fix the vehicle confidently. Here’s how.
Plan the Attack
Diagnosing misfires, just like any other kind of diagnosis, involves coming up with possible causes of the problem, then proving or disproving those suspicions.
That’s why preliminary checks are so important – they help you develop suspicions about what could be causing the problem. Simply measuring voltages and pressures isn’t diagnosing a problem.
Measuring voltages and pressures is the way to prove that suspicions are right (or wrong).
Preliminary checks on vehicle with misfire concerns include checking for service bulletins and programming updates, visually inspecting components for anything obvious (hey, you never know), and looking for trouble codes (DTCs) that are current and also those stored in history – misfire diagnosis is certainly much easier with a compatible scan tool and DTCs and history data to point you towards the misfiring cylinder.
Also notice the conditions under which the misfire took place, such as the amount of load on the engine and the engine speed, the temperature outside, the temperature of the engine, and anything else that hints at what was happening just before the problem occurred. Then use this information to help reproduce the problem and then narrow down the possible causes with a strategic plan of attack.
A good preliminary inspection can save you hours of diagnostic headaches later on.
Something’s Missing
When diagnosing a misfire problem, the first thing to figure out is what’s missing (ha!) from the normal combustion process: ignition, fuel or compression. This takes a bit of practice and a bit of knowledge but it’s really nothing you can’t handle.
From experience, ignition-related misfires tend to show up suddenly rather than gradually developing over time. They tend to be worse under load and on older vehicles these problems tend to act up in damp weather. These misfires can be mistaken for shuddering and shaking in other systems so it’s important to hook up a scan tool and check for related codes and history information as part of the initial inspection. Performing this step might save time and point you in the right diagnostic direction.
Fuel-related misfires, however, tend to develop over time and steadily get worse. A fuel-related problem such as a lean misfire or “bad gas” may move from cylinder to cylinder rather than remain in one place so the related DTCs will usually be general misfire codes rather than DTCs for a specific cylinder or component. Because of their elusive
Mechanical problems can definitely cause misfires, too. These problems are usually present all the time. It’s important to remember that a vehicle with a flashing Check Engine Light should not be test-driven – the light is flashing to indicate a problem that will damage the catalytic converter. Start the diagnosis by plugging in the scan tool and check for DTCs rather than road testing the vehicle to avoid an expensive additional repair.
Using a systematic diagnostic approach to determine which component is “missing” is a better strategy than just diving in and replacing parts (my friend calls this “loading the parts shotgun and giving it a blast!”).
Most service manuals and repair information systems list the things that could cause a misfire (as well as other problems) so if you’re really stuck or you’re having a bad day you could refer to this list and check through each of those potential causes until you find the correct one. But because so many things could be causing problems, it really helps to narrow down the possible causes before diving in.
Spark
Ignition-related misfires are often tricky to diagnose because they’re great at hiding and only showing up during certain driving conditions.
If strong spark is indeed being delivered to the plug, check the plug itself for problems (this can be a real challenge on certain vehicles but sometimes it has to be done). Inspect the porcelain insulator for cracks, a common cause of problems. Even small cracks allow voltage to leak out rather than fire the spark plug, causing a misfire condition. Look for things that indicate healthy combustion isn’t happening such as fouling or wear. Compare the plugs from different cylinders to make sure they’re similar in appearance. If the plug is wet with fuel or oil, it likely hasn’t fired for a while.
If there’s no spark being sent to the plug, check the ignition coil. Coil-related misfires may only occur under certain circumstances, for example when the coil gets hot or only under hard acceleration.
Fortunately, faulty coils usually set DTCs to help point you in the right direction. If a misfire DTC is setting in a specific cylinder or cylinders, that can really help speed up diagnosis.
However, older vehicles might have limited capability to report misfire data. In this case it’s important to have a few diagnostic tricks in your arsenal. Timing lights are a good way to see if a coil is firing: install the timing light and touch the inductive clamp to the coil. The timing light should light up when the coil fires. Another old-but-good trick is to swap the suspect coil to another cylinder or location to see if the problem moves along with it. “Swapnostics” is a bit of a down-and-dirty trick, but it’s also quite effective when used responsibly.
Faulty ignition coils used to be an extremely common cause of misfires, but on newer models this seems to have gotten a whole lot better. However, many European vehicles on the road are a bit older so it’s well worth inspecting the coils for problems.
In addition to checking the ignition components themselves, it’s important to also inspect their electrical connections. Loose or corroded electrical terminals can and do cause many problems, especially ones that occur over rough roads and in damp weather. It’s easy to check terminals: use the opposite type of connector and do a drag test, in other words see if the connector “feels” loose. If the connector’s too loose it can cause problems.
Also check that the connections are completely seated and securely locked into place. Some connectors don’t make solid and complete contact until they’re locked in place, not just inserted in place. And finally, check that terminals are free from corrosion and securely mounted inside the connector. If the terminal isn’t making good contact it can’t do its job. Sometimes, a slight drop of Stabilant 22 is all that is needed to solve the problem.
Air/Fuel Ratio Misfires
Since engines need the right air-fuel mixture for combustion, problems with the fuel supply can cause misfire concerns. Problems with the fuel supply system include things such as injector faults or sensor faults causing an incorrect mixture, but may also include problems with the gas itself such as the wrong octane level or just old or bad gas.
Rich-condition misfires do happen (usually focused in a single cylinder) and should not be ruled out, but from experience lean-condition misfires are more common.
Misfires caused by fuel or mixture problems can be difficult to replicate since they may be felt as a slight stumble that may not even set a DTC. If a DTC is set, it’s likely to be a general one such as P300 since the problem will likely occur across multiple cylinders rather than just one specific cylinder.
I remember one engineer who was working as a technician and would recommend draining the fuel tank and sending the fuel to be analyzed when a vehicle had a bit of a stumble – and yes, it’s possible but if the customer has bought gas at the same place every time and no one else complained of a stumble after filling up there, it might be wise to investigate other causes first.
If the problem is slight misfires that happen at idle suspect a vacuum leak somewhere causing a lean misfire. Stay away from that old trick of spraying intake cleaner or brake cleaner on the intake components while the engine is running to locate the leak. The problem with this is that if there’s an ignition leak somewhere you could start a fire.
Smoke machines used with the engine off work well for finding leaks and they won’t cause a fireball in the process.
Mechanical Problems
Even if the spark is good and the air-fuel mixture is perfectly balanced, it won’t matter a bit if there’s a problem in the engine’s base mechanical operation. Things like bent or burnt valves, stretched or jumped timing belts, or worn rings can all create a misfire condition. Actually, it seems like mechanical problems are becoming more common since newer engines seem to have so many more components and many folks don’t keep up with their maintenance — or they do their own maintenance and create more problems.
Misfires on European vehicles can be tricky to diagnose but there’s no need to panic or load that parts shotgun to chase the vehicle away. By performing a careful preliminary inspection, thoroughly checking the basics, then testing your theories with effective diagnostic methods you’ll seldom meet a misfire you can’t handle. European vehicle owners can be great customers so it’s worth getting to know the systems involved. Your bottom line will thank you.
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