Steps to Properly Diagnose Engine Misfires

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Engine misfires are one of the most common problems in vehicles today. There is a lot of guessing and speculation, among vehicle owners, on how to fix a misfire code.  The steps to properly diagnose miss fires are very similar to what was done in the days of carbureted engines without electronic control modules. The difference is there are more steps now then back then. 

First and foremost, when diagnosing misfires, never assume. There are a lot of variables that cause a misfire and if you replace something without knowing, your just setting yourself up for disappointment. For example, replacing an Oxygen sensor, because you have a P0172, (rich code), rarely fixes a misfire. Yes sensors can fail, however, testing the suspect sensor will tell you if you have to go further. 

When I learned to diagnose misfires we had carbureted engines, distributors with points, spark plug wires, and a lot of vacuum hoses.  Most of the misfires were fixed by replacing spark plugs, wires, distributor cap and rotor, and points.  You know, the old school tune-up. Fuel problems didn’t cause misfires, the engine either struggled to run or didn’t run at all.  It was easier to fix those problems.  Today our engines are computer controlled, they have multiple sensors, and individual coils for each cylinder. However, the steps to diagnose misfires start basically the same.

Step 1. What is the car telling you?  You need a good scan tool to read the diagnostic trouble codes. I recommend a good scan tool because you may need to look at live data later. Miss fire codes start with P030_. Hopefully the misfire code is accompanied by more condition codes. P0171 or P0174 (system too lean – bank ?), P0172 or P0175 (system too rich – bank ?) tell you where to look next.  P030_ by themselves, start at the beginning of diagnosis and go all the way through.

Step 2. I like to start by pulling spark plugs and looking at their condition.  Condition tells me the health of the engine and if there are any other problems.  Do the plugs have oil baked on them?  This is an indicator of oil getting past the rings of the piston. Does the spark plug look wet and smell like fuel?  This tells me to look at an injector issue or a faulty spark plug or coil.  Is the spark plug wet but there is not a fuel smell, maybe a slight antifreeze smell?  This is an indicator of possible a bad head gasket. Do the spark plugs show normal use across all of them?  If they look normal the misfire could be a little more challenging to find. I also like to do a compression test in all cylinders.  Especially if there are any signs of coolant on the spark plugs. 

Step 3. Start at the cylinder ignition.  Code P0301 (cylinder 1 misfire) P0302 (cylinder 2 misfire) and so on.  Look at the spark plug and coil associate with the appropriate code.  Before replacing them, swap them with a cylinder not throwing a code.  If the code follows the plug and coil to the new cylinder, you may have found your culprit. If the code stays on the original cylinder, it’s obviously not the spark plug or coil. Remember, if you have a P0172 or P0175 code, it may be triggered by a plug or coil not firing leading to raw fuel coming out of the exhaust. Don’t rule your wiring.  If a wire or wires going to the coil or injectors have been damaged, it will surely cause a misfire.

Step 4. This is where a good scan tool comes into play. By now the spark plugs and coils should have been ruled out. Spark plugs should be looking fairly normal with the exception of the cylinder or cylinders that are misfiring. Now it’s time to diagnose the fuel system.  P0171 or P0174 have usually lead me to a vacuum leak.  Check all vacuum hoses. P0172 or P0175 lead to fuel injector issues.  Carbon build up is the biggest culprit causing a faulty injector. Start with an injector balance test.  Using the scan tool, verify fuel is being delivered evenly to the injectors. Next do an injector air/fuel test. Most of the time this can be done with the scan tool. Usually, if an injector fails, these two tests will show it. Lastly a good Bore scope will allow you to look inside the cylinder and see the carbon build up and leaking head gaskets. 

Step 5. Hopefully you don’t make it to this step. Codes P0016, P0017, P0018, and P0019 are codes coming from the camshaft position sensor and the crankshaft position sensor. These are timing codes.  Verify the sensors are good before diagnosing further. If these codes show up, then the cylinder valves may not be opening or closing properly sending wrong signals to the PCM. If the sensors are working as designed, then you have internal engine troubles.  Check camshaft lobes, camshaft actuators (for variable timing engines), timing chains, and sprockets. Stretched timing chains or damaged chain guides will throw off timing leading to engine misfires or worse. 

Step 6. Verify repairs. Take the vehicle for a test drive while the scan tool is hooked up. Please take somebody with you to drive while you read the live data.  Set the scan tool up to look at fuel management and cylinder firing while driving the vehicle.  Within a few miles you will know if the misfires are gone, and you get the sense of accomplishment. 

Not everybody has the equipment needed to diagnose misfires completely and accurately.  Every manufacturer has their procedure for diagnosing engine misfires. When looking at different forums and groups online, this question comes up a lot.  There are also a lot of misinforming answers that can lead vehicle owners to unnecessary frustration. In my experience, most vehicle owners don’t understand what goes into diagnosis issues like this.  I’ve even had customers get the codes read at a parts store and tell me I should pay them for diagnosis, or they tell me to replace components that won’t fix their problem.  This article is intended to outline the process and help vehicle owners make an informed decision about trying to tackle an engine misfire or bring the vehicle to a trusted professional.