P0301 thru P0308 Diagnosis

P0301 is a misfire in cylinder #1. P0302 is a misfire in cylinder #2, and so on. Misfires are a cylinder failing to contribute power and are usually caused by a
A. Lack of spark to the combustion chamber,
B. Lack of compression in the cylinder, or
C. Lack of fuel to the combustion chamber.
My first step is always a visual inspection. To do this we must first identify the cylinder in question. If you don’t know your cylinder numbers, search the internet for the firing order of the engine brand, size, and year. You will see crude engine shapes with circles for each cylinder with cylinder numbers inside and a list of cylinder numbers separated by dashes that indicates the order in which the cylinders get sparked. This firing order doesn’t matter in a misfire situation except that the engine computer uses this list and the engine rpm to note which cylinder was supposed to fire when the rpm suddenly dropped slightly. It the computer notes a momentary decrease in rpm at the same cylinder several times, it posts the misfire code.
Once you’ve located the errant cylinder, inspect the ignition wire, the ignition coil wiring, and the injector wiring. Also look for any obvious problems with the manifold gaskets near that cylinder. A lack of spark can be caused by damaged ignition system wiring, and bad or leaking high-voltage ignition wire, or a bad spark plug. If everything looks good, remove the spark plug to see how it looks. Spark plugs use either a 5/8 or 13/16 inch hex fitting. You’ll have to remove the high-voltage ignition wire first or possibly the ignition coil (if your engine has coils directly on the plugs).
If the plug looks worn, I recommend replacing all of the plugs as a matter of maintenance. Note how the sparking end looks and maybe look for pictures online of plugs that misfired. It could be wet from unburned gas or possibly oily or rusty from a bad head gasket. We’ll get around to testing for that, but first check the ignition system by inserting the plug terminal back into the ignition wire. Make sure everything is reconnected and ground the threads of the plug to engine metal while a helper cranks the engine for a few seconds. The help should turn the key off immediately if the engine starts. Hold the plug threads against the engine with an insulated tool, if necessary, to watch for a spark to cross the gap. Click this link to my web page to see a pic of what this looks like:
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If there is no spark, try another spark plug that you know is good. If there is still no spark, try another coil or ignition wire. You’ve narrowed the cause of your misfire to the ignition components related to the bad cylinder. If, on the other hand, if you got a spark right away, go on the the compression or injector test.
If you have or can borrow a compression tester, do that test next. Screw the tester into the spark plug hole, remove the ignition fuse, and crank the engine through a few revolutions. If the pressure is less than around 100-120 psi, the cylinder head must be removed to repair either valves or the gasket.
If spark and compression are ok, finally check the injector by unbolting the fuel rail (I’m assuming a fuel injected engine, otherwise you can skip this test). Lift the fuel rail up without disconnecting it from the wiring harness or fuel lines (if possible). This test will shoot gasoline onto the engine, so be careful. Have a helper crank the engine a couple of revolutions while you watch the injectors squirt fuel. If the injector at the misfiring cylinder doesn’t squirt the same as the other injectors, it’s either clogged or broken or the injector wiring is bad. A partial flow indicates that the injector needs cleaning or replacement. For no flow at all, swap the injector with another injector that was firing normally. Redo do the test to see if the problem is with the injector or the wiring. (If the original injector fails to squirt at a different connector, replace the injector, otherwise repair the wiring at the original cylinder.)
Repair as indicated by the test results.