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I get OBD code P0300 Random Misfire (and no other error codes) on a very poorly maintained car. I changed the valve cover (whose spark plug well gaskets were degraded) and spark plugs, but the engine sounds the same, like only half the cylinders are firing, and it vibrates a lot.

Afterwards, I noticed the car had almost no oil. I haven't changed the oil yet, but I'm wondering: Can lack oil cause cylinder misfire / random misfire? If so, why?

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There's nothing that detects whether the plugs are actually igniting the mix evenly, it is inferred from the acceleration that the crankshaft undergoes each time a cylinder fires. There's plenty of things that can cause uneven firing - vacuum leaks, variations in mixture, uneven fuel pressure, though anything like, say, low compression will soon be resolved to particular cylinders.

As for low oil level - maybe if the engine is on the verge of seizing? I'd look elsewhere before assuming that so long as the engine isn't making odd noises.

Misfire Detection Monitor

The misfire detection monitor runs during normal engine operating and driving conditions, and is used by the PCM to determine whether the engine is malfunctioning to the point where it is releasing excess pollutants into the atmosphere. The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. A single misfire will cause a subtle change in the speed of the crank. The PCM tracks each misfire, adding them up and averaging them over time to determine if the rate of misfire is high enough to cause the vehicle to exceed the federal emissions limit. Whenever crankshaft rotational velocity varies by just 2 percent, one or more misfire codes are stored in the memory of the PCM. If this happens on two consecutive trips, the check engine light will be commanded on to alert the vehicle operator that a misfire problem is occurring. If the misfire causes the crankshaft speed to vary by more than 10 percent, the check engine light flashes two times a second to warn the vehicle operator that a severe, catalytic converter damaging misfire condition is occurring.

When the misfire detection monitor detects a misfire, the check engine light will flash as the misfire is occurring. But the light will not remain on the first time a misfire problem is detected. It will come on only if the misfire continues during a second drive cycle and will set a P0300 series code. A P0300 code would indicate a random misfire (probably due to a vacuum leak, open EGR valve, etc.). If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A misfire code P0301, for example, would indicate a misfire problem in cylinder #1. Of course, misfires can be caused by a lot of things. It could be a worn or fouled spark plug, a weak coil, a bad plug wire, a dirty or dead fuel injector, or a loss of compression (burned exhaust valve or leaky head gasket). Further diagnosis is always needed to isolate and identify the root cause.

https://www.nyvip.org/PublicSite/OBDII/misfire-detection-monitor.html

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  • +1 "it is inferred from the acceleration that the crankshaft undergoes each time a cylinder fires" – Geremia Dec 12 '19 at 19:27
  • "maybe if the engine is on the verge of seizing?" It would seem lack of oil would cause camshaft issues, wouldn't it? – Geremia Dec 13 '19 at 17:28
  • If it has hydraulic lifters, they may not pump up to take up the clearance, which would cause valve opening to be later and probably lower than it should be, but you'd almost certainly hear them clattering, and probably be getting a low oil pressure warning light. Obvious question - you have refilled the oil level back to normal? If so, are the codes still being generated? – Phil G Dec 13 '19 at 17:49
  • "you'd almost certainly hear them clattering" I don't. "and probably be getting a low oil pressure warning light" I'm not (which is strange because there's just a few drops of oil at the end of the dipstick). – Geremia Dec 13 '19 at 18:13
  • The dipstick doesn't go all the way to the bottom of the sump, there may be enough in there for the oil pickup to still be drawing oil in. DON"T run it in this state. – Phil G Dec 13 '19 at 19:24

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