Why does oil in the spark plug hole cause misfires?

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Kia Sportage started normally as usual this morning but the engine was running rough immediately from the get go. I surmised it was a missing cylinder but no CEL showed up. Probably not an injector so I drove her off to work and after 15 minutes she ran smooth. In the evening on my way back home, she ran rough right from the start. I had dinner and waited 2 hours for the engine to cool down. Spark plugs were the main suspect. They are Denso iridiums, 8 years old and 40,000 miles in them. What I found was some engine oil in the 2nd spark plug tube, and the lower lip of the high tension connector rubber was wet with this oil. All 4 spark plugs looked OK at the center and side electrodes.

The unwanted oil was cleaned out, spark plugs reinstalled, and no more rough running engine. I was curious about how the oil in the spark plug tube was shorting out the high voltage. Isn't oil a nonconductor of electricity? I recall transformer oil is used for electrical transmission transformers, but perhaps transformer oil is a special mix?

I wiped some of my engine oil from the dipstick onto a plastic bottle cap but my digital multimeter showed infinite resistance through that oil even with the probes only 1 mm away from each other. I could only surmise that the oil was conducting electricity only when it was in a high voltage field.
 
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Since oil is about 20-25% additives, I'm going with: those additives make it conductive. Or perhaps they've suspended something conductive as they did their job in the engine.

I'll wait for the tribologists to weigh in, but your experience proves the conductivity of used oil.
 
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Pretty strange. Transformers do have mineral oil in them; I don't think there's anything particularly special about those oils, though I'm not positive. Motor oil is generally not a good conductor either. My Accord's engine is notorious for leaking spark plug tube seals, which I've fixed a few times, and I've never had a misfire because of it, nor have I heard of anyone else with a misfire from it. I actually found one of the seals leaking AGAIN recently, so I need to do that. It's still running fine though.

And you're right about the high voltage. Voltage is basically the force that "pushes" electricity through wires/conductors. The higher the voltage, the harder the "push." Something that has poor conductivity can still conduct electricity; it just needs a higher voltage to overcome the resistance and "push" the current through. That's why touching a 12V car battery won't shock you, but touching a 120V wire in your house will.
 
EPDM isn't oil resistant - it swells upon contact with oil or any petroleum product. Spark plugs fire with a few kV of electricity. You need a good seal between spark plug and the atmosphere. Ionized air from a spark discharge is a good conductor of electricity - stray sparks from bad wires or COP boots can cause misfires. They are a smaller version of corona discharge: https://en.wikipedia.org/wiki/Corona_discharge
 
My wife had a 2000 Kia Sportage and it had that problem twice. Removed and cleaned the valve cover the second time, and if I recall I replaced the o-ring seals in there. Problem was cleared after that. It seems that when a spark plug is submerged in oil, the spark has a faster and easier path to ground , then having to jump a gap in the engine. Just a crappy design by Kia.,,,
 
So, you cleaned out the oil, but didn't solve the cause of the leak? If this is the case, I would expect the problem to resume some time soon.
 
Originally Posted by PandaBear
because oil doesn't burn the same way gasoline does, and therefore all the settings are now off (timing).


Right, the spark can't ignite the oil(oh, maybe a little) but, not like gasoline.
 
Originally Posted by MrMoody
Oil isn't conductive but elemental carbon sure is, and if there's one thing in used oil, it's carbon.


I would think the combination of high voltage (40,000 volts) of the spark combined with conductive metals and carbon in the oil make a conductive path.

The higher the voltage the less conductive the path needs to be.

Hopefully you don't have any stripped or unduly worn threads allowing gas and oil to bypass them.

Also check the SP seating gasket if it has one and make sure no hot exhaust gasses have cut a channel.
 
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Originally Posted by gathermewool
So, you cleaned out the oil, but didn't solve the cause of the leak? If this is the case, I would expect the problem to resume some time soon.


It was nighttime and didn't have any spark plug o-rings in hand. They and a valve cover gasket will be purchased soon.
 
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