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i'm riding a 1.8L m10 engine, and the third spark plug doesn't work properly after 10mins of driving the car, i've changed it recently more than once and checked its wire as well, everything seems to be quiet well, would u please suggest what else would cause this issue because i'm very frustrated with the car, it works with 3 cylinders most of the time...thanks in advance

sounds like a distributer related problem

Are you sure its electrical and not a sticky valve?

Agree, how did you come to this conclusion? Did you try a different HT lead? Did you measure compression? What does the spark plug look like after some use (color difference compared to the others?). If it is ignition related the plug should be fouling or wet.

there's a little bit of oil on the old spark plug and only on this particular one , not on other cylinders, what's HT btw? , i didn't adjust the valves for a long time as well,

oil doesn't say anything, you need to compare differences in color (between spark plugs) on the part that goes into the combustion chamber. If you don't have any ignition the spark plug should be wet with fuel.

HT lead is spark plug lead, you checked it? I'd try a different example, even measuring resistance doesn't tell you all
and make sure to check valve clearances and measure compression first, it is useless to start a search if you have no clue if the basis is correct

i've found out what was wrong, the pole of the third cylinder inside the rotor cover was worn out , the rest were the same but that of the third one was vanished, so it doesn't deliver electricity everytime to the spark plug,i've changed the cover and the problem was solved...thnx