My 2003 Peugeot 307 was running rough/missing when cold and under hard acceleration and going up hills. The mechanic said to change the spark plugs. If no change it would be the coil pack. I changed the spark plugs and found oil coating the firing end of one spark plug. Is this a major issue and would the oil on the spark plug be the cause of the missing issue?
A fouled plug could cause a misfire, but it should be resolved by changing the plugs. If one particular plug regularly fouls there could be a problem with that cylinder. It could be a number of things causing it, but you’d need to inspect the cylinder to know for sure.