Q: Cylinder number 4 is misfiring black dry carbon on spark plug code P0304

asked by on November 12, 2016

I have one cylinder that keeps misfiring causing the spark plug to foul out with dry black carbon so far I changed the fuel injectors, cap, rotors, wires, and plugs. Been about a year since I replaced the coil and icm. Seems to misfire going uphill and every now and then on highway and interstate.

My car has 185228 miles.
My car has an automatic transmission.

Hi there. It would appear you have covered all the bases replacing components for your P0304 engine misfire. As you already know, replacing components can be expensive and time consuming without resolution. Assuming the engine is mechanically sound, good cylinder compression and valve train appropriately opening and closing. Proper diagnoses needs to be performed. I would look toward an ignition failure, given the carbon-fouled spark plug and misfire under load.

A faulty spark plug is not out of the question. A leaking fuel injector (although new) may also foul the spark plug. An intake vacuum leak over that runner could affect the air/fuel ratio and proper firing of the spark plug. I recommend having your vehicle’s engine misfire diagnosed and repaired by a certified technician, such as one from YourMechanic, if you need further help.

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