Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Cylinder number 4 is misfiring black dry carbon on spark plug code P0304

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Engine turns over good but won't start what might be the problem? It misfired one time and hasn't started since.

Your engine requires spark, air, and fuel to start and run properly. Air is almost always present so that leaves spark and fuel. Depending on the vehicle, if a crankshaft sensor is faulty, the engine control unit may not receive...

Q: Oil all over engine

Hello. First we need to clarify if the leak was fixed or not. This must be done first. Then after the oil is cleaned, we can see if there was any other damage. Oil contamination can get onto electrical components...

Q: Fuel Pressure

Hi There, It sounds like you may have a problem with maintaining fuel pressure and getting the proper amount of fuel to the fuel injectors. As you know, when you turn the key to on, the fuel pump is activated...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...