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.

A: Hi there. It would appear you have covered ...

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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

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: Master cylinder missing a wire

Hello. The brake light can come on for a number of reasons. If the brake pedal feels fine, then the problem is usually electrical. If the only light that is on is the brake light it is typically caused by...

Q: Misfire in engine.

A misfire on cylinder 7 could indicate that you have a problem with your ignition system. I would have a technician perform a tuneup on the car replacing the spark plugs and wires and see if that does not clear...

Q: Alternator draining battery

Hi there. If the alternator is not properly charging then the battery will die over time. The alternator needs to be load tested to see if it is going bad. Bad connections at the alternator and at the battery can...

Related articles

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...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.