Skip the auto shop - Our mechanics make house calls

Q: Cylinder #4 misfire, PO304

asked by on

Hello, My vehicle is mainly repaired by me. I take pride in the repairing and maintenance of my vehicles. But the repair is a bit out of my league this time. My truck is on idle stuttering and almost shutting off. The exhaust is spitting out light smoke and gas. It smells like its burning rich. I have replaced the spark plug and the wire on that cylinder and checked the coil and its all good. The only thing remaining is the injector on that cylinder. I figure its stuck and dripping constant fuel into the cylinder? The vehicle is parked at home now. I cannot drive it this way. My question is, from my description, would it be the injector #4?

My car has 180000 miles.
My car has an automatic transmission.

In order to know for sure that your problem is with the injector for the number 4 cylinder, is to do a fuel pressure leak down. If the fuel pressure does not leak down after you turn on the key and the pressure rises, then let it sit. The fuel pressure should stay up and not bleed down more than one PSI per 15 minutes max. If you see the pressure go down pretty fast then your injector may be the reason why. You would have to block off the return line on the fuel rail if it has one, to make sure that it’s not the fuel pressure regulator doing the leaking.

If the injector is the suspect, replacing it will be the answer. If you do not have any pressure loss, you will need to do an injector driver test for the cylinder by hooking up a noid tester light to the injector connector and see if the light blinks while the engine is running. If the light stays on, then the engine control module has a shorted injector driver. If the light blinks then you know the injector circuit is okay. Afterwards, a compression test on the cylinder is necessary. You will most likely find out that the injector is leaking and flooding the cylinder out. Make sure to check the oil to make sure that it’s not getting over filled by fuel leaking into the cylinder. If you feel you need help diagnosing the problem then contact a qualified mechanic to assess your misfire and make the necessary repairs to get your car going again.

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: How do I open the hood?

It’s very easy to open the hood, and only takes a few steps: 1. Pull the Hood Release lever, which is located on the bottom left side of the instrument panel. 2. From the front of the vehicle, locate the...

Q: Something broke underneath my car.

Hello, without hearing the noise its hard to make a recommendation. If you hear a rattle sound it could be a loose bracket or loose bolt. If you are experiencing a clunking sound, you might have something loose in the...

Q: My bmw 316i has issues with the gearbox

Hello. From what you describe it would appear that your vehicle may be having an issue with the transmission, possibly with the gearbox. It may be possible that the clutch is worn, though that would likely affect first gear more...

Related articles

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...
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.
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.