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!
  1. Home
  2. Questions
  3. Cylinder #4 misfire, PO304

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: Transmission is slipping

Hello. From what you describe it would appear that your vehicle may be experiencing more than one issue. The trouble code you have listed appears to be for an issue with the vehicle's fuel pressure sensor or circuit. It may...

Q: How do you remove the second row seat cushions?

While you can lower your seats almost to the floor position, you can also remove the seat cushions from the car completely. This offers even more cargo and carrying capacity. Here’s how to do it: Locate the release strap...

Q: engine using oil

I would say that is not normal. Especially after a fresh rebuild, unless only the bearings were changed and nothing was done to the piston rings. On your engine, it is common for the piston rings to get gummed...

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...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
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...