P0304 - 2 Injectors were replaced

Started car up and was getting a CEL and pretty much all systems failure even though the car drove fine. Code was P0304 - misfire cylinder 4. You could hear a slight misfire on idle if you listened at the exhaust. Pulled the neg terminal and the lights came back after a short drive... checked battery voltage which some people have noted could cause the all system faults...which was ~12.4v...

57k miles dealer is replacing 2 faulty injectors under warranty. I always used Premium... most often from WaWa...and usually run a can of seafoam every 8-10k miles.

An engine needs 3 things to function: air, fuel, and ignition (spark)

Replace spark plugs (NGK Laser Iridium), if issue persists then..
Swamp coil packs. (Ex: use cylinder#3’s coil pack and put it in cylinder#4, and use cylinder#4’s coil pack and put it in cylinder#3). Before starting the engine go in and erase the Check engine light. Once it’s been erased start the car and drive around. If the code returns there are 2 ways to go about it.
1) if the check engine light is now for cylinder 3 (Which has cylinder#4’s coil pack) then you know the coil pack that originally in cylinder 4 is faulty and have it replaced.
2) if the engine light remains for cylinder 4 then the next test I would perform is a compression test to see if it can build compression. If it doesn’t hold compression then you have a serious issue. If it holds compression then proceed to the next test.

since you said the fuel injectors were replaced, my next guess would be a sticky/sticking valves in cylinder 4

Also if you want a clean engine forget sea foaming and run a oil catch can.

sesty said:

Started car up and was getting a CEL and pretty much all systems failure even though the car drove fine. Code was P0304 - misfire cylinder 4. You could hear a slight misfire on idle if you listened at the exhaust. Pulled the neg terminal and the lights came back after a short drive... checked battery voltage which some people have noted could cause the all system faults...which was ~12.4v...

57k miles dealer is replacing 2 faulty injectors under warranty. I always used Premium... most often from WaWa...and usually run a can of seafoam every 8-10k miles.

Click to expand...

So you have Cyl #1 & #4 injectors being replaced, good. But, did they walk you through what they checked? Could be as simple as a spark plug. In that case, you would want ALL four replaced (if that is the case).
**Many Civic 1.5T owners are tuned, and in the event you are - you could very well be just going through a spark plug sooner than the manufacturers mileage interval.

How are you using the Seafoam? In the tank, crankcase?
I do not know how frequently or what brand of lubrication you are using - but if you use a brand of lubrication that has higher properties of Detergents and Dispersants you will find that it "darkens" faster. That is ok, because that means that the lubrication is interacting with the carbons in the system. That will truly help keeping the hot spots from forming on the pistons and valve sooting.
If perhaps you change your lubrication after using Seafoam, take precaution and ensure that you pre-fill the Oil Filter so that you negate any cold start friction. The solvent properties of those cleaners can remove the thin film lining of the lubrication that would remain during a drain.

Those codes are generic. It could be coils, spark plugs, injectors or a bunch of other stuff. They need to be properly diagnosed. Most mechanics just change parts until the problem goes away instead of diagnosing it.

Here is Scotty Kilmer doing a proper diagnosis on a similar problem.

Should have specified, the car is stock. The injectors are being replaced today - so I am assuming the issue will be fixed.

Seafoam was going in the tank not crank case... I received free oil changes for the life of the vehicle from my dealer when I bought the car.. they use the Honda Synthetic

sesty said:

Started car up and was getting a CEL and pretty much all systems failure even though the car drove fine. Code was P0304 - misfire cylinder 4. You could hear a slight misfire on idle if you listened at the exhaust. Pulled the neg terminal and the lights came back after a short drive... checked battery voltage which some people have noted could cause the all system faults...which was ~12.4v...

57k miles dealer is replacing 2 faulty injectors under warranty. I always used Premium... most often from WaWa...and usually run a can of seafoam every 8-10k miles.

Click to expand...

Consider a fuel brand change to a Toptier fuel like Shell, Sunoco, ExxonMobil, BP, etc as reccomended by Honda. You won't need the Seafoam. A complete list of Toptier fuels is found on their website, as well as an explanation of exactly why so many manufacturers reccomend or require them in DI engines. Other than Costco, most discount operators like WaWa, Giant, Sheetz and convenience stores have the LOWEST levels of additives & detergents to meet minimum state/federal standards.

**** Dealer changed all 4 injectors. He said it was the first 10th gen civic he has had in for injectors.