Car Questions

Clear all

How do I fix misfiring


Topic starter

Hi Scotty, I have an issue with my daughters car that no one has been able to solve and I need to keep the car on its feet for four more months.

it is a 2088 Hyundai Accent with 150 thousand miles on it (automatic) and the check engine light is on and at times blinks. On occasion it goes off. it codes at 0304 which points to the number 4 cylinder I believe. So I replaced all the ignition coils and the spark plugs and no luck. Tried fuel injector cleaner and nothing. Took it to a mechanic and 300 dollars later they could not fix it. they swapped fuel injectors 3 and 4 and the code did not move. They did a compression test and it was fine. They did a leak down test and it was also fine. They said there was a letter out from Hyundai that this was a computer problem.

Took to a dealer and they heard nothing about it. They of course changed the plugs to OEM thinking that would help. Of course the NGK's I put in were fine, but they charged my daughter 200 bucks for plugs. They then wanted to do a "deep dive" on the fuel injection system and charge her 500 more. She said no.

The number for misfire occurs mostly at idle, I am at my wits end and could use some help.


I grew up working on cars but as the technology got intense I mostly stopped.


Any ideas you have would be great!



4 Answers

Smoke test it for an intake manifold gasket vacuum leak

No leak!

Anyone else have a thought?

Posted by: @luckyone1028

did you mean Carb cleaner?

carb cleaner is for carburettors. I wouldn't use it anywhere near a modern engine intake because it's designed to leave an oily film which will foul your sensors. If you want to look for vacuum leaks you use a smoke test. Simple water mist works too. If you really want something combustible, use propane.


Spray car cleaner around the break master cylinder where it seats with the booster. I had a ford do the same thing. Mine was random cylinder misfire but the brake booster can cause a vacuum leak.

did you mean Carb cleaner?

Yes. I'm sorry for the auto correct. Carburetor cleaner for sure. Not car cleaner. 🤣

Topic starter

Still no luck. I am at my wits end with this car. I need to get 90 more days out of it but can't find the fix for the issue.