Cylinders 1 & 4 misfire

1998 hyundai accent gs check engine light.

i get odb codes 301 and 304. i have changed the plugs then reset code. after a couple days it came back on. so i changed the wires and again reset the code. cel came back on after a couple days again. the car runs fine. it’s only getting about 25mpg though wisconsin winters are pretty harsh and we use that reformulated gas. i’m thinking ,(hoping), that my coil pack is shot. i’ve been told, however, that i could have a compression problem, (which i doubt because like i said, it runs fine), or it could be the o2 sensors. any thoughts?

if it has one the first thing i would do is check the coil pack - you can do a basic check pretty easily with an ohmmeter. you just need to know the proper resistance range for each coil and can probably find the specs & testing procedure via autozone’s online repair manuals ( make sure to start with a simple check of the plug/wiring for the coil.

The coil is most likely the problem. Replace the 1-4 coil, plugs(NGK) and wires and inspect for oil in the plug wells. If needed replace valve cover gasket and plug well O-rings.
Whatever you do, don’t overlook the timing belt on that engine.

My personal opinion is that any time a performance problem is an issue the compression should always be tested. The plugs are generally out anyway and it’s always best to rule out a mechanical fault from the get-go.
Mileage is irrelevant because I’ve seen too many engine problems on low miles vehicles; including a Subaru with not one but two totally trashed cylinder heads; and this on a car with only 7k miles on it.

That being said, if 1 and 4 are adjacent to each other on the coil then the coil could very well be the problem.

I would point out that a vehicle with low compression in one or more cylinders could appear to run fine. If compression is low across the board it may even idle smooth. If compression is low in 1 or 2 cylinders then you often get a slightly rough idle or stumble. Hope that helps.

just out of curiosity; since you go under hood yourself, what was happening regarding maintenance prior to the cel and misfire codes?

were you under hood doing other maintenance just (meaning in the week or so prior) before this code came up?

all i’ve done i’ve written about above. i just got the car about a month ago. i bought it used. it has had 1 previous owner. the cel came on after about a week of driving.

UPDATE: today is -5 degrees with a wind chill of -30. i filled up my tank and the cel turned off. i never let my gas tank get close to empty. (1/8 of a tank is the closest it’s came to being empty) i drove it about 60 miles and the cel is still off. now i’m really baffled!

firing order is 1,3,4,2 and they’re arranged in a line on the coil pack accordingly.

I’m having the same problem with a 2005 Sonata at 75K miles. Problem began right after replacing the fuel pump/filter and relay (and crank angle sensor). Since then, the car has been registering codes 300, 301 & 304 (misfire in cylinder 1 & 4 and random misfire). The car also loses power occasionally at high speeds (in 5th gear on highway), which seems logical given the misfire codes. Mechanics have since replaced one ignition coil for #1 & #4, wires, plugs, throttle position sensor. Problem persists, except now it’s only cylinder 4 that is misfiring, apparently. As far as I know (I bought the car used) the timing belt has never been replaced, and the manual says it should be replaced every 30K miles (huh???) and so I am considering doing that next. Does it seem likely that a timing belt slipping could be the problem?