Cylinder 4 Misfire During Idle - Hyundai Forum: Hyundai Performance Forum
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#1 Old 04-24-2010, 01:33 PM
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Default Cylinder 4 Misfire During Idle

Hi, I've been having a problem with my engine misfiring during idle on my 2004 Hyundai Accent and can't seem to pinpoint the problem. It started a few months ago and when I brought it up to get the codes read it brought back p0304, cylinder 4 misfire. I can hear it backfire a little and stutter and shake when it's idling and the check engine light flashes. If I bring the engine up past like 1500 rpm it smooths out and the CEL stops flashing. I tried changing the #4 fuel injector, cleaning the fuel injector system, changing spark plugs and wires and even the coil pack. I Also did a compression check and all cylinders were perfect. After all that it actually seems to misfire more constantly at idle now than it did before. I'm at a loss as to what else it could be. I do have only 1 O2 sensor because of my custom header, but have never had any misfiring for the last 4 years I've had them on, but even then I'd think more than one cyilinder would be misfiring. I did have my engine replaced a little over a year ago by the dealer too. I searched the forum and other forums for answers, but it seems like I already did most recommendations. Could it be the ECU? I was also reading on the forum about regapping the spark plugs, which I may try or maybe running hotter plugs, but it seems weird I'd have to do that now. Anyone have any advice as to what else it could be? I'm trying to avoid bringing it to the dealer, especially when I'm not sure if I still have my extended warranty.

My current mods are as follows:
Customer header and catback (evofusion)
No #1 O2 sensor (making it run a little rich I believe)
Short Intake


Hyundai Accent 2004
Enkei RS5's on Potenza RE750 195/50/R15's
Ksport Pro Coilovers
Evofusion Front Strut Bar
Evofusion SRI
Mobis Grille
Evofusion 4-2-1 header and 2.25" cat-back
Whiteline 20mm rear adjustable swaybar
Coolant bypass mod

http://www.cardomain.com/ride/2937153
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#2 Old 04-24-2010, 06:46 PM
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wow this sounds familiar lol.

yes its possible that the ECU is fried. also possible that the fuel pump is going, try testing your fuel pressure.

also as i have found with my 2004 accent, i believe that the hyundai spec plug gap is dead wrong. they recommend 0.044". i recommend 0.035". try regapping the plugs to that and see what happens. i don't recommend doing this with platinum or iridium but its possible if you're REALLY careful not to break the center electrode tip.

still, for the price of 4 copper plugs i'd give it a shot. not much to lose here.

other possibilities are the coolant temp. sensor (back of the engine, the one for your ECU not your gauge) and the MAP sensor. have some fun with a multimeter. all the specs are on Hyundai Service Website. you may have to hunt through the DTC section to get the specs and troubleshooting procedure.

good luck! let us know what turns out.
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#3 Old 04-25-2010, 03:59 PM
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i had a header on my 1.5L and no 02's and at idle it sounds like it misses or *pops* idk if its a miss the rpm's dont change and it dont act like it wants to die... so i think its just something with the header and the way it changed the sound or flow.... anyways... idk.... it seems like running with no front 02's makes the car mis cuz ive noticded my car act funny ever since i put the header on... like missing once in a while *proably a fouled plug* sometimes @ 65mph one cyl will miss completley ... im thinking about switching back to stock and see what happens.
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#4 Old 04-26-2010, 07:55 AM
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Originally Posted by zero_gravity View Post
wow this sounds familiar lol.

yes its possible that the ECU is fried. also possible that the fuel pump is going, try testing your fuel pressure.

also as i have found with my 2004 accent, i believe that the hyundai spec plug gap is dead wrong. they recommend 0.044". i recommend 0.035". try regapping the plugs to that and see what happens. i don't recommend doing this with platinum or iridium but its possible if you're REALLY careful not to break the center electrode tip.

still, for the price of 4 copper plugs i'd give it a shot. not much to lose here.

other possibilities are the coolant temp. sensor (back of the engine, the one for your ECU not your gauge) and the MAP sensor. have some fun with a multimeter. all the specs are on Hyundai Service Website. you may have to hunt through the DTC section to get the specs and troubleshooting procedure.

good luck! let us know what turns out.

Thanks for the great suggestions. I'm going to give those a shot sometime this week. I was trying to rule out things that I wouldn't think would cause just one cylinder to misfire, like the fuel pump, but as I already checked the common things I guess it would be dumb to rule out those parts. Also, I do seem to smell some raw gas when it's idling like it's not being ignited so I was leaning on maybe too much fuel, timings off, weak spark or something like that. I'm no mechanic so I may be way off on my assumptions. I'll update you on my results. Thanks again!


Hyundai Accent 2004
Enkei RS5's on Potenza RE750 195/50/R15's
Ksport Pro Coilovers
Evofusion Front Strut Bar
Evofusion SRI
Mobis Grille
Evofusion 4-2-1 header and 2.25" cat-back
Whiteline 20mm rear adjustable swaybar
Coolant bypass mod

http://www.cardomain.com/ride/2937153
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#5 Old 04-26-2010, 01:32 PM
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from what you've said with the raw gas its definately a sparking issue. try those plugs and move on from there.
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#6 Old 04-26-2010, 01:33 PM
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Quote:
Originally Posted by g4ebguysite View Post
i had a header on my 1.5L and no 02's and at idle it sounds like it misses or *pops* idk if its a miss the rpm's dont change and it dont act like it wants to die... so i think its just something with the header and the way it changed the sound or flow.... anyways... idk.... it seems like running with no front 02's makes the car mis cuz ive noticded my car act funny ever since i put the header on... like missing once in a while *proably a fouled plug* sometimes @ 65mph one cyl will miss completley ... im thinking about switching back to stock and see what happens.
well if you're removing your o2 sensors no shit its going to give you trouble. they're there for fuel trim so your mixture is going to be just silly.
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#7 Old 04-30-2010, 08:03 PM
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UPDATE:
Okay so I regapped those plugs to about .035. I cranked the car up and let it idle for a little bit. The CEL doesn't flash, but I can still hear the exhaust popping and the engine shaking a little bit. but not as bad. I put the A/C on to see what happens and it started dragging the engine down, almost sounded like it was going to stall, and then the CEL starts flashing and sounded like it started misfiring more again like usual. I turn the A/C off and the engine idle rpm picks back up to about 1000rpm and the CEL stops flashing after like 10 seconds, but exhaust still pops a little and still smell gas. Not sure if it's related. I haven't checked the sensors yet and I was in the process to start that.


Like I mentioned before also, is I only have one O2 sensor. When I had my custom exhaust put on it only had one O2 bung (precat), which right now is hooked to the #2 sensor plug. I want to eventually extend the wires so I can run it to the #1 sensor plug instead. I'm not sure if the engine will use either sensor reading or needs the #1 sensor for A/F setting. I'm sure I'm running rich with only one O2 sensor, but not sure if it would cause just one
cylinder to misfire, plus I never had any misfiring before this problem came up.

I'll add more once I start trying some other things. Such a frustrating problem


Hyundai Accent 2004
Enkei RS5's on Potenza RE750 195/50/R15's
Ksport Pro Coilovers
Evofusion Front Strut Bar
Evofusion SRI
Mobis Grille
Evofusion 4-2-1 header and 2.25" cat-back
Whiteline 20mm rear adjustable swaybar
Coolant bypass mod

http://www.cardomain.com/ride/2937153
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#8 Old 04-30-2010, 08:24 PM
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my gut feeling is that the o2 sensor isn't the problem here but you really can't rule it out. on stock electronics your ECU is looking for input from that sensor to handle the fuel trim so you're likely running way too rich explaining both the misfires and the gas smell. any colour to your exhaust? maybe a bit dark? that would indicate too much fuel. same with the gas on the plugs as well as having soot collect on them after using them for a short time.

i'd also start going through testing the fuel system in case the fuel regulator is being an ass as well as checking your wires/coils with a multimeter for resistance.
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#9 Old 05-02-2010, 09:46 PM
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Quote:
Originally Posted by zero_gravity View Post
my gut feeling is that the o2 sensor isn't the problem here but you really can't rule it out. on stock electronics your ECU is looking for input from that sensor to handle the fuel trim so you're likely running way too rich explaining both the misfires and the gas smell. any colour to your exhaust? maybe a bit dark? that would indicate too much fuel. same with the gas on the plugs as well as having soot collect on them after using them for a short time.

i'd also start going through testing the fuel system in case the fuel regulator is being an ass as well as checking your wires/coils with a multimeter for resistance.
I'm going to get an O2 sensor extender and move the sensor to the no. 1 plug just so I can rule that out. Like I said, I've never had any misfires since it's been missing for like 3 years, but you never know. I know I am running pretty darn rich though, because the plugs do tend to have mild black soot on them after a short time and the exhaust is just black inside with soot. At this point I'm not ruling anything out. Thanks.


Hyundai Accent 2004
Enkei RS5's on Potenza RE750 195/50/R15's
Ksport Pro Coilovers
Evofusion Front Strut Bar
Evofusion SRI
Mobis Grille
Evofusion 4-2-1 header and 2.25" cat-back
Whiteline 20mm rear adjustable swaybar
Coolant bypass mod

http://www.cardomain.com/ride/2937153
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#10 Old 06-22-2010, 07:25 PM
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I'm experiancing the same problem with my 04 GS. Unfortunately my exhaust system has a leak at the pipe that has the flexable braided part. I'm sure that the exhaust is setting off my engine light due to oxygen sensors, but I also get the flashing engine light and bad idle vibes occasionally.
I'm hoping that when I address the exhaust problem, everything else will cure itself, heh heh.
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