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Discussion Starter #1 (Edited)
2013 hyundai accent gs hatch manual 120k
replaced 4 plugs for regular maintenance but saved old plugs. changed air filter and oil.
replaced 4 connectors to coils because the locking clip broke on all 4
2 weeks later
p0301 cylinder 1 misfire as im driving on the highway
replaced plug cylinder 1 with old plug.
next day, same code.
checked connector on cylinder 1.
swapped coil 1 and 2. put in another used plug.
next day or 2 p0301 again while driving
compression test
1:180 2:186 3:187 4:190 seems okay?
ran techron through tank.
next day p0301 and swapped old plug
removed and cleaned throttle body and map sensor
crc cleaner through vacuum line.
next day, sea foam through vacuum line.
misfires as im driving on the highway again. p0301.
 

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You swapped coils and the problem didn't move that eliminates the coil
Static compression test is good. Swap injectors.
 

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You swapped coils and the problem didn't move that eliminates the coil
Static compression test is good. Swap injectors.
It GDI,, not something for the average Joe to mess with,, need new fuel delivery pipe and proper torque on each end..

Simple "craps & giggles"... cut the paint off all the ground lug points (cable to body,, strap at left frame, strap at right side, and couple multi-black wire grounds adjacent to PCM

Wont cost anything other than some time if DIY,, take that for test drive afterward
 

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It GDI,, not something for the average Joe to mess with,, need new fuel delivery pipe and proper torque on each end..


GDI yeah that's probably not going to be feasible for him.


Simple "craps & giggles"... cut the paint off all the ground lug points (cable to body,, strap at left frame, strap at right side, and couple multi-black wire grounds adjacent to PCM

Wont cost anything other than some time if DIY,, take that for test drive afterward

You can trust sbr711 not to lead you astray. There are some on here that will have you chasing your tail and leave you scratching your head. They will also waste your money and time if you're not careful to take things with a grain of salt. Do the grounds. Like he says....craps & giggles....may just solve your problem without spending any more money.
Let us know.
 

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Who said to swap injectors??

Anyway, do you have a scanner where you can monitor the misfire data PID as engine is running?
Mis-fire appears to be in cylinder 1.
You have done the swap plugs and coils
Would try spraying around intake manifold to check for vacuum leak
 

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Discussion Starter #6
I'll try to get my hands on a better scan tool. I'll cut the paint off of my grounds today too. I have a freeze frame from my scanner. Details below
DTCFRZF P0301
FUELSYS1 CL
FUELSYS2 --
LOAD_PCT % 46.7
ECT°F 181.4
SHRTFT1% 23.4
LONGFT1% 9.4
MAP PSI 8.4
RPM 2719
VSS MPH 65
SPARKADV 28.5
IAT °F 100.4
TP% 21.6
RUNTM SEC 786
FRP PSI 79508.5
EVAP_PCT% 0.0
FLI% 91.8
BARO PSI 14.2
VPWR V 14.017
LOAD_ABS% 42.4
EQ_RAT .999
TP_R% 12.2
AAT °F 77.0
TP_B% 21.6
APP_D% 25.9
APP_E% 25.9
TAC PCT% 16.5
 

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Discussion Starter #7
New observation

I took off the grounds I could find and sanded the body and the connector, added a little dielectric grease and bolted back up. 1 by each strut tower and 2 smaller ones next to the ECU.
Still getting the p0301 misfire. But the last time it happened, I simply shut the car off while it was missing and immediately restarted it. Ran smooth.
Thinking I may not need to swap the plugs every time it's missing. I'll try just shutting it off and restarting next time it happens. Does this info give any new ideas?
 

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I'll try to get my hands on a better scan tool. I'll cut the paint off of my grounds today too. I have a freeze frame from my scanner. Details below
DTCFRZF P0301
FUELSYS1 CL
FUELSYS2 --
LOAD_PCT % 46.7
ECT°F 181.4
SHRTFT1% 23.4
LONGFT1% 9.4
MAP PSI 8.4
RPM 2719
VSS MPH 65
SPARKADV 28.5
IAT °F 100.4
TP% 21.6
RUNTM SEC 786
FRP PSI 79508.5
EVAP_PCT% 0.0
FLI% 91.8
BARO PSI 14.2
VPWR V 14.017
LOAD_ABS% 42.4
EQ_RAT .999
TP_R% 12.2
AAT °F 77.0
TP_B% 21.6
APP_D% 25.9
APP_E% 25.9
TAC PCT% 16.5
Your fuel trims are a bit high. but not critical.
 

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I took off the grounds I could find and sanded the body and the connector, added a little dielectric grease and bolted back up. 1 by each strut tower and 2 smaller ones next to the ECU.
Still getting the p0301 misfire. But the last time it happened, I simply shut the car off while it was missing and immediately restarted it. Ran smooth.
Thinking I may not need to swap the plugs every time it's missing. I'll try just shutting it off and restarting next time it happens. Does this info give any new ideas?
I would scope the ignition system. Not something you are likely to have is a DSO. I'm not sure if you have cleaned all grounds in the system. Would be nice to have the ground distribution diagrams to be sure but lacking that you may need to get a shop who specializes in driveability so they can scope it. Did you try what @avisitor said about spraying to find a vacuum leak? With the higher than normal trims it could be a vacuum leak but also they can be from the misfire. Leak is easy to check. To keep from having a fire you might want to use a propane torch (not lit) around the intake at the number one cylinder. If the engine smooths out you've found the problem.
What I like to do just in case it's to subtle is to look at the fuel trims at the same time as doing the test. If the short term trims start going down during the test and go back up as soon as you pull the torch away.....you've found the problem.

The reason to use trims is the engine may compensate to keep idle speed the same.
 

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Discussion Starter #10
I don't have a dso
I sprayed all around with starting fluid, but it was inconclusive. Maybe watching trims as I do it would be more accurate.
 

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I don't have a dso
I sprayed all around with starting fluid, but it was inconclusive. Maybe watching trims as I do it would be more accurate.
Do it while looking at trims. If there's a leak it will show on the trims.
 

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Discussion Starter #12
Confirmation

Restarting the car last time smoothed out the misfire, but I wanted to try it again to make sure. My car was steady missing today and I shut the car off and turned it back on and it was running smooth. Are we still thinking vacuum leak test again?
 

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I had a cylinder 3 mis-fire.
Whenever I restarted the engine, it seemed fine.
And it would come back sometime later while idling.
Turned out to be a busted in-take manifold gasket.
Never having ever replaced it before, I didn't know if I could do it.
Tried and it took me about three hours to remove and replace.

Unless the vacuum leak is severe or the misfire so pronounced,
It may be difficult to see any difference in the idle while spraying around the engine
Always be careful. Don't want to short things out nor start a fire.
 

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Restarting the car last time smoothed out the misfire, but I wanted to try it again to make sure. My car was steady missing today and I shut the car off and turned it back on and it was running smooth. Are we still thinking vacuum leak test again?
If there is a vacuum leak you'll pick it up on the fuel trims even if the engine runs smooth. The metal expands and contracts which can make the leak better or worse. Do the test with fuel trims and see what you get.
 

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Discussion Starter #15
If there is a vacuum leak you'll pick it up on the fuel trims even if the engine runs smooth. The metal expands and contracts which can make the leak better or worse. Do the test with fuel trims and see what you get.
Someone mentioned using a propane torch to find one. Not lit. I think I'll try that this time. And the metal makes sense. It seems to only miss after it's good and hot. It runs best when I first start the car.
 

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Someone mentioned using a propane torch to find one. Not lit. I think I'll try that this time. And the metal makes sense. It seems to only miss after it's good and hot. It runs best when I first start the car.
The propane is why I suggested it. Not a good idea to spray carb cleaner on a hot engine. Get the engine hot and use propane. If it missing at the time you won't need to monitor the trims but if not monitor the trims.
 

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Discussion Starter #20 (Edited)
the misfire has been getting more frequent and restarting the car isnt always fixing it. i used a good scan tool today and drove around with it plugged in and watched the live data. one thing i noticed was the downstream o2 sensor having "weird" readings. when it was running well, it was around .1-.8v most of the time. when it was missing, it was down at .010 to .020v. not much fluctuation with throttle either. am i running that lean and the o2 sensor is correct or is this a sure sign of a bad sensor?
 
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