Misfire/rough Idle Fixed! - Hyundai Forums : Hyundai Forum
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post #1 of 5 (permalink) Old 02-14-2010, 04:05 PM Thread Starter
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ok gents, the mystery of the misfire, rough idle, and a s*it load of codes has been solved! after reading tons of threads on here about coils, MAF sensor, TB, and every other suggestion i have figured out the problem.

quick synopsis:

p0303- cyl 3 misfire
another code cant remember about bank 1 being too lean and a few others.

car would idle rough, spit and sputter. after swapping coils, fuel injectors, cleaning MAF and TB, changing spark plugs and wires, nothing was working. with car running unplugged each injector to test, cyl 3 had no change. removed fuel rail to check the inj individually by unplugging the other 3 and placing a bag over active cyl and cranked, inj works. i have previously checked the spark by doing the same by unplugging wires and from what i can remember it was giving spark. removed cyl 3 plug and inspected piston head, and found very wet.

after inspecting the entire fuel inj and coil control harness by removing it and checking wires and plugs found no problems. however when reinstalled everything and tested it, it worked perfectly. test drove it with no problems what so ever. so the only thing i can tell you to check is the main connector for that harness to the left of the intake manifold. thats the only thing i hadnt touched b4 now. so if you're having this problem check that plug before you do anything else!!! the dealers couldnt even figure this out!! good luck guys.
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post #2 of 5 (permalink) Old 02-27-2010, 06:09 PM
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What a potentially helpful post. Thanks, Mikec0020!
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post #3 of 5 (permalink) Old 02-28-2010, 09:43 PM
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OMG you did everything but replace the block!

It's really concerning the dealer's mechanics couldn't figure this out. I've had problems with more than a few Hyundai service departments. When my Sonata was misfiring a few years ago, none of the dealers could diagnose it and said all was fine -- it was only detected much later once the misfire got so bad the car started bucking down the highway (dead cylinder) and I had it towed in to the dealer and the codes were found.

Good job with your car!
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post #4 of 5 (permalink) Old 02-21-2011, 08:11 PM
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I hate to bump this up but where exactly is kvthe coil harness plug? I have he hyundai sonata repair manual but im unable to find it in the index. I too have a 2003 Sonata that idles very roughly. I plan on cleaning the throttle body thus week and try to unplug and replug the fuel injectir rail as you suggested but Id also like to try to unplug the coil.
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post #5 of 5 (permalink) Old 02-16-2017, 10:21 PM
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I know this is gonna sound dumb, and maybe whistful thinking, but if a car is overheating it will boil the gas and cause power loss and misfiring..(I had this with my motorhome this summer) Anyway, after having the power loss.misfiring type issue today I just went out to try it again in the cool of the night and no problem, except I noticed the cooling fan was running more than normal..Checked the radiator and it was 1/2 gallon low..Drove it some more and no problem..Check engine light is still on and will have that checked tomorrow..but maybe..it was just an overheating issue..

I also put prmium gas and a can of STP fuel system cleaner (all they had at the gas station) in there today mid trip when it was craping out on monument hill (a bit up a pass/pull, where it noticable worked (went into 2nd gear) to pull 60 up the pass..

Well, know more tomorrow..but overheating will make a car run crappy..

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