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My 2001 Hyundai Elantra XD suffers from misfires during the first 5-10 minutes after starting the car. After this, the car runs as as it should. I'm not sure, but I think the misfires only happen when accelerating, not when idling. The check engine light will sometimes blink when the engine misfires (when it's bad enough to exceed a threshold, I assume).

The severity of the problem seems to be linked to how long the car has not been driven. For example, the misfires may not occur at all when I have driven the car the day before. When the car has stood still for a couple of days, I can be pretty sure it will misfire.

It may be a coincidence, but the problem started when the weather was getting colder (temperatures dropping below 10°C or 50°F).

My dealer replaced the spark plugs, but the problem keeps appearing.

I think the misfires may be caused by a failing catalytic converter. The converter has been causing the check engine light to light up for over a year, but it was decided to not yet replace it since this is a very expensive part and the car was still running fine. The misfires only started occurring a couple of weeks ago. My dealer mentioned he noticed that the catalytic converter rattles.

Could the misfires be caused by the catalytic converter?

Could there be another cause, considering the symptoms described above? The fuel pump? The fuel filter?

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  • The catalytic converter would have nothing to do with whether it misfires or not. You'd see a completely different set of symptoms with a failing cat. You may not have a failing cat, either. This thread may help you with that diagnosis.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 13 '16 at 12:13
  • From what I understand, part of the exhaust fumes are fed back into the engine. That's why I thought the cat could be causing the misfires. This answer also says a cat can cause misfires: Anytime I've had a cat plugged so bad that it affected performance, it caused the vehicle to outright misfire.
    – Brecht Machiels
    Nov 13 '16 at 12:58
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    The system you are talking about is the EGR system, which all occurs prior to the catalytic converter. As for the misfire you are talking about, it's caused from a plugged cat and occurs all the time when you get above a certain RPM range, usually somewhere around 2500 rpm (give or take). You'd have completely different symptoms if it were a plugged cat. I'm fairly positive your issue has nothing to do with the catalytic converter.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 13 '16 at 13:15
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    How's the coolant level? Do you notice any fuel smells when you're starting the car? Try letting the car sit and pull a spark plug, shine a light in there and check for any fluid in the chamber, that would be my first guess either a leaking headgasket or a leaking fuel injector.,
    – Ben
    Nov 13 '16 at 15:38
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    To help take out the guesswork as to what's going on you need to tell us what the long-term/short-term fuel trims are saying. Edward's already given a couple of possibilities and I could probably add three more but it isn't going to help you narrow your list of possibilities. Data will empower us; speculation will leave you clueless.
    – Zaid
    Nov 13 '16 at 16:35
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As @Pᴀᴜʟsᴛᴇʀ2 mentioned in the comments, it seems unlikely that your misfiring issues are caused by the catalytic converter. What would be a useful next step would be to use an OBDII diagnostic reader and see what codes the engine controller has recorded.

The cooling weather and the fact that it seems to be worse if the car is unused for a few days both suggest to me a possible moisture problem.

Theory 1: water in fuel tank

One possibility is that there is some water in the fuel tank. If it's warm and there's just water vapor it might not be noticeable, but when the weather cools, the water vapor condenses and now we have liquid water in the fuel. If the diagnostic codes show misfiring on multiple cylinders, this could be a cause. Adding some gas treatment designed to help with this condition is a cheap and easy way to test this theory. It's not expensive and even if it's wrong it won't hurt anything.

Theory 2: bad plug wires

Another possibility is that your plug wires or coils need replacing. When it's cool in the morning and water vapor in the air has condensed on electrical parts such as wires and coils an existing fault in the insulation will seem worse because the water conducts electricity better than air, so a barely-noticeable crack in a wire becomes a short to the engine block it's touching. Here again, diagnostic codes will help, because they might tell you which plug wire or coil to start with. One simple no-tools diagnostic here is to wait until it's very dark outside (new moon and/or overcast) and look under the hood as the engine is running. Sometimes you can see the blue flashes indicative of arcing which are a sure sign of insulation problems.

With that said, however, if your catalytic converter is faulty it should definitely be replaced even if it's not the cause of this particular problem.

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    Thanks to the suggestion of a possible leak, I noticed that the misfires mostly occurred when it had rained (or the air was moist). I passed on this information to my mechanic, who was able to blame the ignition coil for the misfires; he could see sparks on it when spraying it lightly with water. The coil has now been replaced, and I hope this solves the problem.
    – Brecht Machiels
    Feb 15 '17 at 10:04

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