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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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Brecht Machiels
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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.

Edward
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