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?