For the first 15 to 20 minutes after starting my Elantra it idles very low and every couple minutes, idling our not, it very nearly stalls. Per the tach, the rpm dips significantly for about 2-3 seconds then returns to normal. If I'm idling during that time, like braking or sitting at a red light, it usually stalls.
Exactly one time it lit up the check engine light. The codes indicated a misfire on 3 cylinders (I don't know if the 4th was fine or if the reader could only get last 3 codes.) That explains the loss of power, but not the cause.
So I know the usual suspects with misfires, but there's a kicker that I can't wrap my head around:
It only happens when the fuel tank is more than about 7/8 full. I stopped filling the tank, making sure to never add enough to go over 3/4 and the problem is not appearing at all.
Can anyone explain how a full or nearly full tank could cause a fuel flow problem? Or any other problem that could give these symptoms?
(I also do seem to have some kind if electrical problem too, but doesn't seem related, and I'll ask about that after I see if the new battery fixes that)