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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)

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Have you put a fuel pressure gauge in it to see what it is doing during these episodes? –  Paulster2 Jul 8 '14 at 1:58
So this happens only during the first 15-20 minutes of running (when the tank is close to full)? –  Zaid Jul 8 '14 at 3:15
@zaid Correct. From a fill-up, at around 50-60 minutes of city driving, the tank is reduced enough that the symptom simply goes away. That's 50 - 60 minutes total, in multiple trips. –  Chris Jul 8 '14 at 3:25
So the problem persists even when the engine is warm? –  Zaid Jul 8 '14 at 3:26
@Paulster2 I have not put in a pressure gauge. I didn't know where to start. That's a good idea. I'll look into it. –  Chris Jul 8 '14 at 3:28

2 Answers 2

i know this problem your evap canister needs to be changed its full saturated with fuel it cant take any more vapors in it which will cause a very rich mixture in you manifold and probably will stall the engine you can test this by unplaging the vacuum tube to the canister and plugging it with a screw or some thing if the engine dosent stall then just change the canister or keep it unplugged although it my be illegal in some places.

it also can be your evap solenoid that stays on which will still cause the same problem you can test and fix the same way as above.

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What if bobber (fuel level sensor) in your fuel tank is squeezing fuel hose when tank is about 100% full? In this case you get fuel pressure drop and lean fuel mixture (that causing missfire). Go to service and aske them to open you tank, remove fuel pump system and investigate it.

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