When I fill the gas tank, the car idles poorly, like a miss, stalling if I don’t rev the motor, and when driving it feels like I shut the key off and turn it right back on. It only does this until about 3/4 of a tank is left, then it runs fine. No codes have been set, check engine light is off. It’s a 2008 with 29,000 miles.
sounds like the evaporative system is faulty. Did you “top off” a few times?
Took it to the dealer and they couldn’t/wouldn’t diagnose without a check engine code.
What do you mean “top off”? If you mean add more gas after the hose auto-shutoff, yes I did.
Yes, that is exactly what he meant.
It is NEVER advised to “top off” after the pump auto-shutoff. That will send raw/liquid gasoline into the evap system (which is designed to deal ONLY with gasoline vapors), messing it up.
Any idea why it only happens when the tank is full? If the system was faulty would it happen all the time? Would it set a code?
If you end up with liquid gasoline into your evap system rather than just vapors, when the evap system tries to vent - to release gasoline fumes - it vents and releases raw gasoline. At that point you would be running very rich - basically flooding.
If the vent valve is damaged and/or if you overfill then you can be getting gas in there each time. After a while (like by the time you’re down to 3/4 tank) you’re not getting liquid fuel into the evap system anymore, and whatever is in there has been burned/evaporated.
That’s my guess anyway.
And, if it turns out that the evaporative emissions system was damaged by overfilling the tank, the warranty will not cover this bit of owner negligence.
If you are about to set off on an expedition across the Gobi desert, it might make sense to squeeze every last drop possible into the tank. However, for the typical US driver who passes gas stations many times per day, this practice is really unnecesary and can ultimately be quite expensive. If the carbon canister needs to be replaced, that can easily cost $300-$400.
This is why car makers put warnings on the gas cap about not overfilling the tank.