I fill my gas tank to 100% but I don’t pump it again and overfill it like most people do after the pump stops. I then proceed to start the car. Initially it revs up and starts fine but when it revs back down to idle it does a rough idling (and sometimes it stalls). To get around the stall, I would restart and rev the engine to 3000 rpm for about 2 mins and it would be fine enough that I can drive away without it stalling but after a mile the rough idling stops and everything is back to normal. I replaced the PCV valve and it worked for about a year but it all came back again. I am stumped and every mechanic gives me a different reason and none of the reasons are even definite. It’s one of those “lets try this and see what happens”. One more thing, the car runs fine. It only happens when I pump more than 75% of the tank.
Sounds like a problem with the EVAP system, what specifically I don’t know.
As a test, after a fill up, try pinching off or blocking the vacuum purge line that draws vapors from the EVAP canister into the intake manifold.
If the engine starts and runs well then you know the problem is coming from the EVAP system.
This is only a temporary setup. Keeping that line blocked will disable the EVAP system and trigger the check engine light.
I would do what circuitsmith suggested. It seems likely that your evap system is ending up with liquid fuel in it. Personally, I’d be tempted to pull the vacuum line, plug it, and cap the port on the throttle body - again just for diagnosis. If you do it the next few times you get gas and then just reverse it after you’ve driven a while until the next fill up this should tell you. You could also try not filling all the way a few times. Just go to 3/4 tank or so. What year is the car?
Sorry for the late reply guys. I just got done driving across country to NY. For cigroller, when I fill 3/4 tank I don’t experience this problem. The car is 2007 Toyota Corolla S. I will try the pinching set up and will let you guys know. It might take a while since I’m in NYC now and don’t have a driveway to perform this.
I’m going to go WAY out on a limb here and make a wild guess. I might ne nuts.
The EVAP system contains a number of valves that direct the flow of fumes.
One is the “purge valve”, which is activated by the ECU when you start the engine to allow the fumes collected by the charcoal canister to be drawn into the engine and burned.
There is also a diaphragm operated valve at the top of the tank that opens via pressure created in the tank’s airspace when filling the tank and allows the vapors to flow into the charcoal canister and then to the vent hole next to the fill pipe.
In addition, there is a float valve by the top of the tank that closes the vapor flow to the diaphragmatic valve when the tank fills and raises the float.
I’m going to posit a wild guess that the float valve is sticking open when the tank is filled, allowing some raw fuel to travel through the diaphragm valve and into the charcoal bed, temporarily saturating the charcoal and preventing the engine from breathing in. And when you only fill the tank 3/4, the fuel does not flow through the sticky float valve, no fuel flows to the charcoal bed, and the ability of the engine to break in is not temporarily constrained by raw fuel in the charcoal bed.
In short, I think that float valve that allows the fumes to vent through the charcoal bed is sticking open when you fill, allowing raw fuel into the charcoal bed. Nothing can breath through a wet hanky. Or a wet charcoal bed.
You may want to print this and bring it to a good shop. I no longer have access to the EVAP system diagram for your car, but any shop should via the repair database they subscribe to. With the diagram to follow, my theory should become clear.
I haven’t bumped a thread in years, but this one has my curiosity.