How is this possible? I bought a new gas cap about a 1.5 years ago because of the gas smell from the rear of the car. The new cap was a Stant replacement. I got a 1/4 turn out the cap and it started clicking. I realized it wasn't sealing when gas poured out after a fill up and a spirited right turn. I could actually see the top of the filler neck with the cap on.

So I bought an original GM cap to replace the Stant that wasn't sealing and it would twist down a couple turns before the lock clicking. It's been fine and I haven't had the gas smell since. Well, this weekend, it been close to 100* and I drove to a car show on Friday. After about 50 miles of driving in hot weather, I got to the show and the car started to stumble as I was pulling in and moving 5mph or idling. I parked and the smell of gas was strong near the charcoal cannister. i started thinking that the cap isn't doing it's job again. I released the cap, the pressure (lots of it) released and gas was all over the filler neck opening. Here's the problem....the cap won't go back on. It goes on 1/4 turn and locks like the Stant replacement. Again, I can see the top of the filler neck with the cap on which means the cap is bottoming out on the filler neck, but isn't sealing.

Can anyone tell me why this is happening when the gas cap just came off the car?

 

Sounds like you have two problems. One is the evap system not doing its job. Normally it should allow the engine to consume the fumes and clear the canister to allow it to absorb more fuel vapors. The cycle varies over the years with different ECMs and PCMs but usually the purge valve is open during cruising to vent the canister. The valve or lines may be blocked. Id also check to make sure the heat shielding is still intact above the muffler. Even with it there the tank heats up quite a bit.

Second is that it sounds like the threads on the neck may be damaged or the neck is deformed. Inspect them to see if there is wear or deformation. Sometimes if the gas nozzle is leaned on it can deform the filler neck.

 

yea sounds like 2 problems.
For the pressure in the tank, did you disconnect and plug the vent line? If not its probably a clogged line or bad purge valve
As far as the fuel leaking out the filler. Is this only happening when you fill up in the morning when it's cool and it happens later in the day when it's hot out? Also are you purchasing a vented fuel cap or a regular one?

 

I seem to remember that there was a safety recall for something like this. If so, you can still get dealer to fix for free, as there's no expiration date for safety recalls. Check this site for more info.

 

https://www.thirdgen.org/forums/body...call-read.html

 

I had this exact same gas cap issue with the original factory tank & cap. In hot weather the cap could not be seated shut before it would click off. On really hot days it was so bad the gas door would rest on the cap.

To fix it I modified the threads on the cap. Used a large round file and deepened the gooves. Then used a flat file to reduce the tops of the threads a little. Kept test fitting the cap and filing until it all worked out, then went a smidge more.

Haven't had an issue with it since.

RBob.

 

Just an update to this issue....The gas cap not going on and sealing is because of the evap system issues. I still haven't resolved that issue, but because of the excessive pressure and heat buildup in the tank, the filler neck is expanding and not allowing the cap to seat. After the tank cools, the cap seats just fine. Once the evap system is fixed and the fuel isn't boiling and heating up the filler neck to expansion, it'll be fine.

 

Interesting. I'm also having the problem of not being able to get the cap on when it's hot. ....which unless I'm checking it in the garage, is always, because I have to DRIVE to the gas station! ...how did you determine it was the evap system? I mean wouldn't the heat be an issue regardless? Pressure would make the gasoline less prone to boiling, but the heat would still be present, expanding the filler neck, and thus the cap would still be hard to get on, right? (not that pressure is GOOD thing), but my point is that evap or not, maybe the problem is heat form exhaust, not an evap issue.

 

My car is still original with the factory exhaust and I didn't have this issue years ago. So, it must be something else. I did replace the fuel pump years ago and I'm thinking that could also be part of the issue. If the new pump runs hotter than the factory pump, it'll heat the gas more. Also, gas will heat when pressurized. If the vent isn't working properly, the tank continues to build pressure which builds heat.