# "Don't fill tank to max on hot days" - still true? **URL:** \ **Category:** Factual Questions\ **Created:** [November 7, 2019, 3:25am UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105 "2019-11-07T03:25:15Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 7, 2019, 8:24pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/21 "2019-11-07T20:24:47Z")
> [@am77494](#): > > I understand your skepticism on my post and I really wish Tom (no longer with us) and Ray were around to give a definite answer. > > Here’s the next best thing - cartalk forums. Please read through this post and decide for yourself - [What gets damaged when fuel tank over-filled? - Maintenance/Repairs - Car Talk Community](https://community.cartalk.com/t/what-gets-damaged-when-fuel-tank-over-filled/74965) > > Thanks I’m not skeptical, I was just explaining why the average guy (like me) doesn’t know anything about this. ---
**Author:** ![QuickSilver](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quicksilver/32/7832_2.png) [@QuickSilver](https://boards.straightdope.com/u/QuickSilver)\ **Post date:** [November 7, 2019, 8:41pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/22 "2019-11-07T20:41:39Z")
> [@EdelweissPirate](#): > > Really, this is not something to worry about when fueling your car. As long as you’re not pumping gas into a burning car, you’re fine. 🙂 If I had a new car for every time that’s ever happened. ---
**Author:** ![Daylate](https://avatars.discourse-cdn.com/v4/letter/d/9dc877/32.png) [@Daylate](https://boards.straightdope.com/u/Daylate)\ **Post date:** [November 7, 2019, 9:06pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/23 "2019-11-07T21:06:57Z")
[QUOTEI’ve always filled that way on every car I’ve every owned or used. Brim full every time. Never had an engine light, never had an emissions problem, never had an overflow problem. I’m sceptical that it is a real, widespread problem.] [/QUOTE] I’ll second that post. I always top off the tank as far as possible, and have never had a problem. If I stopped pumping when the gas nozzle first clicks off, there would be two or three gallons of space left in the tank, and the capacity would be reduced to 11 gallons from the published 14 gallons. Maybe because I live out here in the PNW where it almost never gets to 90° F, this makes more sense here than back in St. Louis or some such hellhole. ---
**Author:** ![Willcross](https://avatars.discourse-cdn.com/v4/letter/w/b2d939/32.png) [@Willcross](https://boards.straightdope.com/u/Willcross)\ **Post date:** [November 8, 2019, 5:22pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/24 "2019-11-08T17:22:57Z")
I was a dealership mechanic for awhile. Every couple months I’d see a car with the charcoal EVAP canister flooded with raw fuel and we’d have to replace it. Not a common repair, but it definitely happens. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [November 8, 2019, 5:32pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/25 "2019-11-08T17:32:25Z")
I drive almost 10 miles from the gas station home. That uses over a half gallon from my Ford E250. (my best mpg in the city is 14.2). 16 mpg on Trips. Overfilling isn’t a concern because the extra gas is only briefly in there. ---
**Author:** ![Dr.Colossus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.colossus/32/9423_2.png) [@Dr.Colossus](https://boards.straightdope.com/u/Dr.Colossus)\ **Post date:** [November 8, 2019, 8:39pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/26 "2019-11-08T20:39:09Z")
> [@Dr.Colossus](#): > > So in my van, more than I’ve noticed w previous vehicles, the gas seems to froth or bubble up and clicks the nozzle off very early. If I go slow (like the half depressed lever slow fill) I can put gobs more gas in. > [@kanicbird](#): > > I had a car that if I pumped very slowly I could get another 4 gallons in it. > [@Daylate](#): > > I’ll second that post. I always top off the tank as far as possible, and have never had a problem. If I stopped pumping when the gas nozzle first clicks off, there would be two or three gallons of space left in the tank, and the capacity would be reduced to 11 gallons from the published 14 gallons. Given this seemly common occurrence–that full-speed pumping causes the nozzle to click off apparently early–can someone illuminate on exactly what is happening? What is the mechanism that causes the pump to self-stop? I presumed it was when gas touched the end of the nozzle…but now that I’m saying it out loud, that seems absurd. What kind of sensor would be required, and where would it be located on the end of the nozzle? If it’s simply a case of air getting in the mixture, what would be the ideal procedure? Allow fast-fill until the premature shut off, then slow fill until the first additional shut-off and stop then? ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [November 8, 2019, 9:00pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/27 "2019-11-08T21:00:04Z")
There’s a small air tub inside the nozzle. When gas backs up to the end of that tube, it changes the air pressure that is holding the lever open which shuts off. Note that this is usually purely mechanical. Next up: how a flush valve works in a urinal. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [November 8, 2019, 9:47pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/28 "2019-11-08T21:47:29Z")
I don’t force every last drop into the tank. My dad did that and wasted a lot of time. I get maybe another 3/4 gal (after the nozzle shuts off) in the tank. That’s full enough. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 8, 2019, 10:09pm UTC](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105/29 "2019-11-08T22:09:55Z")
> [@ftg](#): > > There’s a small air tub inside the nozzle. When gas backs up to the end of that tube, it changes the air pressure that is holding the lever open which shuts off. Note that this is usually purely mechanical. > > Next up: how a flush valve works in a urinal. The gas doesn’t even have to touch the nozzle. An increase in back pressure will do it. FWIW, I once filled my F350 to the brim early in the morning, and when I went out at lunch, diesel was leaking from the filler cap. [Previous page](https://boards.straightdope.com/t/dont-fill-tank-to-max-on-hot-days-still-true/843105.md?page=1)