Is it really bad to top off the gas tank?

[Patman]

I've often heard that you should never completely fill your gas tank to the absolute maximum and a quick google search that I did came up with a few people saying something about it causing damage to the charcoal canister. Do modern cars even have these as part of their emission controls anymore?

I haven't been completely topping off my tank, but I really want to start doing it as I think my MPG calculations would be a lot more accurate this way. This would help me fine tune the calibration on my Scangauge and once that is done I really wouldn't need to completely top off the tank anymore as I could then rely on the gauge better.

 

[Eddie]

I believe the top off problem is very real but, we are talking about the additional quart or two. This amount is not going the cause issues with the mileage calculation. My 2007 has a charcoal canister and the associated valves to go with it. I assume most modern vehicles have this as part of the evaporated emission control system.

 

[Artem]

It's not good to do so. You need to leave some room in the tank for vapors to expand safely.

I also don't see how topping off the tank will help you with measuring MPGs more accurately, unless you literally keep fueling until you see fuel almost coming out of the filler neck

When i was calibrating my Scan Gauge 2, i used the same gas station / pump for 2-3 fill ups and calibrated the gauge.

From then on, it was 99% accurate. I'd be on a business trip, filling up at any name brand gas station and as the pump would click and shut off at say 11.8 gallons for my 07 Civic, the Scan Gauge would say i need... 11.8 gallons to fill up. It would fluctuate from 11.7 - 11.9 but i feel that's still DEAD ON accurate if you round the remaining numbers up or down (11.850 gallons for example)

DON'T DO IT!

 

[Corvette Owner]

Patmman,

How accurate is your Corvette's Average Mileage on the DIC (Driver's Information Center)?

Usually, GM's average mileage is a little optimistic. It calculates fuel used by fuel injection pulses.

 

[ChiTDI]

Patman - I don't know about GMs, but Toyotas get poisoned from overfill and the charcoal "canister assembly" approaches a thousand dollars for replacement.
Ask me how I know.

 

[Trav]

Modern canisters are located in the rear of the car underneath or behind the bumper these days, they can be a pretty complex affair with valves and solenoids and so on.
They don't like raw fuel and some are expensive so its safest to fill the tank only till the pump shuts off.

One thing that can happen if it does get filled with fuel is unmetered fuel can get sucked into the engine when the evap runs, not a good thing.
Getting soaked once or twice is not likely to damage the unit but i wouldn't make a habit of topping off the tank.

Hot summer days when the tank is topped off and parked without burning any fuel off is usually when this will happen, the expanding fuel has to go somewhere.

 

[Quest]

It is fur-real if you are dealing with cars like Corolla (circa 99 and onwards) or Hondas (since 2K), where frequent top-ups to the brim (thus leaving no room for expansion) will ruin the Evap canister and/or check valve, and will cost you quite some serious dollars as they throw you a CEL and aircare centre failed you right on the spot.

Q.

 

[qr25de]

First click whilst filling up at 10-15% pump speed. The first click at a reasonable speed isn't actually "FULL" but has met the appropriate vapour pressure and liquid. If you keep clunking away like most people do AND if you jam the pump to full speed you end up in the "over fill" situation that you're talking about.

 

[cchase]

On long trips, I religiously try to get every last drop into the tank of the truck. If I drive well I can usually get about 700 miles on a tank. It is 18 years old and has the original charcoal canister. I'm thinking maybe it's a practice I should stop.

 

[D189379]

I always top up my vehicles and have never had an issue in 20 years of driving.

 

[surfstar]

YES ITS BAD
http://www.epa.gov/donttopoff/

Please don't do it for the extra 10 miles before you have to refill. Also, although your tank to tank MPG may vary slightly, you'll still be accurate over many tanks without topping off.

 

[KrisZ]

Originally Posted By: Trav
Modern canisters are located in the rear of the car underneath or behind the bumper these days, they can be a pretty complex affair with valves and solenoids and so on.
They don't like raw fuel and some are expensive so its safest to fill the tank only till the pump shuts off.

One thing that can happen if it does get filled with fuel is unmetered fuel can get sucked into the engine when the evap runs, not a good thing.
Getting soaked once or twice is not likely to damage the unit but i wouldn't make a habit of topping off the tank.

Hot summer days when the tank is topped off and parked without burning any fuel off is usually when this will happen, the expanding fuel has to go somewhere.


I think that's what happened to my dad's 06 Altima. He said that right after filling up, that car stalled at the next red light and wouldn't restart. It re-started after a while but wouldn't idle, he had to rev it to keep it from stalling. It turned out that there was a seal in the intake that blew out, most likely from fuel ingestion and was allowing unmetered air into the engine.
He said that he fills to the brim every single time, of course when I said that it is not good he wouldn't listen because all that time everything was OK, so in his mind this incident cannot be linked to all those years of overfilling

. He was luck the canister was OK.

 

[MNgopher]

I'm a tank topper offer. Never had an issue with any vehicle and have always done it. Part of the trick is that while I may fill all the way up, I still end up using a gallon or so right after I leave the fueling station. Space remains in the system prior to thermal expansion becoming an issue for me.

I don't spill gas all over (if a pump is too agressive, then a full top off isn't possible). We don't use vapor recovery equipment that could be fouled.

And for those who think there is little to be gained, my '97 Explorer takes 4 gallons past the first fuel pump clickoff (thats over 65 miles of additional range). Given the miles I drive, that's just over a days worth of fuel.

 

[Nick R]

I only do it to get an even number. So instead of $47.83 I'd just pump it to $48.00. Usually to the nearest $0.50c just to make quicken at the end of the month easier.

 

[mechtech2]

Every car is different.
Some respond with no symptoms, and others do indeed saturate the lines , vapor canister, and EVAP system. This is usually temporary, as vacuum and purging clean things out in a while.

Long term, a first click stop will give you consistent MPG readings.
Short term , like one tank, can vary.

 

[91344George]

Anecdotal evidence aside from those that claim they've never had a problem---

NEVER TOP OFF! WHEN THE PUMP CLICKS OFF STOP FILLING!!!!!

If you don't you eventually WILL do serious and permanent damage to the charcoal canister.

READ YOUR OWNER'S MANUAL IT WILL ALMOST CERTAINLY MENTION THIS.

 

[MNgopher]

When does the eventually start? Have a mileage number or timeframe in mind? Running vehicles between 7 and 25 years old with 125,000 to 198,000 miles, I'm just curious when I should expect to have problems.

Owners manual number one says nothing on the subject. Owners manual number two states to not overfill the system to prevent the possibility of over pressuring the system that could splash back on operator. Three clicks past the initial shutoff is fine (interstingly what usually allows for a full fill on pumps that have some control on them).

 

[Patman]

Originally Posted By: Corvette Owner
Patmman,

How accurate is your Corvette's Average Mileage on the DIC (Driver's Information Center)?


When I first got my Corvette I compared the actual mileage vs the trip computer mileage for the first 3-4 months and it was very accurate, to within a few tenths of a MPG.

 

[Patman]

Originally Posted By: Artem


I also don't see how topping off the tank will help you with measuring MPGs more accurately, unless you literally keep fueling until you see fuel almost coming out of the filler neck



I use the same gas station and even the same pump # every time, but we don't have those "nozzle locks" up here on our gas pumps, so we have to hold the handle down while filling up. I try to be consistent in my filling method, I start out filling the tank with the nozzle at full pressure and when I know I'm a few liters from the top, I'll ease back on the pressure and let it stop at that point. But I don't think that's consistent enough, because from one fill up to the next I might use slightly more or slightly less pressure on the handle, so the flow would stop at a slightly different spot. But if I were to keep topping it off until it was very near the top, I could definitely be more consistent.

Eventually once I get the Scangauge right on on the money, then I won't need to bother topping it up this way.

So I'm going to venture a guess that it can't be all that harmful to top it right to the top for just a few fill ups?

(FWIW, the vehicle in question is my Civic, I already have the trip computer in the Corvette for my MPG, so the Scangauge is attached to the Civic, so that's the one I'm going to be "topping up" in)

 

[Tegger]

Originally Posted By: Quest
frequent top-ups to the brim (thus leaving no room for expansion)

Gas tanks made since about 1971 (first year for EVAP) have an unfillable area in them which makes up about 20% of their total volume. It is impossible to completely fill a post-1971 gas tank unless you tip the car on its side at some point during the fillup.

As for the EVAP canister itself, it's theoretically possible to push raw gas into it through overfilling, but I've never actually seen that happen.