Consequences of topping off fuel tank.
- Thread starter Papasonic
- Start date
J T
New member
I've always wondered about that myself. It seems like this sentiment has been shared on car forums for many years. You'd think by 2012 auto manufacturers would have designed a fuel pump that didn't depend on a bunch of gas in the tank to keep it cool.
Oh noes! I just topped off my fuel tank this morning after running it all the way down to, nay, past the low fuel light. I just cant win with this car, can I?. 
I was always under the impression that the fuel pump was cooled by the fluid passing through and an almost empty tank (certainly not 1/4...) wouldn't harm it. I drove my last car down to the light most of the time, most people I know let it get fairly low, and I've done the same in this car. I've never seen a single problem related to driving with low fuel. Not counting completely running out of course.
I was always under the impression that the fuel pump was cooled by the fluid passing through and an almost empty tank (certainly not 1/4...) wouldn't harm it. I drove my last car down to the light most of the time, most people I know let it get fairly low, and I've done the same in this car. I've never seen a single problem related to driving with low fuel. Not counting completely running out of course.
mitdr774
also a dinner prick
The pump does not burn up, the internals simply overheat and either short out or break. The fuel pump is housed in a cup that is always full of fuel. This is to keep the pump cool. The pump has to be kept cool somehow and if it is inside a tank there really is no other option. If you ever notice on modern OEM fuel pumps there is a small hose that comes off of the top and runs down into the bottom of the cup. This hose is connected to a venturi (sp) that draws in fuel. This happens at a greater rate than the fuel is pumped out of the tank. As a result the cup is always full of fuel to cool the pump and feed the engine (the pump draws its fuel from the cup as well). If you run the tank to low the fuel itself can become to warm to effectively cool the pump.
sx sonic
New member
It is true that the pump is cooled by fuel as many others have stated. It is a pump that runs non stop as long as the engine is running and motors heat up and heat is bad for electronics so why wouldn't they utilize the liquid that is in the tank to assit in cooling the pump.
I've replaced a few fuel pumps and I've noticed a trend in premature fuel pump failures. The owners rarely had more than a 1/4 of a tank of gas. In other words they were the individuals who drove everyday with their gas light on and the extra thermal loading shorten the life span of there pumps.. most other pump failures I saw were just do to a long life..
That said I do not worry about running my tank to E and refilling (which is what I do). The thing to avoid is always haveing a low tank, the pump MAY last but its lifespan is being significantly cut into.
-tyler
I've replaced a few fuel pumps and I've noticed a trend in premature fuel pump failures. The owners rarely had more than a 1/4 of a tank of gas. In other words they were the individuals who drove everyday with their gas light on and the extra thermal loading shorten the life span of there pumps.. most other pump failures I saw were just do to a long life..
That said I do not worry about running my tank to E and refilling (which is what I do). The thing to avoid is always haveing a low tank, the pump MAY last but its lifespan is being significantly cut into.
-tyler
SSpeckerwood
New member
sx sonic
New member
SSpeckerwood lmao. I'll ask my math teacher that wends and get back to you haha.
I wouldn't call it ocd it takes like a few secs to get to a whole number and then I don't have to deal with coin change if I pay cash and balancing the checkbook is easier if I go that route
I do have a phobia of pennys thou. I hate them and want them all to be melted down and thrown into a grand canyon haha.. uggh change so little value can take up so much room!
I wouldn't call it ocd it takes like a few secs to get to a whole number and then I don't have to deal with coin change if I pay cash and balancing the checkbook is easier if I go that route
I do have a phobia of pennys thou. I hate them and want them all to be melted down and thrown into a grand canyon haha.. uggh change so little value can take up so much room!
rdss
Junior Member
Aircraft have been using fuel to cool hydraulic lines for a long time, it's simply! Free cooling through something you're going to use to power your vehicle. Fuel running through the pump is being "pumped" causing heat so that is false as well and this pumping is why people prefer a return less fuel system that way the fuel isn't heated and dumped back into the tank to be repumped and heated again.
LS1notFOUND
New member
No matter how low I think I've gone, there always seems to be 2 gallons or so in the tank when the little fuel light comes on. I've always kinda did the fill as much as I can click click click and so far haven't had this issue in 10K miles of service.... "ready CEL"
FREEYOURMINDSS
New member
Topping off is dissuaded for two main reasons:
1.) Excess fuel is drawn into the emission system, primarily the vapor canister, which poisons the charcoal inside and sets an emissions code necessitating the replacement of said canister. You can't pass any kind of emission inspection/check with this code being thrown. Most cars still use a vapor canister, including the sonic.
2.) The more fuel that gets squeezed into the tank after the pump shuts off the more fuel is siphoned back into the gas pump after shutoff, effectively wasting any money one is trying to save by topping off.
This is the reason almost all vehicles have stickers inside the fuel door that states "Warning: Do not top off".
All manufacturers are going to returnless fuel systems because not only are our electronics and pumps fast and cheap enough to implement this type of system, but the manufacturer saves the cost of running just one fuel line instead of two for every vehicle that uses this system (cruze, sonic, etc..). Multiply that X hundreds of thousands of vehicles and you get a significant reduction in cost of production.
And for the sake of argument, your fuel pump will be "happier" if one keeps the tank 1/4 full or more. The pump works harder (uses more voltage) the lower the fuel level is.
Any experienced/educated mechanic, automotive fuel systems textbook, or 2 min. of research on the internet will confirm this.
1.) Excess fuel is drawn into the emission system, primarily the vapor canister, which poisons the charcoal inside and sets an emissions code necessitating the replacement of said canister. You can't pass any kind of emission inspection/check with this code being thrown. Most cars still use a vapor canister, including the sonic.
2.) The more fuel that gets squeezed into the tank after the pump shuts off the more fuel is siphoned back into the gas pump after shutoff, effectively wasting any money one is trying to save by topping off.
This is the reason almost all vehicles have stickers inside the fuel door that states "Warning: Do not top off".
All manufacturers are going to returnless fuel systems because not only are our electronics and pumps fast and cheap enough to implement this type of system, but the manufacturer saves the cost of running just one fuel line instead of two for every vehicle that uses this system (cruze, sonic, etc..). Multiply that X hundreds of thousands of vehicles and you get a significant reduction in cost of production.
And for the sake of argument, your fuel pump will be "happier" if one keeps the tank 1/4 full or more. The pump works harder (uses more voltage) the lower the fuel level is.
Any experienced/educated mechanic, automotive fuel systems textbook, or 2 min. of research on the internet will confirm this.
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LS1notFOUND
New member
Return-less systems are generally higher pressure too do not bring heat which makes vapor (better emissions), back to the gas tank, thus keeping fuel temps cooler. This actually also keeps the pump running at one constant state rather than adjusting at the return side. Its a win win!
Swordsmith
242hp/252 ft-lbs BNR EFI
Fuel has weight. If a car is designed so it should always be kept at least 5 gallons full, then you're hauling around an extra 40 lbs of weight at all times. That in turn impacts your MPG.
Driven logically, which is to say, fill tank, drive till you only have a gallon or so left, repeat, you'd spend nearly 1/4th your driving time at less than 1/4th a tank, lowering the average weight of your car, thus raising average gas mileage. Sure, by a tiny fraction... but multiply that by the number of cars on the road, and that fraction would add up to millions of gallons.
Further, it costs fuel to get to a station, and to get back up to speed again after stopping. Fill at half tank means you do this twice as often. Again the gain is miniscule on a per driver basis, but millions of gallons saved if everyone in the country does it.
Wear and tear on the fuel pump is silly, the thing should be designed to run at any level of tank, and probably is, but if it isn't, it's going to be covered by your warranty. If GM has to cover thousands of pump replacements every day because people are driving their cars logically, that will encourage them to design a more robust pump. However, I've driven cars using the "low fuel light" plan for 30 years, and haven't noticed an unusual rate of pump replacement (two pumps in 30 years) so I don't really believe this is a problem, unless it is a new one. In which case, I really want to punish whatever bean counter thought a cheaper less robust pump was a good idea.
When exactly to fill up depends on your situation, it might make sense to fill sooner if you're in some back country area where you might have to drive miles to find a station, and you happen to be passing near one at 1/4th tank. But if you're somewhere where stations are plentiful, there's no real reason not to run down to Low Fuel light every time.
Driven logically, which is to say, fill tank, drive till you only have a gallon or so left, repeat, you'd spend nearly 1/4th your driving time at less than 1/4th a tank, lowering the average weight of your car, thus raising average gas mileage. Sure, by a tiny fraction... but multiply that by the number of cars on the road, and that fraction would add up to millions of gallons.
Further, it costs fuel to get to a station, and to get back up to speed again after stopping. Fill at half tank means you do this twice as often. Again the gain is miniscule on a per driver basis, but millions of gallons saved if everyone in the country does it.
Wear and tear on the fuel pump is silly, the thing should be designed to run at any level of tank, and probably is, but if it isn't, it's going to be covered by your warranty. If GM has to cover thousands of pump replacements every day because people are driving their cars logically, that will encourage them to design a more robust pump. However, I've driven cars using the "low fuel light" plan for 30 years, and haven't noticed an unusual rate of pump replacement (two pumps in 30 years) so I don't really believe this is a problem, unless it is a new one. In which case, I really want to punish whatever bean counter thought a cheaper less robust pump was a good idea.
When exactly to fill up depends on your situation, it might make sense to fill sooner if you're in some back country area where you might have to drive miles to find a station, and you happen to be passing near one at 1/4th tank. But if you're somewhere where stations are plentiful, there's no real reason not to run down to Low Fuel light every time.
mitdr774
also a dinner prick
I have replaced enough pumps over the years as a mechanic that I have no desire to risk the life of my fuel pump. Most of them went out after warranty had expired. I drive right by the gas station I use everyday on my way to and from work. I use no measurable extra fuel to stop in. The miniscule amount of increased economy is not worth it to me.
nocoolnames
New member
I always fill at 1/4 tank or more because I like the reserve for emergencies. What happens when you get up one day an gas is being rationed and the long line makes you late? It happened in the 70s and very well could happen again. What if power goes out an no one can pump due to pumps running on electricity. Not all stations have generator. Plus the fact that it helps your fuel pump last longer is another bonus. I'll suck up the 1-2 mpg I loose I'm exchange for always having some gas for when I need it.
