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Problems with topping off tank?

I was curious as I accidentally filled one of my trucks from a gas can to where the gas was almost up to the gas cap. I would have spilled gas had I not been paying attention. I know they always say not to top off at the gas station. What harm could this do to the truck?

I live a ways from town and use one of my gas guzzler trucks to move firewood. Since it would burn several gallons driving to town and back to fill it up, I usually just add a gallon or two at a time after using it. I drive the 50+ mpg Geo Metro to town most of the time and just take the empty gas cans with me when I am about to fill it up and fill them at the same time.

Since the gauge doesn’t really move when only a gallon or two has been burned, I don’t have a good way of knowing exactly how how much gas it can take. There are times where I have a lot of idling in 4WD low range and this burns more gas than you would expect.

Anyway, I was just wanting to see how bad of a thing to do this is. If bad, I may just add a gallon when the gauge starts to drop and repeat as needed. I like to keep all my tanks close to full so there is less air space for oxidation and condensation to occur and also I am currently concerned the the situation in the Middle East could take a dramatic turn for the worse anytime.

It will probably work out fine. Condensation in the tanks is not as big a problem in the newer vehicles compared to older vehicles, 60’s through 80’s era, You are concerned, I am concerned, evidently you have a plan B, sure you know better than I what is prudent.

It’s no longer necessary to worry about condensation in the tank for a couple of reasons.

First of all, the ethanol in today’s gasolines breaks the surface tension in the water molecules to allow them to mix with the gas. The turbulence of normal driving promotes the mixing.

Second, fuel systems have changed. In olden days, fuel was drawn from the tank by a pump mounted to the engine. This caused reduced pressure in the fuel line, creating an ideal environment for vapor lock to form in hot weather and for water molecules to separate from the fuel causing fuel line icing in cold weather. Today’s systems are complete sealed from the gas pump, which is now in the tank, to the injectors, and operate at pressure typically in excess of 40psi. That is a deterrent to vapor lock and to icing.

Tanks themselves are pretty rustproof these days, many not even being metal. Corrosion protection has come a very long way over the decades. You need not keep your tank topped off.

Regarding your concern about damage from overfilling, that happens to some vehicles because the gas can find its way to the charcoal canister and saturate the bed. Since the gas tank needs to breath in through the charcoal bed as gas is pumped out, that can cause a vacuum to form in the tank, preventing the pump from drawing gas out and even causing premature pump failure. Modern systems are designed to prevent this, but without knowing what vehicle you overfilled, it’s impossible to offer an opinion.

If, per chance, you did saturate the charcoal bed, you’ll experience the symptoms of fuel starvation once you get some miles on it. Power reduction and eventually maybe sputtering to a halt. You can temporarily relieve it by removing and reinstalling the fill cap.

I hope this helps.

About overfilling. With older vehicles … say early 1990’s and before … overfilling might cause some problems with the charcoal canister, but if you drove the car right away enough miles to bring the fuel level back down to below the charcoal canister level it usually wouldn’t do much in the way of permanent damage. On those cars there’d be an evap tube from the top of the tank to the canister, and (through a simple coolant temperature actuated valve) from the canister to the intake manifold often via the throttle body. That way gas vapors at the top of the tank would be drawn in and trapped in the canister, then when you drove they’d be sucked from the cannister into the engine and get burned up.

With newer cars the EPA won’t allow such a simple system I guess. With the old way, there could be a leak in the evap tubes, or just the gas cap seal could leak, allowing raw gasoline vapors to vent into the air, and there’d be no way for the engine computer to tell. So newer cars have a more sophisticated system. They are constantly testing for evap leaks – that’s how they can tell the gas cap is loose – and in order to do this they use various pumps, electronically actuated valves, etc. And those it seems from comments here don’t take so kindly to being saturated with gasoline or over-pressurized due to over-filling. You could search this forum for “evap purge valve” and maybe get some ideas what problems might ensue when overfilling.

Gotcha! None of the vehicles here are too new. The truck in question is a 1997 F-250 Light Duty with the 4.6L engine. This is basically a beefed up F-150 from the same year. This truck has some oddball parts that only it uses. I like the truck for the purpose it is used for but would not buy one because of the odd parts issues I have encountered. This is the one with the 7 lug wheels.

The other vehicles of interest are a 2000 Chevy S-10 with the 4.3L, and a 1992 and 1994 Geo Metro with the 1.0L engine. The 1994 is the “advanced emissions” model which is somewhat of an oddball compared to the rest of these. It is kinda like dealing with the 1997 F-250 in some ways but not a biggie.

Either way, it sounds like overfilling is a bad idea. I will be modifying my gas can fills to not bring the gauge all the way to the top.

Because this is a closed system and there is no need to weorry about moisture entering the system.
Why not just keep the tank at the 3/4 level. If you’re hauling wood and notice the guage below the 3/4 level, then add a couple of gallons when you’re done.


I think that is pretty much my plan. The gauges all stay at full until a couple gallons are burned so I will just add a gallon each time the needle starts to drop.

I guess moisture isn’t a hug concern. My main reason was to have all the gas I can in case something blows up in an oil producing region. This looks more probable by the day if you look at this news.

I don’t know if this will make sense but.

Gasoline shelf life is not what it used to be. If you fill up today with 20 gallons and only use 2 gallons every week…by the time you dilute that old gas enough it will be 3-4 months old.

Why not just buy 4 …5 gallon cans fill them up as a back-up. Then every 60 days empty the cans into both vehicles and drive the Gas guzzler to town with the cans to refill and top off the guzzler.


True! All the alcohol seems to really hasten the demise of modern gas. One thing I have found with things that sit but get used every so often is to keep them TOTALLY FULL. That way air doesn’t get into the gas with moisture and all. I am sure a sealed system on a car is different than a chainsaw or generator but that is my luck with those simple items. I do tend to take a decently long trip in the truck every 3-4 months at least that uses a lot of the gas so it does get refilled from time to time.

AS for the cans, I do try to cycle them through every 60-90 days in a rotation, if not sooner. The cans get used for mowers and other devices around the farm that need gas and get refilled a couple at a time when I get gas in the car. So far all this seems to work pretty well.