Never go past 1/4 tank?

[Teufelaffe]

I know that many people recommend that you do not let your gas tank get below a quarter full. The idea being that this will help keep the sediment and schmutz that settles in the bottom of the tank from getting into your fuel system. However, I have talked to a couple of people, including my brother-in-law Jim (who is a mechanic), that say this is dead wrong. A couple of years ago, when my wife and I were visiting her family for a few days, he showed me an example using two jars and some sugar water. It went like this:

He filled both jars with sugar water and sealed them both (to cut down on evaporation). Every couple of hours, he would stop by and siphon some water from the top of each jar. When one jar got down to a quater full, he would refill it with more sugar water. The other jar he would let get pretty close to empty (but not all the way!) before he would refill it with more sugar water. The day we were leaving, he showed me the two jars. The jar that he had let get almost empty each time, looked pretty clear. There was a bit of cloudyness near the bottom of the jar and a thin layer of sugar at the very bottom. However, in the jar that he never let get below a quarter full, almost the entire jar was cloudy and there was a full half inch of sugar at the bottom of the jar.

Jim said that this same thing happens in your gas tank, albeit at a much slower rate. He said that for anyone who's going to have their car for more than four or five years, that the whole "don't go below 1/4 tank" thing is a very bad idea. Not only do you end up with more overall sediment in your tank, if you should happen to dip "too low" in the gas level after you've been driving a few years, you're almost guaranteed to get a considerable amount of crap running through your fuel system. He recommended that you should let your gas tank get pretty low before filling it, and just use fuel system cleaning additives every other oil change. According to him, that will be much better for your fuel system over the long haul.

By the way, he still drives the '78 Toyota pickup he bought when he was a teenager. 250k miles on it, and still going.

I'm wondering if anyone else has heard of this?

[JonRav4]

I wouldn't go below a 1/4 tank in our '96 because the tank is disproportionate to the gauge. 1/4 is not really 1/4 capacity. At 1/4, we run our of gas fast.

As far as dirt, use a cleaner occasionally and replace your fuel filter when suggested. Otherwise, go as low as you want. Any dirt will end up in the engine eventually. The sloshing of the gas while you drive mixes it all up.

OTOH, when storing a vehicle you should fill the tank up to prevent rusting.

[clutterbucker]

I may not know the real reason concerning RAV4's about this rule, sinc emy mother is the one that owns one. I am a BMW driver (old ones...cheap ones =). I have heard two theories including reasons for why you shouldn't drive below 1/4 tank of gas. One being that the added pressure of the 3/4 tank on top of the bottom quarter helps push the fuel through the fuel pump, thus making everything run more smoothly. The second, and equally reasonable theory is that when you extract fuel fromt he air-tight confines of the fuel tank, it is left with a vacuum. As everyone knows, it's hard ot suck the fluid out of a glass bottle. The same theory works here. Your fuel pump(s) struggle to get the remaining gasoline out of the tank far more than they would on a full tank. This kills your fuel pump much more quickly.
I think, personally, that the second is more plausible but that both may be true.

[Bossman]

I'm no mechanic by any stretch of the imagination but I heard of one other reason not to run below 1/4 tank. I 've heard it said that doing so is very injurious to your catalytic converter. Why? Beats me but I've heard this reason far more times than any other.

[viper03860]

The fuel tank and fuel system before the fuel pump already have a vaccum in it because the Federal Laws require that fuel systems not release raw fuel vapor into the air. The Vaccum is regulated by engine vaccum and all of the vapors are burnt in the engine.

The 1/4 Tank Rule does not apply to any new cars, if the fuel system has a filter, the filter will catch most of the dirt and rust. Just be sure that you maintenance the vehicle to Manufacturers Specs.

[typesix]

It can be bad for the converter if one runs too low on fuel and the engine misfires while gasping for the last amount of gas. Another reason for not running low during the summer is that the gas gets warmer returning from the engine(fuel pump supplies gas at 100% capacity and unused portion returns to tank) and having more gas allows for less chance of vapor lock(although less likely to happen with today's fuel injected cars). During winter, running with more gas lessens condensation in tank.

[flyguy123]

I think its a wise tale. Your fuel pump has a screen covering the end of the pick up line, then you have a fuel filter that filters prior to being sent to the engine. Now I have seen the screens clog on the fuel pump but only from years of setting. I run all of my vehichles down to E and in the 13 years of driving I have yet to have problems.

[RIP]

Ditto to flyguy123 but, make it after 30+ years of driving.

[RavSav]

I always try to keep my RAV4 full or at least half full of gas because:

1. It's better to have it and not need it than to need it and not have it;
2. It keeps the tank from rusting;
3. Pressure;
4. I only run it close to empty when I'm traveling long distances (200+ miles), meaning driving continuously until almost empty.

Great thread and interesting reasons BTW.

[iampbdb]

You know! The darn thing sucks off the bottom anyway. !/4 tank or not. And drive a car once, you'll see, it bounces. Shake up that test jar of sugar and suck off the bottom. You're gonna get all the junk that's in there mixed up and sucked out.

One main reason for keeping your tank full is that there's less air in it, (if the space is taken up by gas), air that has moisture in it that could "condense" and cause water in the bottom of your tank, (that, by the way, gets sucked up right away), 'cause it sucks from the bottom.
IAMPBDB

[Brian R.]

Keeping your tank 1/4 full or more is throwback advice from when engines had carburetors and suction fuel pumps. Then vapor lock and pressure to the pump meant something.

Not filling your tank when the tank truck is filling the gas tanks at the filling station is the best preventative measure you can take to keep sediment out of your tank. Since your fuel pump picks up from the bottom of the tank, and puts out 40-50 psi, you will not see any substantial increase in fuel pressure from increasing the amount of fuel in the tank. Modern push-type pumps have no problem with vapor lock. They pressurize the fuel lines instead of sucking the gas from the tank as did the old style suction pumps.

There is no air in your fuel tank, no water from the air, and therefore no rusting at any stage of tank fillage, except totally empty. viper03860 is mistaken. The tank with gasoline in it holds pressure, not vacuum, and filling the tank pushes all the air out (from opening the gas cap) as it is filled. The pressure in the tank is supplied by the vapor pressure of the gasoline. That is why you get escaping gas vapor when you remove the gas tank cap. You will get a DTC (P0440) when your cap cannot seal and the system is not kept pressurized. Try it. Loosen your gas cap and see that you get a "Check Engine" light. Being under pressure, the only thing that can happen if there is mass transfer is that vapors escape, not water or air to get in. If you cap doesn't seal, then you will have the possibility of air getting in to the tank, but you should investigate the "Check Engine" light and the P0440 DTC and never have this problem. Winter or summer has no effect on this. In fact, winter air is much more dry than the summer air. I think typesix is thinking of a frozen gas line from summer rain being carried into the winter and freezing in the gas line. Don't fill your tank in the rain and you should not have this problem.

Running out of gas is not going to hurt your cat converter. Running rich may overheat it, but running lean is not a problem, particularly for the few seconds your engine is running out.

[mmcpeck]

Wow, there are a lot of theories and ideas on here, if I didn't know any better, I would think I was reading a boating forum or one on older cars.

I would agree with everything that Brian R. just said, modern fuel systems have almost no exposure to air and don't vent to the atmosphere. Going below 1/4 tank will do absolutely nothing to your car or fuel system, and also agree that taking your tank to Empty is actually better. In all the years I've worked on cars, I would have to think really hard to think about any rusted fuel tanks I have ever seen. Boats are a different story, I see them a lot on boats, but we're talking a whole different environment.

There is no chance of vapor lock in a modern pressurized fuel system, and fuel injection doesn't run off gravity feed from the tank, or mechanical vaccum diaphram fuel pump, so extra fuel is not helping anything run better.

Cars are made now to be almost idiot proof, if the average person had to worry about any of these issues, or even know what they were, you'd need to give a course in owning a car.

Change you rfilter every 30K miles and use a fuel additive every now and then and you will fine for the life of the car, no matter when you fill or how empty you let it get. And I can confidently say you will never have a rust problem in your fuel tank.

[RIP]

...and aren't most fuel tanks plastic now? What's to rust?

[G.A.S.]

wives tale.
You cant use sugar as a representitive of gas.
the properties are different.
anyways.
The only thing I can come up with on the 1/4 tank threoy is.
You dont want to leave your car just set for a long period of time with a close to empty tank.
This is so condensation will not build up within the tank. Hot to cold temps cause this.
If a tank is full, condensation levels are lower.
Gas is picked up from the bottom of the tank continously.
Water settels imeditly to the bottom, dirt will mix with gas but the filters take care of that.
Bottom line.
There is no ruel of thumb in what level of fuel is best in a tank.
When I was a kid we sometimes went gas station to gas station and never got above a 1/4 tank.
40 years of driveing and never and issue and never herd of an issue.

worked in a service garage for to many years

[mmcpeck]

There isn't going to be ANY condensation in the fuel tank no matter what level you leave it at....again this isn't a boat or a 30 year old car that vents the fuel system to the atmosphere, it is a Completely sealed and monitored system. The ECM monitors the EVAP system, fuel vapors are stored in a charcoal canister and then released back into the fuel system to be burned when the engine is at cruise.

The #1 cause of the Check Engine light coming on that I see (85% of the time) is people who filled up their gas tank with their car running, most likely to keep the heat or A/C running. When this is done, the EVAP system loses all pressure, while the ECM is turned on, so it does it's job and notices pressure drop and flips the Check Engine light on. The EVAP system is an emissions system, so if anything inconsistent is detected the Check Engine light is turned on to alert the driver to a possible emissions problem.

The same can happen from a faulty fuel cap or obviously any other leak in the EVAP system, as designed.

The point being there is no "air" in your fuel tank at any point. Not in modern cars.

Read Brian R's post above again and pay attention to what he says about fuel vapor pressure, it is 100% correct. This is the positive pressure that is released when you open your fuel cap, it is not "air" rushing in.