Below a quarter tank - how bad?

Factual Questions
21

And this is intentionally so. This makes the needle much more sensitive on the lower part of the scale, where it is most important to have a sense of how far you can travel on the fuel remaining, and plan a fuel stop accordingly.

Not just as you used gas. There is a regular daily cycle of barometric pressure change that produces “diurnal pumping”, and of course the ambiant pressure changes about 1"/1000’ as you drive over hill and dale. The volume of air pumped in and out of a vented container is proportional to the empty space in the container. Most light aircraft fuel tanks are still vented, and SOP dictates filling the tanks when you are done flying for the day.

22

So, if one has an in-tank fuel pump, how high is the intake off the bottom of the tank? What are the tank’s dimensions? Knowing those two things will give you an estimate of the minimum safe fuel volume, ignoring sloshing. There are a multitude of answers given the different tank shapes and sizes that are out there.

Anyone know what year in-tank fuel pumps became de rigeur?

23

In my experience with fuel injected vehicles that normally use an intank fuel pump

the fuel also provides some of the cooling for the pump itself which in vehicles

that I have observed is relatively high on the module. That being the case ,

running 3 dollars of gas in your car all the time will eventually cause premature

failure of the pump . If your lucky , perhaps Rick or

Garyt will give you their more qualified opinions.

24

Thank you, that makes lots of sense. Consider my ignorance fought.

That fifteen dollar membership fee is looking like more and more of a bargain.

25

Forget about vapor lock. That went out with slide rules, even with a carbureted engine. And with modern sealed tanks, condensation formation due to low fuel level is extremely minimal. The mileage issue is sorta true: you can save a very small amount of fuel by running your car with only a little gas in it to save weight, instead of keeping it full. The problem here is the savings is pretty small and constantly keeping an eye on the fuel gauge is inconvenient.

26

From a personal safety point of view, you shouldn’t let your tank go below half-empty. We teach this in our Sexual Harrassment, Assault & Rape Prevention seminars. If you don’t ever come close to running empty, you’ll never run out of gas. When you do run out of gas, you expose yourself to unnecessary risk.

27

I have had one issue with a low fuel tank that no mechanic I have spoken to (3 or 4 now) has been able to definitively explain.

If I have a bit less than a 1/4 tank and I either; a.) accelerate hard from a standing start or b.) accelerate aggressively through a sharp turn, my car stalls and will not restart for 5 or 6 minutes.

I have assumed that the gas sloshed away from the pump momentarily and allowed it to suck air, killing the engine. Or the fuel pump got a little warm and tripped out and wouldn’t restart until it cooled sufficiently or … ?

All I know for sure is that if I have less than 1/4 tank I do not punch it off the line or take corners too quickly.

FWIW, I have a 2001 Saturn L300 V6.

28

OK first off all fuel tanks are vented. otherwise as you burned fuel you would suck a vacuum in the tank as you burned fuel. Think about it. Way back when the tank was directly vented, now it goes through a charcoal canister, which BTW does not remove any moisture.

As far as excess condensation from low fuel causing problems, I gotta tell you that in over 30 years in this business I have never heard of a genuine case, just FOAF stories. Now in the interest of full disclosure, I live in LA which is not the humidity capitol of the know western world.

As far as vapor lock goes, it is not as much a thing of the past as you might think. If you have a fuel injected car that has the main pressure pump external to the fuel tank, and a helper pump in the tank you can have vapor lock if the helper pump fails. I haven’t seen two pump system in about 12 years or so, but there are fuel injected cars on the road that have them.

Where is the fuel pump pickup? One of two places. Either flat on the bottom of the tank, or flat on the bottom of a small canister that is flat on the bottom of the tank. The purpose of the canister (about the size of a coffee can open at the top) is to hold a supply of fuel for when the level is low and the fuel sloshes. This also allows the car to start when the fuel level is low, and you park on a grade where the fuel runs over to a corner.

The fuel cooling the pump. yes it does, but it is cooled by the fuel flowing through it, it does not need fuel around it. As long as fuel is flowing trough it, the pump is being cooled. If the fuel flow stops, the engine stops, the pump stops, so no danger of overheating of the pump.

Now if you run the tank so low that the engine is misfiring, and just barely chugging along you are doing damage to the car. Misfire is bad, very very bad. Misfire destroys catalytic converters in very short order. If you run your car to that point you might wind up killing the converter trying to get to the next gas station.
If on the other hand you get gas before you get to this point, I can see no problems, and I have never seen an authoritative cite to the contrary.

Rhubarb If I was working on your car I would look for either a leak above the in tank pump (inside the tank of course) or pump that was not flat against the bottom of the tank, or a plugged fuel filter. In the interest of cost change the fuel filter first.

29

:dubious: Maybe if you’re driving a Hummer with a 12 gallon tank but most cars can go at least 60 miles on the last 1/4 tank. At least in the Eastern US I’m not sure if it’s possible to be more than 60 miles from the nearest gas station.

If you’re driving late at night and almost at your destination I’m sure it’s more dangerous to stop to get gas than to let your tank go below half and wait till tomorrow to fill up. And according to your recommendation, driving on the interstate would require nearly 1/3 more stops for gas than is necessary, and more stop = more danger both in crime and accidents.

30

I have a Chevy Trailblazer that I don’t allow to fall below half of a tank, but it’s for reason other than the OP’s questions.

In my experience, GM’s tank level indicator is not reliable below half of a tank. The first half goes slowly, and the second quickly. To fall below 1/4 courts the portential embarrassment of running dry going on a short errand.

I also have a Mercury Sable wagon, and the tank level is nearly linear. I can run it down to 1/4 without fear that the reading is false.

31

My Ford Contour’s gauge is not linear. The first 1/2 tank gets me around 200 miles, and the second half gets me a little over 100. Of course when you fill the tank it’s over F, I’ve never figured that one out.

32

I think the idea here is that actually running out of gas before getting to the gas station in the middle of the night is a risk that far outweighs planned daytime stops. I am not sure if I buy that, and I’m not exactly in the risk group, even though I tend to actually push my car into a gas station at least once or twice a year.

33

Only had two cars in my life and they were both the same model so not a great deal of experience but FWIW, my tank is rated as 35 litres. If i fill up bang on the empty mark it’s roughly 35 litres that goes in. However i rarely think about filling up until it’s well beyond the empty mark. My record so far is 39.04 litres.

I play this game of chicken pretty much every month coming up to pay day :slight_smile:

Incidentally, can anyone tell me how much warning i would get before my car would just stop or start damaging itself running on fumes? I’ve never noticed any difference in behaviour with the level i run it to. When it’s this low i’m rarely more than a mile or two away from a service station, although the whole fill a canister and walk back to the car thing is something i really don’t want to have to do. Am i playing a dangerous game?

34

About 0.1 seconds As soon as the engine starts to misfire from lack of fuel, you are in danger of converter damage. You might want to consider stopping for fuel just a little sooner.

35

letting a diesel run dry will get air in your fuel lines, and may require bleeding (though mine didn’t on the one occasion I let it get that far)

36

Rick, I tip my hat to your three decades of experience, and I agree with most of what you said. I disagree with your first two points, though.

There’s more to the vapor recovery system than a charcoal canister. The main purpose of it is to turn vapor back into liquid. As I said, I don’t know how it works, but it does.

About the time you started your career, I had gas-line freeze on three different vehicles. In L.A., you probably don’t have much experience with subzero temperatures. In my hometown, we do. An alcohol-based additive will bond with the water in the tank and carry it to the engine. If you forgot to do that, though, and the line freezes, there’s no cure but pushing the car into a warm garage. It doesn’t happen anymore.

37

You’re forgetting that some of us in the risk group probably couldn’t push our cars. I’m not sure I could (I very much doubt I have the sheer strength to do it). So I try to keep my car above or at least not much below 1/4 tank at all times.

38

IMHO 1/4, or somewhere between 1/4 and 1/2, is much more sensible than 1/2, especially if you’d have to fill up at night. It just seems that over the long run the increased risks of accidents and crime wouldn’t make the extra refuelings worthwhile in terms of increased personal safety in the form of less chance to run out.

Of course that’s assuming your fuel gauge is accurate, you have a reasonably decent fuel/range ratio, and you’re not in BFE.

39

Try driving from your hometown to mine. I’ll bet there are a few stretches in Indiana that might push 60 miles between gas stations.

40

Well I do know how the vapor system works, and you are incorrect. The canister has three connections on it. At one end there is a hose to the fuel tank. Next to that is another hose that goes to the evap valve on the engine. At the other end of the canister there is a connection that goes to the atmosphere. Vapors from the tank go into the canister where they are trapped by the activated charcoal. As you are driving the engine management computer will open the evap valve and admit air from the canister. since the gas cap is sealed, air is drawn in from the atmosphere through the canister picking up the fumes and conducting them to the engine where they are burned. If you ever get liquid fuel in the canister you will have problems. Liquid fuel will probably require the replacement of the canister.
As far as your second point goes, yes, I have seen cars with water in the fuel tank. However it has always been from the fuel supply not condensation. When I first started in this business we had one gas station in town that had a leaky regular tank. Every so often his pumps would put out H2O and we would get towed in died on the road cars. After the first one or two we learned to ask where they bought their last tank of gas. Buying gas at car washes used to be a major source for water introduction.
If yours was in fact condensation, I stand corrected. I find it interesting that these problem happened 30 years ago when gas tanks were not required to be encapsulated like they are now.