Repeatedly Running On A Low Tank?

I see that thing about running low on fuel damaging fuel pumps posted all over the internet, but personally I think that's total b.s.

All the critical parts in the fuel pump - bearings (bushings), armature, brushes/commutator, pumping elements (vanes, rotors, or rollers) - are constantly bathed in the fuel as it flows thru the pump. That lubricates and cools the parts regardless of fuel level in the tank.

With regulator bypass pumping/circulation that modern cars have, there is full volume of fuel going thru the pump at all times it is running regardless of engine demand. The only effect of low fuel in the tank is a slight temperature rise of the volume of fuel in the tank (due to same electrical power dissipated in the pump being absorbed by less mass of fuel), and that rise will be very small - power used by fuel pump is small - temperature rise of the fuel in the tank and the tank itself will be very small - lots of mass compared to the power being dissipated.

*BUT* - again - the fuel is constantly flowing thru and around all internal components of the pump whenever it is running providing cooling (unless you actually run out and the engine stops - but that is a different scenario altogether, and even then, the pump will still be full of fuel at that point with a full column of fuel from its lowest end to the fuel rail - only the pickup will be filled with air, and there won't be any flow - and most cars turn the pump off when the computer senses that the engine is no longer running).

If anyone wants to argue this, be sure of your facts beforehand - I used to design automotive fuel pump components.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

"Bill Putney" wrote in message news: snipped-for-privacy@mid.individual.net...

As I understand it this concern mostly applies to older systems that included a return line, not more modern systems that use returnless systems and variable delivery pumps. The concern is that continually running with low fuel levels can lead to the fuel temperatures becoming much hotter than desirable. Older systems with a return line pumped much more fuel than necessary to the engine compartment - particularly during low speed / low power operation. The excess fuel is diverted back to the pump at the pressure regulator under the hood through the return line. However, this process of pumping the fuel to the front of the car, through the hot engine compartment, and through the pressure regulator heats the fuel. With a full tank, the warmer fuel being returned to the tank only changes the overall temperature of the fuel in the tank slightly. However, when the tank is nearly empty, the hot fuel coming from the return line has a much greater effect on the average temperature of the fuel in the tank. Repeatedly running the tank with low fuel levels results in a much higher average fuel temperature, particularly in hot climates. Higher pump operating temperatures can shorten the life of the pump. I can't say how much hotter the fuel can get, or how much this can shorten the life of the pump, but I do understand the theory. I suspect it was much more of a problem 20 years ago than now. I know my Sister, a chronic low tank driver, had to replace two in-tank fuel pumps on her mid 80's car, but her late 90's car never had a problem.

And even if the fuel never get so hot so as to contribute to a pump failure directly, running around with very low fuel levels can lead to running out of fuel altogether, which is definitely not good for the fuel pump.

Here are some interesting references:

Ed

This angle actually could have some validity. Back during the gasoline crisis of the 70's, related to the Arab oil embargo, I heard it claimed that one of the factors that caused the shortage was public hysteria. Instead of the average car running around with a half a tank of gas, people suddenly tried to keep their tanks near full. This caused a surge in demand, which made the shortage far worse.

I can see where someone speculating in gasoline futures would like to encourage people to increase the amount of gas they are buying in the short run to keep the demand up long enough so that they can get out with a big fat profit. However, if you are following this strategy, you need to get out before people revert to old habits...

Ed

I had a friend who would run his tank near empty all the time. He would stop and put in $2 every so often just to keep the car running.

I always kept my tank near full all the time (still do). I stop and put in (now, $6) every so often.

What's the difference?

(HINT: What happens when either one of us runs out of money?)

Think about what you just said Ed. Why would burning 5 gallons of fuel out of a full tank increase demand over burning 5 gallons out of an empty tank? 5 gallons is 5 gallons.

You are missing the point. While it is true that people didn't use any more fuel because they were keeping their tanks three quarters full instead of half full, it is also true that the sudden increase in the amount of gas being "stored" in the tanks of millions of car created a temporary supply problem. Suppose tomorrow everyone in America decided they needed to fill-up (all on the same day). Don't you suppose that might create a temporary shortage? This is supposedly one of the factor that lead to long gas lines during the fuel crisis of the 70's. When the oil embargo was announced, some people who might have normally waited a few days until they purchased gas ran down and purchased gas immediately. This sudden surge caused some stations to run out. Other noticed that stations were running short, so they too decided they needed to go buy gas immediately, resulting in more station running short. This was only a temporary problem but the up tick in demand was enough to upset the normal fuel supply system. It took day and alternate fueling rules to get the daily demand back into line with the ability of the system to meet the daily demand. Of course eventually demand dropped back to the normal and as people dropped back to their old habit of running closer to half a tank on average, there was actually a dip in demand.

Ed

After an initial surge in demand to top off, assuming no changes in driving behaviour, demand would quickly return to normal. Sustained increased demand would have had to be caused by hoarding OUTSIDE of what you can fit in your tank.

Simple math.

Hmm - are they variable delivery pumps, *OR* do they dump the excess back into the tank immediately from the regulator outlet (IOW - just like a recirc system, but with a very short (translation: No) loop to the engine compartment.

Serious question: Are there actually many cars with a variable delivery pump like you described, or they in fact actually like I describe in that the excess dumps back into the tank instead of first circulating thru a loop to the engine compartment like both my Chrysler LH cars do?

Good point - I had not considered that (picking up heat from the engine compartment in the older circ loop).

Similar to what I described in my previous post. Again, I maintain that the temp. rise even with only 2 or 3 gallons of fuel in the tank (along with the mass of the tank acting as a radiator dumping some of the heat into the outside air) is of little consequence to fuel pump longevity.

Makes sense with the later elimination of the engine compartment loop as you described. Of course one of the reasons they did that was to reduce likelihood of vapor lock due to ever-rising engine compartment temps. - the consant recirc. helped keep fuel rail temps. down (while - as you pointed out) slightly raising the temps. in the tank from the return.

Not as bad as you might think. The pumping section of the pump is at the bottom end of the pump assembly, so worst-case, air would hit the pumping blades (or whatever type of pumping section the particular pump has), and the fuel would still be completely filling the pump internals

- but - yeah - no flow for added cooling. But only lasts for a breif period until the engine stops running and the computer shuts the pump off. So - again - even running completely out is not as bad as you might first imagine.

No real problems with those links - except the last one: "if you werent so broke, you could afford to not wait that long. yes its true, especially for people who put only 5$ in their tank. since its always low, it could rupture your gas tank from overheating. this happened to the old hustlers down the street. they dont know how to take care of anything." Sorry - that is pathetic. :)

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

I suppose it might. About the same as if everyone decided to buy a dozen eggs from the grocery store. That wouldn't reflect anything except a spike in gasoline or egg demand. Once people had filled their tanks or gotten their eggs, demand would return to normal.

No, the fuel shortage was caused by Nixon's price controls. Nothing else. Once the price controls were removed supply came back to normal but the prices were higher.

The oil embargo was imposed by the middle east cartel. We only got about 25% of our oil from them at the time so the embargo hurt and put a crimp in supplies for a while but we could have lived through it. The oil companies wanted to let the laws of supply and demand work to balance it out. But the congress insisted that "We Have To Do Something" so Nixon put price controls on almost everything made from crude. If you can't make a profit supplying something, then were is the incentive to supply it?

The funny thing was that oil tankers were laying at anchor just off Houston waiting for the price of crude to go up, I saw pictures. There was no shortage of crude because most of the middle east countries ignored the embargo and what short fall we had was made up by other suppliers.

Jack

I made it up to 31 MPG with the tC...

Driving like an old man with a hat...

WTF fun is that?!?!

So, to make up for it I took a 95 MPH rip on the highway Saturday...

Nice theory, but the wage and price controls had been in effect for several years before the arab oil embargo tirggered the first wave of gas shortages with the long lines. I had no problems buying gas in 1971, 1972, and most of

1973. I am only claiming that the sudden (but temporary) up tick in demand related to people trying to keep their tanks full made the problem worse. It was not the root cause, but a factor that contributed to the long gas lines that appeared quickly after the Arab oil emargo started.

We did live throught it, but not without the pain of long lines at the stations. Suppose someone cut off 25% of our oil today? How long would the lines be tomorrow?

Again, a fine theory, but the price controls were in effect for a couple of years before the long lines appeared. I do agree that the price controls had an effect. However, the controls did not apply to imported oil. They did discourage US exploration and production, so they increased our dependence on foreign oil, which made the effects of the Arab oil embargo worse. BTW, oil prices were not actually frozen under the Nixon policy - "Nixon's cost of living council creates a price ceiling that allows stored oil to be sold at 35 cents above the current prices and that newly produced oil could be sold at uncontrolled prices. " Price controls on domestic crude oil actually were actually more restrictive after the 1973 oil crisis had passed. I suppose you could blame the 1979 shortages on these controls, but I can't seee ow you can blame the 1973 crisis on the wage and price controls since they didn't actually control oil prices.

Well now you are saying there was no shortage....please explain. I've heard the story of tankers sitting off the coast from others (one ex-Coast Guard guy in particular), so I assume it is true. But isn't that how an emargo works? When the Arabs imposed their embargo, some of their oil was in tankers under contract to them, so they stopped deliveries until their demands were met, or when they needed the money (which is what actually ended the embargo).

Ed

I am surprised. I know Datsun 280Zs had plastic tanks in 1976. I am thinking that plastic tanks are actually a good thing.

Ed

Possum is better....

Ed

I don't think it is a fake. I read a SAE Paper that drew similar conclusions but with a lot of caveats. As I recall the authors speculated that the new oil acted like a solvent and that it dissolved the wear reduction materials from the surface of components. This led to increase wear rates for those components.

You might want to read the follow abstracts of SAE papers:

Regards,

Ed White

on 6/5/2008 7:06 AM C. E. White said the following:

Yes, they are easier and safer than metal when drilling holes in the tank to steal gas, as has been happening lately. Drilling into the tank with cordless drills won't cause a spark which may result in injury or death to the thieves, and thereby limiting the amount of damages you may have to pay for injuring the thief from other reasons, like dropping the drill on their face.

Are you sure about that? As I recall the wiring diagram for the SO's RAV4, there is not a separate feed for a low fuel level switch. I'll check the wiring diagrams this evening when I get home.

Ed

Take your cordless drill into a completely darkened room and hit the trigger, and tell me there isn't a spark! You still have a rotor, a stator and brushes, and any time you have those, you can generate a spark.

Maybe a few will start blowing these bastards to hell...

I think the chief reason they are in the tank is to prevent vapor lock. If the pumps are external they have to pull fuel through a line from the tank to the pump. This "suction" lowers the fuel pressure and can lead to vaporization in the lines, followed by vapor lock, and bad things. By placing the pump near the bottom of the tank, there is very little pressure drop from the fuel inlet to the pump, so little chance for vapor lock.

You could pull a line off the bottom of the tank to accomplish the same thing, EXCEPT, this would be a bad idea since everything in the bottom of the tank would go directly into the bottom opening. In the real world, the pump is slightly off the bottom, allowing water, rust, dirt, whatever to fall to the bottom and not be picked up.

My sister had a 1986 Jetta. It had two fuel pumps, an internal and an external. The external pump was a high pressure positive displacement pump needed to feed the Bosch CIS fuel injection system. The in-tank pump feed the external pump. She had the in-tank pump fail twice. Both times it was in hot weather. The car would start fine in the morning, but the first time you stopped in traffic during the heat of the day, the car would vapor lock and stall.

Ed