Re: Repeatedly Running On A Low Tank?



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:
http://findarticles.com/p/articles/mi_qa3828/is_199807/ai_n8795921 http://www.autocarepronews.com/default.aspx?type=art&id 095& http://www.athomemechanic.com/archives/2005/06/keep_your_fuel.html http://answers.yahoo.com/question/index?qid 080513130941AATbVqN
Ed
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C. E. White wrote:

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')
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