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
Here are some interesting references:
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. :)
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')
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