Myth buster: There's no need to worry about sludge in the bottom of your gas tank.

Dear Car Talk

Dear Car Talk | Oct 01, 1997

Dear Tom and Ray:

I am looking to you to validate or refute a family myth. My
father-in-law and mother-in-law believe in the phenomenon of sludge at
the bottom of the gas tank. The family myth goes that you should never
allow your tank to go below one-quarter full or all the sludge will get
sucked up into the engine and destroy it.

I always thought this was a myth designed by parents who didn't want
their kids to run out of gas, and I dismissed it completely. But
recently, my older sister revealed that our dad had told her the same
thing! Is it true, or did two sets of parents on opposite coasts come up
with the same urban myth? -- Sharon

RAY: I guess this myth has gone bicoastal, Sharon. While there usually
is some condensed water at the bottom of the tank, that small amount of
water doesn't do any harm in the engine. And while there are often
flakes of rust because of that water, there's a filter that prevents
them from getting sucked into the engine and ruining it. So it is a

TOM: Plus, the thing that most people don't realize is that you're
ALWAYS sucking gas from near the bottom of the tank. Why? Because that's
where the pickup sits.
RAY: It has to sit there. If the gasoline pickup (the tube that sucks
the gasoline out of the tank) was at the top of the tank, it would only
work when the tank was completely full, right? Think about it.

TOM: And I think you're right that this "never let it go below a quarter
tank" myth served the interests of parents, who a) didn't want to have
to pick up the kid when he ran out of gas in East Armpit at midnight,
and b) didn't want to get in the car the morning after junior borrowed
it and find no gas in it (a teen-age tradition celebrating its 100th
anniversary this year along with the automobile).

RAY: Now, having said all that, we should add that while running down
below a quarter tank doesn't do any harm, running completely OUT of gas
can do some damage (and we're not just throwing this in for the sake of
all the parents of teen-agers who got mad at us in the last paragraph).
We've seen a number of cases in which the electric fuel pump has been
ruined by having been run on empty. Why? Probably because the pump uses
the fuel as both a lubricant and a coolant.

TOM: So here's the story in a nutshell, Sharon. You have our permission
to run your car down below a quarter tank as often as you want to. Just
don't expect either set of parents to be real sympathetic when you call
them for a ride -- or a new fuel pump -- when you DO space out and run
out of gas, OK?

* * *
TOM: Well, you asked for it, and here it is. My brother and I sat down
and wrote down everything we know about how to make your car last

RAY: And it only came out to eight lousy pages!

TOM:But now this gold mine of information can be yours....

RAY: Get your copy of "Ten Ways You May Be Ruining Your Car Without Even
Knowing It!" by sending $3 and a stamped (55 cents), self-addressed,
No.10 envelope to Ruin No.1, PO Box 6420, Riverton, NJ 08077-6420.

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