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DEBUNKING THE GAS TANK MYTH

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 getsucked 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 myth.

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 work only 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 didn't want to have to pick up the kid when he ran out of gas in East Armpit at midnight, and 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?

A belt worn too soon

Dear Tom and Ray: Recently, I took my 1993 Buick Century to a local dealer for servicing prior to starting on a road trip. The odometer reading was slightly over 22,000 miles, and at the time I had had the car 33 months. I was shocked when the service department informed me that a belt changeover was needed involving the "serpentine" belt.

Is it normal to have belts replaced on today's automobiles at 22,000 miles? I can appreciate that belts may deteriorate, but I'm wondering about the kind of material used in these belts. Could they be made of some low-grade material, possibly including cork granules?
-- Carl
Tom: Our chief investigator, Paul Murky, of Murky Research, has looked into this for us, Carl. And he was shocked to discover that belts for the '93 Century are, in fact, made out of old Boone's Farm Strawberry Hill corks, used Hartz flea collars and petrified giraffe droppings from Busch Gardens.

Ray: Actually, belts should last 30,000 to 40,000 miles, Carl. So 22,000 is a little on the early side. But there are several possible explanations.

Tom: One is that the belt had just worn out prematurely. The belt could have been defective, or may have been damaged somehow. Or perhaps -- in addition to your 22,000 miles -- you spent the equivalent of another 15,000 miles idling the engine, waiting for your wife to come out of the betting parlor.

Ray: Another possibility is that the dealer was being less than honest with you. Maybe you didn't really need a new belt. Maybe it just showed some early signs of wear, but he had a boat payment due and needed a few extra bucks that week.

Tom: More likely, however, is that he was just being cautious. You said you came in for service prior to a road trip. My guess is that he detected some signs or wear in the belt -- maybe a little bit of cracking -- and recommended that you play it safe and replace the belt before you hit the road.

Ray: Sure, it's possible that the belt could have lasted another 10,000 miles. But on the other hand, it could have broken -- and left you with no power steering -- while you were on your way down Pike's Peak.

Tom: And then you'd be writing to us complaining that you took the car in for service, and the morons at the dealership didn't have the good sense to tell you to change the belt that was about to break.

Write to Car Talk in care of The Buffalo News, P.O. Box 100, Buffalo, N.Y. 14240. Tom and Ray can't answer your letter personally but will run the best letters in the column. Their radio show airs at 7 and 10 a.m. Saturday on WBFO (88.7).

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