Just thought I'd ask the pros here about this. I've been told that it is bad practice to run fuel injected gasoline cars close to empty as the fuel pump needs the extra gasoline for cooling purposes. Specifically, the guy that explained that to me was referring to my Toyota Corolla.

My question then is, why is this so? He wasn't able to explain it, but said that he was told so by his mechanic. And does it apply to M-B gasoline cars like my '84 500SEL and the diesels as well?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

The fuel pump uses the fuel to cool itself and the pump does not shut off when the tank runs dry. It does not take to long to burn out the pump if the tank runs dry. Also some say that when you run the tank low enough you risk sucking in the dirt that may be sitting in the bottom of the tank, but there is a filter there as well so it depends. That and why would excess crap be in the tank!

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

It's not a problem until you run out of gas. Fill the fuel tank as soon as possible when the low fuel light illuminates.

Duke

Well won't turning off the ignition stop the fuel pump from pumping?

Does this apply to both gassers and diesels?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

Diesels you don not want to run out of fuel! Not only will they be hard to start but parts of the IP's are lubed by fuel.

On a fuel injected gas car from my understanding the pump starts pumping when you turn the key to a certain position and until the engine dies. It doesn't take long to fry a pump.

With the new Marine gas engines you have to fill the fuel filters with gas! Dry starts will ruin the pump in short order and they are not cheap.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

No. They are usually energized for a second or two when you turn the ignition switch to on. They then shut off until you turn the key to the crank position and the pump control circuit detects that the engine is turning -usually via an impulse for the ignition system. For the same reason the pump circuit will usually be de-energized if the control circuitry detects that the engine is not running such as if the engine stalls.

Duke

I've ran mine out of fuel twice. Each time, all it took was some priming and the beast cranked without any problems. But I do know it is wrong to do so due to the need for lubrication by the fuel.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

Besides the fuel pump cooling effect.
Another consideration is the comtanination in your tank. When you have 17 gallons of diesel all the particles and debris are "diluted" but when you have only 2 gallons in the tank everything settles to the bottom of the tank and the debris will be sucked closer to the screen. So the chances of clogging the screen and getting particles in the fuel system are greater.
Second item. When a fuel tank is near empty and the air cools overnight the moisture in the "air" in the tank will condense.This will in turn add water to your diesel fuel. Something you don't want. A full tank does not have as much "air" in it, so there is less moisture that can be condensed out.
We see this all the time in aircraft. So much that on every preflight you drain fuel from each tank to check for water in the fuel. The water will settle to the bottom of the tank and that is where the drains are. Of course we also check for contamination and the correct color to ensure you have the correct octane. You would be suprized how often water is found.
So there are a couple of reasons for keeping the tank above 1/2.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Must be nice to just reach over your head and open the drain.

Sure beats crawling underneath the third member and trying to disconnect fuel hoses when lying on your back.

Yes, and it sure is interesting to pull water out of an aircraft that has been recently flown.

If you are running the car so close to empty that you are worrying about whether or not there is enough fuel in the tak to cool the fuel pump then you should probably also be worried about running out of gas and getting stranded.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record