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Running on Empty

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Old 07-10-2018, 12:02 PM
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Fuel indicator said "0 Km, Refuel Immediately". I refueled 3 km (2 miles) away using 51 L. Full tank is said to be 55 L. Using average mileage and the distance to the gas station, there was enough gas for about 43 km (27 mi) to bone dry from the 0 Km alarm. So maybe a safe absolute margin is about 20 Km. I had a part filled jerry can in the trunk (for lawn mover) so was not worried. Did anyone else do a similar experiment? This is in a 2015 S3
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Old 07-10-2018, 04:29 PM
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Originally Posted by morris39 View Post
Fuel indicator said "0 Km, Refuel Immediately". I refueled 3 km (2 miles) away using 51 L. Full tank is said to be 55 L. Using average mileage and the distance to the gas station, there was enough gas for about 43 km (27 mi) to bone dry from the 0 Km alarm. So maybe a safe absolute margin is about 20 Km. I had a part filled jerry can in the trunk (for lawn mover) so was not worried. Did anyone else do a similar experiment? This is in a 2015 S3
Every time you run your car near empty you are putting unnecessary stress on the fuel pump....I've only seen my low fuel light once since new for this reason.
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Old 07-10-2018, 05:17 PM
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+1. The fuel inside the tank helps keep the fuel pump cool and within operating temp. Running a tank to empty may cause unnecessary stress to the fuel pump especially when it is not completely submerged while the car is moving, i.e. fuel sloshing around. I do not really understand why one want to know how much fuel is left in the tank when the gauge is at Empty. The gauge is there to warn Owners to gas up soon. I make it a habit to gas up when the gauge hits the red zone. I never wait until Empty before gassing up.
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Old 07-10-2018, 05:21 PM
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Originally Posted by M4D View Post
Every time you run your car near empty you are putting unnecessary stress on the fuel pump....I've only seen my low fuel light once since new for this reason.
Do we know that or is that a guess? If stress, then can it be quantified in a rough way? I am skeptical, unless you have actual pump design experience? Do you or know someone?
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Old 07-10-2018, 05:30 PM
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Originally Posted by morris39 View Post
Do we know that or is that a guess? If stress, then can it be quantified in a rough way? I am skeptical, unless you have actual pump design experience? Do you or know someone?
It is not a guess, it is fact. Haha, skeptical......do what you want. Not our problem...
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Old 07-10-2018, 07:32 PM
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Originally Posted by morris39 View Post
Do we know that or is that a guess? If stress, then can it be quantified in a rough way? I am skeptical, unless you have actual pump design experience? Do you or know someone?
Here's a nice article with illustrations about submerged electric fuel pumps. Excerpt taken from: AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics - What Causes Fuel Pumps to Fail

To cool and lubricate the internal components, fuel must flow continuously through the fuel pump. Insufficient flow limits cooling and lubrication of the pump. A leading cause of fuel pump failure is running the fuel tank low. This is particularly critical on late model vehicles without a fuel pressure return system. Running such a vehicle out of fuel once can permanently damage the fuel pump.


When the fuel level is low, the pump has to work much harder to produce the same pressure. This is because the reduced fuel weight no longer pushes fuel into the pump. Instead the pump must draw the fuel in. A low fuel level also means less fuel to dissipate heat and lubricate the pump. The combination of an overworked pump, reduced cooling and lubrication will likely damage the fuel pump.
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Old 07-10-2018, 07:39 PM
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Some more reading concerning electric fuel pumps from the Bosch website: https://www.boschautoparts.com/en/au...ump-assemblies

What are the most common causes of fuel pump failures?

Some of the top causes for fuel pump failure are contamination (which is caused by dirt and debris entering the system from re-fueling), overheating, and fuel pump wear over time. Frequently driving on a low fuel tank can accelerate wear on the fuel pump.

How can fuel pump failures be prevented?

To prevent fuel pump failure, always replace the fuel filter and clean the tank to reduce debris and contamination when replacing your pump. Addditionally, avoid extended periods of low fuel levels to help maintain the durability of the pump.

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Old 07-11-2018, 05:49 AM
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Originally Posted by plat27265 View Post
Some more reading concerning electric fuel pumps from the Bosch website: https://www.boschautoparts.com/en/au...ump-assemblies
Good points I grant you. Nowhere do I recommend running fuel tanks to empty. This was an experiment in case that an inadvertent low fuel condition occurs. Some question still remain . At what level is harm being done? Is reported damage based on self-reports of low fuel or how? A number of contributing factors are listed, how do they scale, additional dirt ingress at low fuel may be a biggie. Why do the manufacturers not warn about this?
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Old 07-11-2018, 07:16 AM
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Originally Posted by morris39 View Post
Good points I grant you. Nowhere do I recommend running fuel tanks to empty. This was an experiment in case that an inadvertent low fuel condition occurs. Some question still remain . At what level is harm being done? Is reported damage based on self-reports of low fuel or how? A number of contributing factors are listed, how do they scale, additional dirt ingress at low fuel may be a biggie. Why do the manufacturers not warn about this?
It is common automotive knowledge...stop asking questions and move on dude.
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Old 07-11-2018, 08:41 AM
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Originally Posted by morris39 View Post
Good points I grant you. Nowhere do I recommend running fuel tanks to empty. This was an experiment in case that an inadvertent low fuel condition occurs. Some question still remain . At what level is harm being done? Is reported damage based on self-reports of low fuel or how? A number of contributing factors are listed, how do they scale, additional dirt ingress at low fuel may be a biggie. Why do the manufacturers not warn about this?
Bosch is a fuel pump manufacturer. By design, they would know the conditions that will jeopardize its life. Concerning the scale, they (the pump manufacturer) cannot tell because car manufacturers design fuel tanks differently from one another. There is no one shape fits all kind of scenario.

The Owners Manual of my '16 S5 contains warnings against running the gas tank until its completely dry which is not the case here. However, it does say that once the 'Please Refuel' indicator lights up for the very first time, there is still around 2.1 to 2.4 gallons or 7 to 9 liters of fuel left in the tank. This is for the B8.5 A5/S5. Your car's owners manual should have similar info.

I think the key here is to never have to see the 'Please Refuel' indicator light to remind you to fuel up. Be proactive by planning ahead and fuel up whenever you are close to Empty or when the gas gauge is in the red zone. Especially on long trips where the next gas station is miles away and you only have 3 gallons left to spare just because you procrastinated by skipping the last gas station on your way to your destination.
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