Author Topic: How much fuel left when fuel light on  (Read 9522 times)

peteo48

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Re: How much fuel left when fuel light on
« Reply #15 on: January 16, 2022, 10:26:48 AM »
My somewhat obsessive personality means I rarely get near the end of the tank. That said, I remember on my Mk2 the fuel light came on when the miles left was 54 miles. It's stuck in my mind because it seemed the light was coming on prematurely.

I've just refilled my Mk4 when I got down to 3 bars (there are 10). I got a smidgeon under 30 litres in so a good 10 litres left. I might live a little next time and let it go down to 2 bars 8) 8)

ColinB

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Re: How much fuel left when fuel light on
« Reply #16 on: January 16, 2022, 11:23:36 AM »
As ColinB already said, the fuel pickup point in the tank is fixed, it does not float around - so however much fuel in the tank the petrol is being drawn from same spot, so if there is dirt there with an almost empty tank, there would have been dirt there with a full tank. 
But this ignores floating debris.

Read the previous comments. SteveiD said:
Best not to run to the bottom of the tank as there will inevitably be a bit of dirt or even water down there!
That refers specifically to water or dirt collecting at the bottom of a nearly-empty tank, and perpetuates the myth that those contaminants will somehow evade the pump until the tank's nearly empty. In reality anything in that position would already have clogged the filter or been ejected via the engine because it would have been first to get sucked into the pump, not the last, because that's where the pump inlet is. The point is that it's not somehow worse if the tank's empty. We both pointed out that the only thing that would concentrate at the bottom of a nearly empty tank would be floating debris as the free surface gets closer to the pump suction.

And, personally, I wouldn't rely on the range calculation to judge how much fuel is left, you only have to look at how that number jumps around as you drive.

Lord Voltermore

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Re: How much fuel left when fuel light on
« Reply #17 on: January 16, 2022, 12:34:01 PM »
I think the main risk is not so much debris getting sucked in but air.   With a flat,under seat   tank mounting   fuel depth is spread over a wider area.

Think of it like a standard nato type 20 litre jerry can.  (about 470mm x360 mm x 165mm  )   
5 litres of fuel would be about 117mm deep when the can is stood upright  ,90mm  deep if the can is laid on edge,  but only 41mm deep if the can is laid flat on its back.(and a 40 litre  underseat tank may spread it even flatter  with less depth  than this)

Thus with just 5 litres remaining there is an increased risk the fuel will surge when cornering exposing the the pick up intake to air. Also a car on a significant uphill or downhill gradient might start  'sucking on fumes'  significantly sooner than one on level ground.
  Trust a dog to guard your house  , but not your sandwich

Neil Ives

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Re: How much fuel left when fuel light on
« Reply #18 on: January 16, 2022, 02:30:36 PM »
.... a car on a significant uphill or downhill gradient might start  'sucking on fumes'  significantly sooner than one on level ground.
This reminded me of driving one of my first cars, a convertible split-screen Morris Minor, (like the picture) The electric fuel pump was inside the engine bay attached to the bulkhead. I didn't have much money then so fuel top-ups of a few gallons were normal. When the fuel level was very low the pump would start ticking as it began to suck air through. The ticking got more and more frequent as the fuel level got lower. To eak out the miles I would swerve the car to one side to make a wave of fuel to be sucked up by the pump. Such fun!
« Last Edit: January 16, 2022, 02:33:26 PM by Neil Ives »
Neil Ives

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