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The mkiv Supra Owners Club

why?


grimreaper
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Guest gzaerojon

and another thing while we're on the subject of fuel.

 

why is it a bad thing to run your tank to near empty, when the fuel pump pick up point is at the bottom of the tank??

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and another thing while we're on the subject of fuel.

 

why is it a bad thing to run your tank to near empty, when the fuel pump pick up point is at the bottom of the tank??

 

Because the less fuel in the tank the more the dregs will accumulate near that fuel pick up id say. More fuel means the crap is spread out.

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The tank is sort of shaped like an egg on its side. The level sender is a float arm like the float in a toilet cistern. It runs a contact against a variable resistance, giving an output to the guage.

 

When the tank is full the weight of the fuel causes the bottom of the tank to sag a little, increasing the tank's volume. As fuel is used the shape of the tank changes, and the sag reduces as the fuel is used. This reduces the volume of the tank and sort of offsets the drop in fuel level so that the sender can almost stay in the same position... Hence the needle hanging around the full mark for ages.

 

There comes a point where the tank shape stops changing so much and the float starts to drop. At about half a tank the shape is back to normal and as fuel is used the level drops in the narrowing tank (lower half of the egg on its side shape), meaning that the level drops faster the lower it gets. So the last 1/4 disappear really fast!

 

And you shouldn't run a tank really low as sediment in the bottom of the tank can get drawn into the pump inlet screen, blocking it and also blocking the fuel filter. Each time you run really low this blockage occurs more and more until it may cause a restriction.

 

EDIT - Oh, and this saggy tank effect will happen more in summer than winter!

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saggy tank theory made me laugh! revisit the bi metal strip /heat theory, all cars have this saggy tank then? some cars have a linear guage but they use a chip ,and it costs more to produce so a coil/ bi metal strip is used and because it uses heat which is retained ,it stops the needle bouncing up/down when the fuel sloshes about which would happen if the guage was a moving coil instrument or moving magnet instrument-you have your answer to the initial why?

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Polyethylene tanks do go saggy with the weight of fuel jagman.

 

Honest. I've seen it for myself on the Ford Focus. I spent 6 months designing the run dry system that stops them running out of fuel. Its possibly a stack up of things, resistance tolerances, arm positions, guages etc too.

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poly tanks may distort a bit , but the fuel level float and its pivot are fixed to the unit which drops in the top of the tank,so even if the tank sagged the fuel level would drop relative to the top of the tank so the guage would underread when full and over read when empty the opposite of what you now get,- back to the bi metal heat thing which has another benefit that the needle will not fluctuate when the cars voltage varies between battery voltage and alternator voltage . as a side note on instruments also of little value,the water temp guages are not linear so the distance between C and N is usually not the same temp difference between N and H , so if you show a reading over the normal setting car running hot and start pannicking it may be only a few degrees C a sign that all is not well but not imminent melt down about to happen,should it continue to rise then panic

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It's deliberate by design.

 

When you are near empty you need more accuracy to see when you are about to run out

 

When you have nearly a full tank do you care if it is 11 gallons or 12 - not really

 

the scale is deliberately non-linear with more accuracy at the low end on every car I've ever driven/owned I think - it's like a standard design feature on analog fuel guages..

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