92-95 Civic owners -a Q

[Nerds laugh at me]

I notice that my gas guage barely moves untill I hit around 100 miles on the odometer. It drops almost a good 1/4 inch or so the next time that the car is started, but not really all that drastically when I drive a constant trip of over 100 miles. It only seems to just plummet when I take short trips and restart the car every 20 or so miles. ( And I know that starting the car does not burn up that much fuel )
What is causing this ?
Is the float getting hung up on something ?

[thisisntjared]

my civic would do that. its like the gas gauge is maxed out and it doesnt read almost 3 gallons past the full mark.

[Sigifrith]

Same here. Not a very precise unit.

[GasSavers_TomO]

I forgot the exact name for the type of circuit that honda uses for its fuel gauges....it updates slowly and isn't linear though. It's basically a reference.

[jbum]

same here. it's not for accurate readings, just general status of your fuel amount.

[GasSavers_Ryland]

the reason for the sharp drop after 100 miles when the engine is cold is that your gas is cold as well, if you drive 20 or so miles in a single shot, almost all of your fuel will have passed thru the fuel rail that is bolted to the head of your engine, where it has had a chance to warm up, and then is sent back to your fuel tank, warm fuel expands.

[GasSavers_bobski]

^ I doubt that's it.

Honda uses a simple gauge and wire coil variable resistor, operated by a float. The gauge moves so slowly because it has a fairly weak coil to move the needle, and a volume of viscous fluid tied to the needle shaft to dampen movement. Mild pressure against a thick fluid results in slow progress. If they didn't dampen the reading somehow, the gauge needle would bounce around whenever the fuel sloshed around in the tank.

[GasSavers_Ryland]

Originally Posted by bobski

^ I doubt that's it.

Honda uses a simple gauge and wire coil variable resistor, operated by a float. The gauge moves so slowly because it has a fairly weak coil to move the needle, and a volume of viscous fluid tied to the needle shaft to dampen movement. Mild pressure against a thick fluid results in slow progress. If they didn't dampen the reading somehow, the gauge needle would bounce around whenever the fuel sloshed around in the tank.

look at how much fuel expands when heated even just a few degrees, then get under your car and feel the gas tank, and how cold it is, drive 30 miles or more and feel the gas tank again, it will be warmer, I've done it.

[GasSavers_bobski]

Originally Posted by Ryland

look at how much fuel expands when heated even just a few degrees, then get under your car and feel the gas tank, and how cold it is, drive 30 miles or more and feel the gas tank again, it will be warmer, I've done it.

So you're suggesting... What? That the density of the gasoline changes enough to make the fuel sender float neutrally boyant? Or just that it changes the level in the tank?
I think your solution is way more complex than it needs to be. The variable resistor used for fuel level senders is far from a precision instrument. Combined with the odd body contours that fuel tanks generally follow, you get strange patterns of needle movement relative to actual fuel volume.

[thisisntjared]

Originally Posted by Ryland

look at how much fuel expands when heated even just a few degrees, then get under your car and feel the gas tank, and how cold it is, drive 30 miles or more and feel the gas tank again, it will be warmer, I've done it.

yea but its not 3 gallons worth. realistically how much fuel will ever be in the fuel lines?? how much fuel will ever get above 100degrees?