Fuel gauge not accurate
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Thread: Fuel gauge not accurate

  1. #1
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    Default Fuel gauge not accurate

    As an example, I drove home last night, and the fuel gauge read 1/2 tank when I turned the car off. Woke up this morning (no, I don't sleep walk/ sleep joy ride, better get in there before some other smart ass does ), and turned the car on and now I have less that a quarter. I know that when I drive home from work I'll probably have half a tank again. Any ideas? I do park on a slight incline...

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    Think of the relationship of the fuel tank to the angle of the dangle of the gauge sender. What position was the rear of the car in when you arrived home? Where was the rear of the car when you got in it this morning?

    Yes you more than likely had half a tank once the car was started and ready to drive away.
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    I get what you're saying. But it stayed at less than 1/4 the whole 30min to work. I would expect it to correct itself?
    Parking on a flat at work and then driving home was back to 1/2

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    1000+ Posts gerry freed's Avatar
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    The float rod is sticking to a baffle when at an angle ?
    It is very unusual to expect an accurate fuel gauge, not normally a feature of cars. You know when the tank is full because hopefully the bowser nozzle cuts off and you know when it is empty becuase the engine won't go. What else could you ask for?
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    I do reset the counter and refil after 550km. Just would be good to get at least one good reading so I know when I really do need to fill up if I can't get to fuel.... don't like the idea of carrying a can in the back and running till dry.

    Anyone tell me how far they get on a full tank for a 2003 2.0l Petrol, driving just a bit harder than miss daisey?

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    Quote Originally Posted by garethb View Post
    I get what you're saying. But it stayed at less than 1/4 the whole 30min to work. I would expect it to correct itself?
    Parking on a flat at work and then driving home was back to 1/2
    It is a system with a float operating a resistor controlling a galvanometer so don't expect too much! However what you describe is not as it should be.

    Older ones misbehave if the float gets heavy if petrol gets through the coating, for example.

    I've just pulled my Renault one apart and improved things a lot by cleaning the resistor and wiper.

    Good luck!
    JohnW

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    You're playing it about right at 550km refill intervals.
    Craig K
    2009 C5 HDi Exclusive

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    550 km sounds right to me (for commuting with freeway). Anything above 550 in city driving and you'd be taking chances.

    You would probably be able to stretch to 600 km but the chances of running out (or as worse just sucking in the junk sitting on the bottom of your tank) just ain't worth the stress.

    Assuming we're talking about C5 check here: Citroen C5 Fuel Tank Capacity

    Looks like the capacity of your tank is 66 litres. So fill up. Reset the trip computer to 0 and do your 550 km and see how many litres it takes to re-fill. 66, less the amount you put in the second time, will give you a rough idea of how much you had left in your tank
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    Sorry JohnW, pulling it apart is not on the top of my list! might get around to it one day?

    Thanks for the tip, didn't think of looking at the tank capacity after a refill. Should give me the info I need

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    Some cars I've owned have quite reliable fuel gauges and can be trusted as they approach empty. I once ran out of fuel in a Xantia because the resistor windings were worn and it would stay near 1/2 for ages and then drop rapidly. It caught me out on a big hill. Cars with a BSI, such as the C5, allow for limited calibration of the fuel gauge, but if that doesn't fix it, you have to look at living with it, possibly cleaning up the winding or replacing the tank unit.

    Parking some cars, including the Xantia, on an angle can also prove a problem when you go to start it next time and there is no fuel at the pickup. By contrast, a 'proper' car such as the Jaguar XJ-S has a separate sump/surge tank, so you have to be down to the last couple of litres for it to be starve of fuel. A sensible design.

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    My 205 seems to take ages to move off the full mark on the guage, then I can almost watch the guage go down to empty... I know however that as soon as the fuel warning light goes on there is 15litres left - almost a third of a tank so plenty to make it to the next servo before running too low and sucking junk from the bottom of the tank.

    Being a motorbike rider, Im used to no fuel guage so I am also in the habit of keeping a good handle on kms between tanks and lts/100kms for city/hwy cycle. I use a great app on my iPhone called Gas Cubby, fantastic for keeping tabs on all this stuff, servicing and vehicle maintenance costs etc. Check it out...

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    Quote Originally Posted by garethb View Post
    As an example, I drove home last night, and the fuel gauge read 1/2 tank when I turned the car off. Woke up this morning (no, I don't sleep walk/ sleep joy ride, better get in there before some other smart ass does ), and turned the car on and now I have less that a quarter. I know that when I drive home from work I'll probably have half a tank again. Any ideas? I do park on a slight incline...

    Thanks,
    Hi garethb
    I have a 2001 C5 petrol with the same or very similar system. When I got it(not new) the gauge read the top end well but seemed inaccurate at the bottom. That caused me some concern as it seemed to be getting very close to really empty some times when the light came on.
    However when I had to clean the tank after some very shitty fuel + rusty water, I bent the arm to change it the other way. So now it does not get to really full on the gauge but has plenty when the warning light comes on. I could probably have spent some more time and my patience but am happy with that result and it is safe for other casual drivers (which i had then)
    My other comment is that the gauge does not read the float input directly as in a "normal" car but is driven by the "computer" using that input. So it evens out the reading and also the fuel warning light arrival. So it decides on a gauge reading and sticks to it for a while before it decides to change it because the conditions have changed, eg a slope or fast cornering or whatever. It does also remember the reading when switched off
    My mileage is very different between just city day to day use and highway use. say 350 to 600+ kms
    jaahn

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    I don't know why running low on fuel would increase the chance of "sucking junk from the bottom of the tank". If the fuel outlet is near the bottom of the tank any junk would be sucked out as it appears regardless of the fuel level. After 20 years the bottom of the tank in my 504 was shiny clean when I looked while changing the gauge float.
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    I have also come to the conclusion that my series 2 Xantia also uses the BSI to interpret the fuel sender before sending a digital signal to the dashboard computer to set the fuel gauge to. It seems to start off about the level you would expect based on how the car has been parked but then quickly adopts the lowest signal that the sender sends it after you start to drive around.

    The gauge on the C5 X7 seems to be pretty reliable though which is good as they obviously use this to calculate the km to empty display based on what the sender sees is left in the tank and current fuel rate of use through the injectors.

    Cheers,

    Ken W

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    Quote Originally Posted by sfrawley View Post
    I don't know why running low on fuel would increase the chance of "sucking junk from the bottom of the tank".
    Good challenge. I don't know either and I'm wondering why I even said it

  16. #16
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    The junk might already be there as water and other impurities. Keeping more fuel in your tank just improves the ratio (quality) of fuel overall. Hence the often heard advice to have more, not less, fuel in the tank.

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    Quote Originally Posted by jaahn View Post
    Hi garethb
    I have a 2001 C5 petrol with the same or very similar system. When I got it(not new) the gauge read the top end well but seemed inaccurate at the bottom. That caused me some concern as it seemed to be getting very close to really empty some times when the light came on.
    However when I had to clean the tank after some very shitty fuel + rusty water, I bent the arm to change it the other way. So now it does not get to really full on the gauge but has plenty when the warning light comes on. I could probably have spent some more time and my patience but am happy with that result and it is safe for other casual drivers (which i had then)
    My other comment is that the gauge does not read the float input directly as in a "normal" car but is driven by the "computer" using that input. So it evens out the reading and also the fuel warning light arrival. So it decides on a gauge reading and sticks to it for a while before it decides to change it because the conditions have changed, eg a slope or fast cornering or whatever. It does also remember the reading when switched off
    My mileage is very different between just city day to day use and highway use. say 350 to 600+ kms
    jaahn
    How long did it take you to do the clean?

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    Quote Originally Posted by sfrawley
    I don't know why running low on fuel would increase the chance of "sucking junk from the bottom of the tank". If the fuel outlet is near the bottom of the tank any junk would be sucked out as it appears regardless of the fuel level. After 20 years the bottom of the tank in my 504 was shiny clean when I looked while changing the gauge float.
    Quote Originally Posted by C5_V6_2006 View Post
    Good challenge. I don't know either and I'm wondering why I even said it
    Hi
    This used to be a common idea in my earlier days. I thought it likely then

    However now all cars continuously circulate the fuel because the pump is a fixed large delivery and the unused fuel just goes back to the tank. This means the fuel does not rest for the dirt to settle out in the bottom.

    But one problem that does occure if you run close to empty all the time, is the pump wears out because it is often running dry as the fuel surges away from the inlet.
    jaahn

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    Quote Originally Posted by garethb View Post
    How long did it take you to do the clean?
    Hi garethb
    You answered while I was posting.
    It's a while back now but it is not hard to do. A couple of hours I guess with the float adjustment. The tank is not square but molded to fit, however the inside is smooth and the hole is large. The access port with the gauge/pump cover is under the rear seat drivers side. There is a plastic round cover in the body hole.
    Not a good job for a smoker???
    Jaahn

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